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Title: The Romance of the Microscope
- An interesting description of its uses in all branches of science, industry, agriculture, and in the detection of crime, with a short account of its origin, history, and development
Author: Ealand, C. A.
Language: English
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*** Start of this LibraryBlog Digital Book "The Romance of the Microscope
- An interesting description of its uses in all branches of science, industry, agriculture, and in the detection of crime, with a short account of its origin, history, and development" ***
Transcriber’s Notes:
Text enclosed by underscores is in italics (_italics_), and text
enclosed by equal signs is in bold (=bold=).
Additional Transcriber’s Notes are at the end.
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* * * * *
THE ROMANCE OF THE
MICROSCOPE
* * * * *
[Illustration:
_By the courtesy of Messrs. F. Davidson & Co._
AN EXAMPLE OF A MICRO-TELESCOPIC PHOTOGRAPH
“Nanda Kot.” Height, 22,510 feet; distance, 60 miles. The trees at
the lower right-hand corner are only 20 yards from the photographer.
A remarkable photograph, showing the great depth of focus of the
micro-telescope.
]
* * * * *
THE ROMANCE OF THE MICROSCOPE
AN INTERESTING DESCRIPTION OF ITS
USES IN ALL BRANCHES OF SCIENCE,
INDUSTRY, AGRICULTURE, AND IN THE
DETECTION OF CRIME, WITH A
SHORT ACCOUNT OF ITS ORIGIN,
HISTORY & DEVELOPMENT.
BY
C. A. EALAND, M.A.
AUTHOR OF “ANIMAL INGENUITY OF TO-DAY,”
“INSECTS AND MAN,” &C., &C.
WITH 39 ILLUSTRATIONS & NUMEROUS DIAGRAMS
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=By Prof. G. F. SCOTT ELLIOT, M.A., B.Sc.=
The Romance of Savage Life
The Romance of Plant Life
The Romance of Early British Life
=By EDWARD GILLIAT, M.A.=
The Romance of Modern Sieges
=By JOHN LEA, M.A.=
The Romance of Bird Life
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=By SIDNEY WRIGHT=
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=By ARCHIBALD WILLIAMS, B.A. (Oxon.), F.R.G.S.=
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The Romance of Insect Life
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* * * * *
CONTENTS
CHAPTER I PAGE
EARLY DAYS OF THE MICROSCOPE 17
CHAPTER II
SOME EARLY MICROSCOPISTS 29
CHAPTER III
THE ACTION OF LIGHT 40
CHAPTER IV
THE COMPOUND MICROSCOPE 50
CHAPTER V
ANIMAL LIFE IN PONDS AND STREAMS 66
CHAPTER VI
PLANT LIFE IN PONDS AND STREAMS 83
CHAPTER VII
THE MICROSCOPE AND PLANT LIFE 97
CHAPTER VIII
ANIMAL LIFE AND THE MICROSCOPE 112
CHAPTER IX
THE STUDY OF THE ROCKS 125
CHAPTER X
THE MICROSCOPE AS DETECTIVE 137
CHAPTER XI
BACTERIA 152
CHAPTER XII
MEDICAL WORK WITH THE MICROSCOPE 167
CHAPTER XIII
THE MICROSCOPE AND AGRICULTURE 178
CHAPTER XIV
THE MICROSCOPE AND INSECT LIFE 192
CHAPTER XV
THE MICROSCOPE BY THE SEASIDE--ANIMAL LIFE 208
CHAPTER XVI
THE MICROSCOPE BY THE SEASIDE--PLANT LIFE 225
CHAPTER XVII
MICRO-TELESCOPE AND SUPER MICROSCOPE 239
CHAPTER XVIII
CHEMISTRY AND THE MICROSCOPE 248
CHAPTER XIX
USE OF THE MICROSCOPE IN MANUFACTURES 260
CHAPTER XX
THE MICROSCOPE AND CAMERA ALLIED 274
CHAPTER XXI
HOW THE GLASS USED IN MICROSCOPES IS MADE 282
CHAPTER XXII
THE CHOICE AND USE OF APPARATUS 291
LIST OF ILLUSTRATIONS
PAGE
NANDA KOT--AN EXAMPLE OF
TELE-PHOTOGRAPHY _Frontispiece_
HEAD OF DOG FLEA 56
STARCH GRAINS OF POTATO 72
PHOSPHORESCENCE ANIMALCULÆ 72
PROTEUS ANIMALCULE 72
CYCLOPS 72
BLADDERWORT 88
SPORES OF HORSE-TAIL 88
HAIRS ON A POTATO LEAF 88
SPIROGYRA 88
THORN INSECT 120
HEAD OF PALM WEEVIL 120
LEAF INSECT 120
HEAD OF STICK INSECT 120
FORAMINIFERA 128
DIATOMS 128
STINGING HAIRS OF NETTLE 144
BUTTERFLY WING SCALES 144
CRYSTALS FROM HUMAN BLOOD 168
CRYSTALS FROM THE BLOOD OF THE BABOON 168
CLUSTER CUPS 184
RUST OF WHEAT 184
POLLEN GRAINS ON A GRASS FLOWER 184
LOWER SIDE OF A FERN FROND 184
HEAD OF A BEETLE 200
HEAD OF HERCULES BEETLE 200
A CICADA 200
HEAD OF MANTIS 200
FACE OF A FLY 216
SECTION OF HUMAN SKIN 216
FEELER OF COCKCHAFER 232
VIEW WITH ORDINARY CAMERA 240
VIEW WITH MICRO-TELESCOPE 240
EYE OF A COCKCHAFER 256
HOOKS ON A BEE’S WING 256
SPIDER’S FOOT 264
FLY’S FOOT 264
FLY’S EYE 296
IMAGES SEEN BY A FLY 296
* * * * *
The Romance of the Microscope
CHAPTER I
THE EARLY DAYS OF THE MICROSCOPE
It is certain that lenses were used as early as the thirteenth century,
and it is probable that they date back to far earlier times. The
ancient gem cutters probably used spheres of glass filled with water as
magnifiers, their work could hardly have been accomplished without some
artificial aid. We know, from early writings, that burning glasses were
used by physicians in their work, and Seneca, the author, who wrote in
A.D. 63, says: “Letters, however small and dim, are comparatively large
and distinct when seen through a glass globe filled with water.”
Euclid, whose name at least is familiar to everyone, was, as shown
by his writings, perfectly well acquainted with the fact that curved
mirrors may be used to magnify objects, and that was so long ago as
the third century B.C. Convex glasses, used as spectacles, were first
mentioned by Bernard de Gordon, about 1307, but, as far as we know,
they were never used for the purpose of studying minute living objects.
To Leonardo da Vinci belongs the honour of seriously investigating,
for the first time, the properties of concave and convex lenses,
and several alchemists, as the early chemists were called, used
flasks filled with water, concave mirrors or glass balls to gather
together the rays of the sun. “Long before the dawn of the seventeenth
century, the principle of the lens was both comprehended and applied
to scientific matters by the Englishmen, Leonard Digges and his son
Thomas, and by the Italian, Giambattista Porta.”
Towards the end of the sixteenth century and the early part of the
seventeenth century, interest in the minute structure of natural
objects appears to have developed. As early as 1590, Thomas Mouffet
used magnifying glasses in studying small mites, and in 1637 Descartes
invented a single lens microscope in which the rays of light were
reflected on to the object by means of a concave mirror. This method of
illumination, it is interesting to note, is still used in some forms of
pocket magnifiers. Most of the early discoveries were made with single
lenses, for in the compound microscopes which were first made, it was
only possible to view such a small portion of an object at one time
that the advantage lay with the less complicated instrument.
The earliest microscopes were simply short tubes of any material which
would not admit light; at one end there was a lens, at the other a
glass plate on which the object to be examined was placed. Because
these crude instruments were chiefly used for the examination of
insects they were known as “Vitrea pulicaria” or “Vitrea muscaria.”
Later they were called “Engyoscopes,” and, after the invention of
compound microscopes, they were described as “Microscopia ludicra,” as
opposed to the latter instruments, known as “Microscopia seria.”
The next stage in the development of the microscope consisted in the
introduction of lenses of very short focal length, and, in 1665, Robert
Hooke used small glass balls, formed by fusing threads of drawn glass,
for this purpose.
It was Antony van Leeuwenhoek, however, who perfected these
instruments. He brought an extraordinary skill and industry to bear
on the grinding and polishing of minute lenses of short focal length.
Already in 1673 Regnier de Graaf wrote to the Royal Society in London
that Leeuwenhoek was making glasses far superior to those of the great
Italian lens maker, Eustachio Divini. Leeuwenhoek’s success was largely
due not only to his method of grinding, but also to the skill with
which he mounted his lenses, which were accurately fitted into a minute
hole in a metal plate. The object to be examined was firmly held in a
stand and adjusted by means of a screw movement. By this means, and by
the use of hollow metal reflectors, he succeeded in availing himself
of transmitted light in the case of transparent objects. Leeuwenhoek
was able to make immense advances with these instruments, the minute
pond animals he could see with ease, and by 1683 he had even attained a
sight of the bacteria. His researches represented the high-water mark
of work done with the simple microscope, most of the later work was
carried out with the compound instrument.
The earliest history of the compound microscope is difficult to
separate from that of the telescope and, in any complete account, the
two instruments must be considered together. It appears that the first
scientist that conceived the idea of using a series of lenses, rather
than a single lens, was Leonard Digges, whom we have already mentioned.
In a book by Porta, a writer who though not himself original, was
gifted with great curiosity and industry in the collection of the ideas
of others, we read: “How to make plain a letter held far away by means
of a lens of crystal,” and also that “with a concave lens you see
things afar smaller but plainer, with a convex lens you see them larger
but less distinct. If, however, you know how to combine the two sorts
properly you will see near and far both large and clear.”
Shortly after the publication of Porta’s book the method of combining
two lenses into a microscope or telescope was discovered, quite
accidentally, by a Dutch boy named Zacharias, who worked in the shop
of his father, a spectacle maker. The event was described by Willem
Boreel, Dutch Ambassador to France, in a letter written in 1655. He
wrote: “I am a native of Middleburg, the capital of Zeeland, and close
to the house where I was born, there lived in the year 1591 a certain
spectacle maker, Hans by name. His wife, Maria, had a son, Zacharias,
whom I knew very well, because I constantly as a neighbour and from
a tender age went in and out playing with him. This Hans or Johannes
with his son Zacharias, as I have often heard, were the first to invent
microscopes, which they presented to Prince Maurice, the governor and
supreme commander of the United Dutch forces, and were rewarded with
some honorarium. Similarly they afterwards offered a microscope to
the Austrian Archduke Albert, supreme governor of Holland. When I was
Ambassador to England in the year 1619, the Dutchman Cornelius Drebbel
of Alkomar, a man familiar with many secrets of nature, who was serving
there as a mathematician to King James, and was well known to me,
showed me that very instrument which the Archduke had presented as a
gift to Drebbel, namely, the microscope of Zacharias himself. Nor was
it (as they are most seen) with a short tube, but nearly two and a half
feet long, and the tube was of gilded brass two fingers’ breadth in
diameter, and supported on three dolphins formed also of brass. At its
base was an ebony disc, containing shreds or some minute objects which
we inspected from above, and their forms were so magnified as to seem
almost miraculous.” So this was the first compound microscope!
Although Zacharias invented the microscope, it was Galileo who
introduced it to the scientific world. He published a book in 1610
in which he wrote: “About ten months ago a rumour reached me of an
ocular instrument made by a certain Dutchman, by means of which an
object could be made to appear distinct and near to an eye that looked
through it, although it was really far away. And so I considered the
desirability of investigating the method, and reflected on the means by
which I might come to the invention of a similar instrument. I first
prepared a leather tube at the ends of which one placed two lenses
each of them flat on one side, and as to the other side I fashioned
one concave and the other convex. Then holding the eye to the concave
one, I saw the objects fairly large and nearer, for they appeared three
times nearer and nine times larger than when they were observed by
the naked eye. Soon after I made another more exactly, representing
objects more than sixty times larger. At length, sparing no labour and
no expense, I got to the point that I could construct an excellent
instrument so that things seen through it appeared a thousand times
greater and more than thirty-fold nearer than if observed by the
naked eye.” Galileo had his enemies, who accused him of having picked
Zacharias’s brains; he admitted that he had taken his idea from the
Dutchman’s invention, but further than that he would not go; in fact,
he replied that the invention of Zacharias was a mere accident but that
his own instrument was discovered by a process of reasoning.
It would serve no good purpose to tell the story of all the scientists
who have helped to bring the microscope to its present state of
perfection, although many of their descriptions of objects and
apparatus are as quaint as the latter. Scheiner, for example, who wrote
in 1630, mentions “that wonderful instrument the microscope, by means
of which a fly is magnified into an elephant, and a flea into a camel.”
To Kircher belongs the credit of being the first worker to construct
an instrument with coarse and fine adjustment and with a substage
condenser, which could be used either for concentrating the sun’s rays
or those from a lamp. With an instrument of this pattern Malpighi saw
the circulation of blood in a frog’s lung. By 1685, when instruments
with four and six lenses were being used, the compound microscope was
firmly established as a help to scientists, and the simple lens was
used thereafter as an adjunct but not a rival to the newer instrument.
History makes a strong appeal to many people, and those who are
fascinated thereby will find endless amusement in reading old books on
the microscope and its objects. In the preface to Mouffet’s _Insectorum
Theatrum_, one of the earliest books on insects, we read the following
quaint lines: “If you will take lenticular object glasses of Crystal
(for though you have Lynx his eyes, they are necessary in searching
for atoms) you will admire to see the Fleas that are curasheers, and
their hollow trunk to torture men, which is a bitter plague to maids,
you shall see the eyes of Lice sticking forth, and their horns, their
bodies crammed all over, their whole substance diaphanous, and through
that, the motion of their heart and blood. Also little Handworms, which
are indivisible, they are so small, being with a needle prickt forth
from their trenches near the pool of water which they have made in the
skin, and being laid upon one’s nail, will discover by the sunlight
their red heads and feet they creep withal.” The creatures called
Handworms are itch mites, which tunnel in the human skin.
In our chapter on Nature Study and the Microscope we refer to the brown
patches to be found on the backs of fern fronds; it is interesting to
note that so long ago as 1646 Sir Thomas Browne had quite a good idea
of their structure. Describing them, he said: “Whether these little
dusty particles, upon the lower side of the leaves be seeds we have
not yet been able to determine by any germination. But, by the help
of magnifying glasses we find these dusty atoms to be round at first
and fully representing seeds out of which at last proceed little
mites, almost invisible, so that such as are old stand open, as being
emptied of some bodies firstly included, which though discernible in
Hartstongue, is more notoriously discoverable, in some differences, of
Brake or Fern.”
Two years earlier a noted scientist, Hodierna, had made a special study
of the eyes of insects and, considering the crude instruments with
which he must have worked, his descriptions are wonderfully accurate.
Of the house fly he wrote: “The head is all eyes, prominent and without
lids, lashes or brows. It is plumed with hairs like that of an ostrich
and has two little pear shaped bodies hanging from the middle of the
forehead. The proboscis which arises from the snout can be extended
freely and stretched forth to suck up humours and can afterwards
be directed back through the mouth and taken into the gullet. This
instinct nature has given the creature according to its need, for it is
without a neck and cannot stretch forth its head to obtain its food,
as is also the case with the elephant.” The author’s knowledge of the
house fly was evidently greater than his knowledge of the ostrich,
for the bird has anything but a plumed head. The eye of the insect he
compares to a white mulberry.
Another of these early workers, writing about the same time, gives
a concise account of cheese mites, heading his description “On the
creatures which arise in powdery cheese,” he wrote: “The powder
examined by means of this instrument (the Compound Microscope) does not
present the aspect of dirt, but teems with animalcula. It can be seen
that these creatures have claws and talons and are furnished with eyes.
The whole surface of their body is beautifully and distinctly coloured
in such sort as I have never seen before, and which indeed, cannot be
seen without wonder. They may be observed to crawl, eat and work and
are equal in apparent size to a man’s nail. Their backs are all spiny
and pricked out with various starlike markings and surrounded by a
rampart of hairs, all of such marvellous kind that you would say they
are a work of art rather than of nature.”
At about this period the microscope was used for the first time for
medical work and, as far as can be ascertained, Pierre Borel was the
first to use it for this purpose, and he learned a great deal about the
structure of flesh and the appearance of blood.
Of all the early writers on microscopy the man who spread abroad his
knowledge of the instrument and its capabilities, more than anyone
else, was Kircher, who died in 1680. He was an energetic writer, and
wrote on a large number of subjects. His books dealt with magnetism,
designs for a calculating machine, light, sound, history of plague,
the philosopher’s stone, Egyptian antiquities, a history of China and
a grammar. To all who read his book on the plague, it is clear that
he had a good idea of infection; he was, in fact, the first writer
who suspected it, though the microscope he used could not show him
bacteria. In his book he wrote: “Everyone knows that decomposing bodies
breed worms, but only since the wonderful discovery of the microscope
has it been known that every putrid body swarms with innumerable
vermicules, a statement which I should not have believed had I not
tested its truth by experiments during many years.” The experiments he
performed to prove his statement are so quaint that we give them in his
own words.
_Experiment I._--“Take a piece of meat which you have exposed by
night until the following dawn to the lunar moisture. Then examine
it carefully with the smicroscope and you will find the contracted
putridity to have been altered by the moon into innumerable wormlets
of diverse size, which, however, would escape the sharpness of vision
without a good smicroscope. The same is true of cheese, milk, vinegar
and similar bodies of a putrifiable nature. The smicroscope, however,
must be no ordinary one, but constructed with no less skill than
diligence, as is mine which represents objects one thousand times
greater than their true size.”
_Experiment II._--“If you cut up a snake into small parts and macerate
with rain water, and then expose it for several days to the sun and
again bury it under the earth for a whole day and night and lastly
examine the parts, separated and softened by putridity, by means of a
smicroscope you will find the whole mass swarm with innumerable little
multiplying serpents so that even the sharpest eyes cannot count them.”
_Experiment III._--“Many authors claim that unwashed sage is injurious,
but I have discovered the cause of this. For when, by means of the
sun, I minutely examined the nature of the plant, I found the back of
the leaves completely covered by raised work as with the figure of a
spider’s web, and within the water appeared infinitesimal animalcules,
which moving constantly came out of little buds or eggs.”
_Experiment IV._--“If you examine a particle of rotten wood under the
sun, you will see an immense progeny of tiny worms, some with horns,
some with wings, others with many feet. They have little black dots of
eyes. What must their little livers and stomachs be like?”
In the light of modern discovery much of the writing of these early
microscopists seems absurd. Kircher’s experiments, for example, prove
nothing, and he is often hopelessly vague and sometimes incorrect in
his statements. We must not be too critical, however, for some of this
early work was excellent, the microscopes in use would not be tolerated
at the present day, and without these pioneers microscopy would not
have reached the stage it has. Rather than laugh at their efforts, we
should marvel that they did so well.
CHAPTER II
SOME EARLY MICROSCOPISTS
Of the early British microscopists, Robert Hooke must not pass
unnoticed. He was appointed Curator of the Royal Society two years
after its formation, and the terms of his appointment were somewhat
one-sided. He was required to “furnish the Society every day they meet
with three or four experiments”; for this no pay was to be his till the
Society accumulated sufficient funds to reward him.
Although compound microscopes had been invented in Hooke’s day, it is
noteworthy that he remained faithful to the single lens, in fact it was
not till very many years later that the simple lens was supplanted, in
general use by the more complicated, if more perfect instrument.
In his book on Microscopy, entitled _Micrographia_, Hooke gives
a quaint account of the making of a microscope. “Could we make a
microscope,” he writes, “to have only one refraction, it would _cæteris
paribus_, far excel any other that had a greater number. And hence it
is, that if you take a very clear piece of a broken Venice glass, and
in a Lamp draw it out into very small hairs or threads, then holding
the ends of these threads in the flame, till they melt and run into a
small round Globul, or drop, which will hang at the end of the thread;
and if further you stick several of these upon the end of a stick with
a little sealing wax, so that the threads stand upwards, and then on
a whetstone first grind off a good part of them, and afterward on a
smooth Metal plate, with a little Tripoly, rub them till they come
to be very smooth; if one of these be fixt with a little soft wax
against a small needle hole, prick’d through a thin Plate of Brass,
Lead, Pewter, or any other Metal, and an Object, plac’d very near, be
look’d at through it, it will both magnifie and make some Objects more
distinct than any of the great Microscopes.”
This early worker was noted for the variety of his investigations
rather than for the depths of his learning. Amongst the so-called
Observations, in his book are many that are not connected with
microscopic work. The following are interesting and, in the curious old
book _Micrographia_, there are an extraordinary number of well executed
illustrations. Early in his book Hooke compares various man-made
objects, such as a razor edge, the point of a needle and a piece of
cloth, with various natural objects, and always to the detriment of
the former. He examined _Foraminifera_ with his microscope, and was
probably the first man to draw these beautiful little creatures.
Petrified wood and charcoal also came under his notice. When he studied
cork, he observed that it was made up of “little boxes or cells,” and
the name cell has survived to this day despite the fact that it is by
no means an appropriate term. That Hooke’s knowledge was not very deep
is shown by the fact that he presumed cork to be a fungus growing on
the bark of trees.
Many of the objects we have described in our pages were described and
illustrated by Hooke more than two hundred years ago. The sea mat,
despite his accurate observations, he mistook for a seaweed, as many
later naturalists have done. The stinging hairs of nettle he made
out in every detail. Fish scales, bee stings and birds’ feathers all
came under his notice. The foot of a fly he described with wonderful
accuracy; the scales of a butterfly’s wing and the head of a fly were
all studied and described in detail. On the life history of the gnat
he made many blunders, but he saved his reputation by remarkable
observations upon the _Chelifer_, a curious parasite of the fly which
we mention in our pages, and upon the silver fish, a little creature
which frequents sugar and starch. Neither of these organisms had been
described before. Fleas, lice, vinegar-eels and spiders were also
studied by this indefatigable worker, a worthy collection indeed, but
Hooke, like others of his time, was an observer first and foremost. As
a methodical, scientific worker he was of little account.
Living about the same time as Hooke, the celebrated Italian, Malpighi,
laid the foundations of much of our present-day knowledge of plant
structure. Various romantic stories have been told concerning certain
imaginary events which led Malpighi to take up the study of plant
structure, but the scientist himself refuted these picturesque stories.
Suffice it to say that his book on the subject, _Anatome Plantarum_,
though imperfect in many respects and, as might be conjectured
in so early a work, often inaccurate, contains a large number of
astonishingly good drawings; many of the original drawings, by the way,
executed in red chalk, are in the possession of the Royal Society.
It is interesting to note that this botanist compared the falling of
leaves to the shedding of an insect’s skin, in this respect at any rate
he had advanced no further than Aristotle, who compared leaf-fall to
the moulting of a bird. On the other hand, the Italian was the first
scientist to describe the pores (stomata) of leaves, though he never
discovered that they occurred on all leaves. He, first of all men,
showed that nectar was formed by the flower and not transferred thence
from other sources as had previously been believed; he too explained
accurately for the first time the process of germination in the seed.
It was not alone as a botanist, however, that Malpighi was celebrated.
He elucidated the various changes which take place during the hatching
of an egg; he was the first man to give an accurate account of the
structure of an insect, and this he did in his work on the Anatomy of
the Silkworm. Using a simple microscope for his investigations, he
contracted an eye affliction during this period from which he suffered
more or less severely all the rest of his life. He discovered the
breathing tubes of insects and that when they are covered with grease
the insect will die “in the time that one can say the Lord’s Prayer”;
the heart, the silk glands, the development of wings and legs were all
discovered for the first time by this untiring worker, aided by his
simple microscope.
Pages could be filled with accounts of Malpighi’s other scientific work
on the structure of the lung, the liver and kidney, the life of the
liver fluke and a hundred and one other subjects. Though undoubtedly a
great and clever microscopist, the general estimate seems to be that
his work had little influence upon the scientific world. The main
reason is that he was ahead of his time; men of the day concluded, for
instance, that in his Anatomy of Plants he had said the last word on
the subject, that there was no more to be learned. An English worker,
Nehemiah Grew, carried the Italian scientist’s studies of plant
structure a little further and his Anatomy of Plants contains many new
and often accurate observations. His studies also led him to discover
the structure of the ridges and sweat pores of the human hand, in fact
Grew may be looked upon as the originator of the study of finger prints.
A Dutchman, Jan Jacobz Swammerdam by name, and a contemporary of Grew,
was undoubtedly the most accurate observer amongst these old-time
microscopists. Despite ill health, his enthusiasm was unbounded, and a
friend wrote concerning him: “Swammerdam’s labours were superhuman.
Through the day he observed incessantly, and at night described and
drew what he had seen. By six o’clock in the morning in summer he
began to find enough light to enable him to trace the minutiæ of
natural objects. He was hard at work till noon, in full sunlight, and
bareheaded, so as not to obstruct the light, and his head steamed with
profuse sweat. His eyes, by reason of the blaze of light, became so
weakened that he could not observe minute objects in the afternoon,
for his eyes were weary.” If only for the fact that the Dutchman made
clear the processes involved in the transformations of insects, his
name would be famous. He described the structure and habits of the hive
bees, male, female and drone with wonderful accuracy, and illustrated
his work with plates which “would do credit to the most skilful
anatomists of any age.” Swammerdam was sarcastic at times; he had
shown that the facets of a bee’s eye are six-sided and, as so commonly
happened in those days, some naturalists jumped to a conclusion, in
this case that the fact explained the six-sidedness of the cells in the
honey comb. By the same reasoning Swammerdam remarked that men, having
round pupils, should build round houses. It is not only for his study
of the minute structure of insects that this microscopist is noted,
he worked upon the tadpole and the snail. He it was who discovered
the red blood corpuscles of the frog, and he described his discovery
in the following terms: “In the blood I perceived the serum in which
floated an immense number of rounded particles, possessing the shape
of, as it were, a flat oval, but nevertheless wholly regular. These
particles seemed, however, to contain within themselves the humour[1]
of other particles. When they were looked at sideways, they resembled
transparent rods, as it were, and many other figures, according, no
doubt, to the different ways in which they were rolled about in the
serum of the blood. I remarked besides that the colour of the objects
was the paler the more highly they were magnified by means of the
microscope.” Of the snail he made a number of strikingly accurate
studies, in all of which he was aided by his lenses, so that it is the
more remarkable that he considered snails to be insects.
Leeuwenhoek, another Dutchman, we have already mentioned in our
previous chapter. He of all men brought the simple microscope to its
highest state of development. His instruments were one of the sights
of Holland, and many eminent personages made a point of seeing them.
Though he had not the advantage of any scientific training and spoke
no other language than his own, he made some remarkable additions
to the scientific knowledge of the time. Like Hooke, he was not a
methodical worker, he was impelled by an unbounded curiosity. “When
we are inclined to disparage Leeuwenhoek’s hasty methods it is well
to recollect that he initiated biological inquiries of the greatest
interest, _e.g._, the parthenogenesis of aphids and the revivification
of dried microscopic organisms, while he gave the first notices, or the
first worth mention, of rotifers, Hydra, infusorians, yeast cells and
bacteria.”
We may here explain the meaning of the term “parthenogenesis of
aphids.” The female aphids or green flies are able to bring forth
generation after generation during the first two-thirds or so of each
year without the assistance of males. This form of increase, which by
the way accounts for the extraordinary numbers of green fly, is known
as parthenogenesis.
Leeuwenhoek thought that no one but himself could use his lenses
properly, in consequence, when he sent any interesting object to a
friend for him to examine, a lens was always affixed in place so that
the object could be seen to the best advantage. He gave a set of his
lenses and objects to the Royal Society, and described his gift as
“a small black cabinet, lackered and gilded, which has five little
drawers in it, wherein are contained thirteen long and square tin
boxes, covered with black leather. In each of these boxes are two
ground microscopes, in all six and twenty; which I did grind myself,
and set in silver; and most of the silver was what I had extracted from
minerals, and separated from the gold that was mixed with it; and an
account of each glass goes along with them.”
Kircher, whose work we mentioned in our last chapter, was overwhelmed
with the notion that various living creatures are generated from
non-living matter. Fleas, for example, he was certain, came from dirt,
and it remained for Leeuwenhoek to prove that they arise from eggs and
grubs, in the manner now so well understood.
He carefully studied the structure of a garden spider, and for the
first time explained its wonderful feet, its jaws and poison gland,
its spinnerets and silk. He studied _Hydra_ first of all men, and
said that, under the microscope, its tentacles appeared to be several
fathoms long. Although sadly at sea over the correct position of his
snails in the animal world, he was clever enough to include _Volvox_
amongst the plants and fortunate enough to see the young forms escape
from the parent colony.
Concerning this microscopist’s early studies in bacteriology we may
quote from Professor Miall’s _The Early Naturalists_, a book by the
way of the greatest interest to those who would learn something of
the struggles of the men who laid the foundations of our present-day
biological knowledge.
Professor Miall says: “In 1683 Leeuwenhoek wrote a letter to the Royal
Society which contains the first mention of bacteria. He had been
writing and speculating upon saliva, and had searched the saliva of
the human mouth for animalcules without finding any. It then occurred
to him to ask whether the teeth might lodge animalcules discharged
from the salivary ducts. He tells us that, though his own teeth were
scrupulously clean and particularly sound for his age (about fifty),
the lens revealed a white deposit upon them. This deposit was found to
contain minute rods, some of which showed either a steady or gyratory
movement. Others were very minute, of rounded form, and moved with
remarkable velocity. The largest of all, which were either straight
or bent were motionless. The teeth of an old man, which were never
cleansed, contained among others large rods which exhibited snake-like
undulations. Rubbing the teeth with strong vinegar did not kill the
moving bodies, but they became quiescent when detached and placed in a
mixture of vinegar and saliva, or vinegar and water. Nine years later
Leeuwenhoek returned to the subject. Living particles were no longer
met with in his teeth, and he was at a loss to explain why, until
it occurred to him that he was accustomed to drink hot coffee every
morning. This, he thought might have killed the animalcules, and his
conclusion was confirmed by finding that on the back teeth, which were
less exposed to the hot drink, plenty of them were still to be found.
In 1697 he tells how he pulled out a decayed tooth, and found that the
cavity abounded in moving particles.” Nearly a hundred years elapsed
before anyone else took up the study of bacteria.
From the time of Leeuwenhoek onwards, scientific discoveries were
announced in rapid succession, so that in one short chapter it is
impossible to keep pace with the progress that was made. Among the
great men who owe much of their success to the microscope we may
mention the Frenchman Réaumur, whose memory is kept green for all
time by his thermometer; as a worker upon problems of insect life
he was indefatigable; the Swede, Linnæus, to whose early efforts we
owe the orderly arrangement of living creatures and plants, known as
classification. This arrangement has been considerably modified, more
modern ideas have upset much that he initiated, yet he remains the
parent of orderly arrangement.
Buffon, a great naturalist, was followed by Cuvier, the first serious
student of fossils; by Humboldt, naturalist and traveller; by Robert
Brown, the founder of modern Botany; by Darwin and by Pasteur in turn.
How much these men owe to the microscope can never be known; certain
it is that without its assistance our world, the world we know and can
see, would have been smaller than it is to-day.
FOOTNOTE:
[1] Humour is here used in its original sense, meaning moisture or a
liquid.
CHAPTER III
THE ACTION OF LIGHT
It is hardly necessary to remark that the wonderful properties of the
microscope depend upon light. Without light, lenses would be useless,
objects could not be illuminated and we could not see them. In this
short chapter we propose to give a brief outline of the action of
light; if our words appear to savour of the school-book, we shall try
to avoid it, but, we repeat, if they do so we would remind our readers
that the more one knows of the action of light the better use one can
make of one’s instrument. As a well-known microscopist has remarked
we may be able to afford a costly harp or a costly microscope, but
although we may be able to strike a few notes on the former and examine
a few objects with the latter, we can only make the best use of either
by thoroughly understanding and practising upon it.
The first thing we learn when we study light is that it travels in
straight lines. The chief source of light to the inhabitants of this
earth is the sun. Now the sun is so far away that, for all practical
purposes, the rays of light coming from it may be looked upon as being
parallel to one another. That we must always remember, when dealing
with the sun, though, of course, it does not apply when we are dealing
with lights near at hand, unless they are specially constructed to
throw parallel beams or rays, whichever we elect to call them. To
prove that light travels in straight lines is not difficult, and we
may devise a number of experiments for the purpose. The doors and
ventilators of many dark rooms, in which photographic operations are
carried on, are constructed on the assumption that light cannot travel
round corners. An arrangement as shown in the diagram will allow air,
but no light, to pass. If light were capable of going round corners,
some other arrangement would have to be devised for the ventilation of
dark rooms.
[Illustration]
Having learned so much about light, we come to the most important fact
of all, as far as the action of light concerns microscopic work. When
rays of light travel, from a substance like air into a substance like
water, they are bent out of their straight course. Without any desire
to introduce a number of unfamiliar words, we may venture to remark
that, any substance through which light passes is called a medium.
Some media are clearly more dense, more compact or solid--dense is the
proper word--than others. Water is more dense than air and glass than
either. The bending of light rays is known as refraction. So now we may
state our second law a little more concisely, thus:--When light passes
from a medium into one more dense, or _vice versa_, it is refracted,
and the more dense the medium into which or from which the light passes
the greater the refraction.
[Illustration]
A diagram and an experiment should make matters clear. Suppose AB is a
ray of light traveling in air and that it falls on a sheet of water,
WXYZ, the ray will be bent along BC and its course from air to water
may be represented by ABC. Suppose again, WXYZ represents, not water
but glass; as glass is more dense than water the course of the ray AB
is represented by ABD, it is refracted or bent to a greater extent
than the ray which passed from air into water.
For our experiment we need only plunge a stick into water and notice
that, owing to this property of light, the stick appears bent, from the
point where it comes into contact with the surface of the water.
[Illustration]
Some of us may be old enough to remember that once, on either corner of
nearly every mantlepiece, there stood an ornament of doubtful utility
from which there hung a dozen or more glass prisms. Now the only beauty
about these otherwise hideous contraptions was to be seen when light
played upon them. Then patches of violet, green, yellow and red were
thrown upon neighbouring objects. White light, ordinary sunlight that
is to say, is really composed of various colours--violet, indigo, blue,
green, yellow, orange and red--which, when combined together, make
light as we know it. When white light passes through a prism of glass,
it is not only bent out of its course, but broken up into all these
colours. A prism, as we all know, when examined at either end, is seen
to be triangular in shape. Putting aside for a moment the question of
the breaking up of light into its component parts, the path of a ray
of light through a prism is shown in the diagram. As the ray passes
from air into glass it is bent, because glass is more dense than air;
it is bent once more on leaving the prism because air is less dense
than glass.
[Illustration]
Now lenses are made of various shapes, and those with two outwardly
curved surfaces are known as double convex lenses. A double convex lens
is usually made with both its surfaces equally curved and in the finer
optical work great care is taken to ensure that this is the case. For
certain purposes, however, as we shall learn in a moment, one or other
of the faces only may be much more curved than its companion and this
may be carried to such an extreme that one face is flat, the lens is
then known as plano-convex. Lenses may also have inwardly curved faces,
if both are of this design they are called double concave; if one face
is flat and the other inwardly curved they are known as plano-concave.
There are other combinations, for example, one face may be inwardly
curved and the other outwardly curved, but the four kinds we have
described are all that need trouble us.
[Illustration]
It does not require a great amount of imagination to recognise that
the double convex lens, that is the lens with two outwardly curved
faces is little more than a pair of prisms placed base to base, or more
accurately, a number of prisms so arranged as shown in the diagram.
Parallel rays of light falling upon such an arrangement of prisms
would be bent from their course, as shown by the arrows, and this is
just what happens with a double convex lens. Now rays of light from
an object, passing through a lens of this shape may follow any one of
three courses, according to the position of the object with regard to
the lens. In one position and one only the rays after passing through
the lens will be parallel to one another, as shown in the diagram.
The only position of the object for the above to take place is when
it coincides with a point known as the principal focus of the lens,
conversely the parallel rays of light from the sun, after passing
through a double convex lens, will come to a point at its principal
focus.
Suppose now that the object be placed at a point beyond the principal
focus of the lens, the light rays therefrom will, after passing through
the lens, converge to a point thus:--
[Illustration]
In the diagram O is the object and P the principal focus of the lens.
The third case occurs where the object is nearer to the lens than its
principal focus, then the rays after passing through the lens, diverge
and never meet.
[Illustration]
We have already stated that when white light passes through a prism
it is broken up into different coloured rays varying from violet to
red. The reason for this is that all the light rays composing white
light are not bent equally as they pass from one medium to another.
The violet rays are bent the most, the indigo next, blue next, down to
red, which is least bent. Once more, considering the double convex lens
as made up of a number of prisms, let us represent, by a diagram, the
course of parallel rays of white light through it.
[Illustration]
A A′ represent the parallel rays of white light falling on the lens, L
L′. The blue rays are bent more than the red, so the principal focus
of the former is at C and of the latter at D. The consequence of this
difference in bending of the various coloured light rays would be
most serious in microscopic work were not means devised to overcome
it. Objects for instance at C in our diagram would appear blue, at D
they would appear red, whilst at E E′ though no single colour would
predominate they would be illumined with many coloured rays, though
less strongly than at C or D.
This chromatic aberration, as it is called, depends amongst other
things on the nature of the glass used in lens construction. It has
been found, however, that a combination of flint and of crown glass
will overcome the difficulty. In a later chapter we shall explain
the difference between these two kinds of glass. In practice, a
plano-concave lens of flint glass is combined with a double convex lens
of crown glass and, if the nature of the glass is satisfactory, as
also the shapes of the lenses, there is full correction for chromatic
aberration, and objects viewed through such a lens will not appear with
coloured margins.
There is one further trouble likely to occur in such, or any lens. We
write of the rays meeting at a point. In our diagrams we represent the
rays by straight lines, really they are much more complicated than they
appear in a diagram. It is quite easy to take a ruler and make our
imaginary light rays meet at a point, as a matter of fact, where real
lenses and real light rays are concerned, it is very difficult, if not
impossible, to make the latter meet at a single point. One more diagram
may make the matter clear.
[Illustration]
Parallel rays A pass through our lens and, as we know, they should all
meet at a point P, the principal focus of the lens; the majority do so,
but some meet at other points, such as P′. In consequence of this it
is difficult to obtain a clear image of an object at P, and the lens is
said to suffer from spherical aberration. The perfect simple lens would
be one fully corrected for chromatic and spherical aberration.
CHAPTER IV
THE COMPOUND MICROSCOPE
In our chapters, dealing with, the history of the Microscope, we
attempted to trace the gradual development of the compound instrument
from the simple lens; we stated that the latter, in a crude form, had
been known and used from very early times and that the former developed
side by side with the telescope. We have also said a few words in
Chapter III. concerning light for the reason that the microscope can
be better understood and used more efficiently when we are acquainted
with the phenomena due to light. The simple lens, sold under the name
of pocket magnifier, in its cheapest form consists of a double convex
lens, that is to say, a lens with two outwardly curved surfaces. Better
quality pocket magnifiers consist of two or more lenses, which may be
either double convex; plano-convex, _i.e._, with one surface perfectly
flat and the other outwardly curved, or they may be constructed of a
combination of double convex and plano-concave lenses, such as were
described on p. .
The object of both the simple and compound microscope is to make
objects appear larger than they do to the naked eye. When we buy our
pocket lens we shall find that these little instruments are constructed
to give different degrees of enlargement, some make objects appear
five times larger than they do to the naked eye, some ten, some
fifteen and some twenty times larger. Twenty times is about the limit
of magnification for the ordinary pocket lens. If we are observant we
shall notice something else--the greater the magnification the nearer
we must hold the lens to our object. Within certain limits, this is
not a very serious matter, but a point is reached where we must hold
our lens so near to the object that we cannot see it, and that is why
we cannot obtain very great enlargement with a pocket lens. Despite
this fact, as we read in our opening chapter, some very wonderful
discoveries have been made with these simple microscopes.
Now we wish to show how a compound microscope works and, having done
so, to explain the uses of its various parts. We shall consider the
lenses of the instrument to be double convex; we do this for the sake
of simplicity. Even in the cheapest compound microscopes of to-day
simple convex lenses are never used, for the reason we explained in
our last chapter. To understand the course of the light rays passing
through our microscope, however, we may look upon the lenses as being
merely double convex.
Let us try a simple experiment first of all. For the purpose we require
two double convex lenses, one capable of magnifying more than the
other, a sheet of paper and a candle. We must darken the room in which
we make the experiment and, having lighted the candle, we may proceed
to make a compound microscope, for that is really what we are about to
do. Taking the lens which gives the greatest magnification, we look
through it till we can see a clearly defined image of the lighted
candle, then we fix the lens at that spot, so that, during the rest of
our experiment, the candle and lens remain at the same distance from
one another. Now we put the piece of paper as nearly as we can in the
position of our eye, moving it nearer or further from the lens till we
have a perfectly clear image of the candle thrown upon it. The first
thing to strike us is that the image is upside down; it is known as a
real, inverted image. Real because it can be thrown upon a screen and
inverted--well because it is upside down. There are some images, as we
shall learn in a moment, which can be seen but which cannot be thrown
upon a screen: they are called virtual images.
Having fixed our sheet of paper in position, we take our second lens,
focus it sharply upon the back of the sheet of paper, being careful to
keep the centres of the two lenses as far as possible in a straight
line with one another. Having obtained a sharp image we remove the
paper and gradually advance our second lens towards the first. We soon
reach a point where we have a very much larger image of the candle than
the first lens gave us; we must fix our second lens at this point
for we now have a compound microscope, a very crude one certainly and
without the trimmings which make the microscope so useful. Before we
proceed to explain what has happened to the light rays we must take our
paper screen once more and place it as near as possible to the spot
where our eye was situated when we saw the second image. We shall find
that, however much we may move our screen to or from the second lens we
can never manage to obtain an image upon it for the reason that this
second image is virtual, but unlike the first image it is not inverted.
[Illustration]
One or two diagrams will help to explain our experiment and, instead of
the lighted candle, we will suppose that our object is an arrow--it is
easier to draw and serves just as well. The magnification of the object
by our first lens may be represented by the diagram below, where AA is
the lens, CD the object and D′C′ its image.
The arrow C′D′ shows the point at which we placed our screen, and as
our diagram shows, the image is magnified and inverted.
Our second lens, we remember, was focussed on the back of the paper,
placed at C′D′; for practical purposes we may ignore the thickness of
the paper and say that it was focussed on the image C′D′. Had we left
it at that, the further course of the rays through the second lens
would be represented by a replica of the diagram we have just given.
But, in our experiment, we moved the second lens nearer and nearer to
C′D′ till we obtained a clear much magnified erect image of C′D′, let
us call this second image C″D″, and represent the course of the light
rays by a diagram.
[Illustration]
We may well ask, why did the lens AA, our first lens, form a real
image whilst the second lens BB, which is precisely similar to AA,
except that its magnifying power is not so great, form a virtual
image? The formation of a real or a virtual image is nothing to do
with magnification, so we repeat--why do two similar lenses form
different kinds of images? Let us refresh our memories with the
remarks concerning the principal focus of lenses in the last chapter,
then we may try another experiment. The principle focus of a double
convex lens, we remember, is the point to which parallel rays of
light converge, after passing through the lens. If now our object is
further away from the lens than its principal focus, a state of affairs
that existed in the case of our lens AA and the object CD, we obtain
a magnified, real but inverted image; if, on the other hand, using
the same lens if we wish, the object is nearer to the lens than its
principal focus, we obtain a magnified virtual and erect image. The
form of image then depends on the relative positions of lens and object
and not on the magnifying powers of the former.
After this digression, we will see what happens when we combine the
diagram showing the real, inverted image, formed by the lens AA with
the virtual erect image, formed by the lens BB. In reality we will
draw a diagram showing the path of the light rays through our compound
microscope.
[Illustration]
We have used the same lettering as in our previous diagrams and we
see that, also as before, a real, inverted image C′D′ of the object
CD is formed by the lens AA and a virtual, erect image C″D″ of the
image C′D′ is formed by the lens BB, the object CD being further from
the lens AA than its principal focus and the image C′D′ being nearer
to the lens BB than its principal focus. One very important point we
must notice before we leave the diagram. We have mentioned several
times that the image formed by BB is erect and so it is, but it is an
erect image of an already inverted image, so that the final image of
CD, as seen by the eye E is inverted. The fact that objects viewed
through the microscope appear upside down is puzzling at first. To all
intents our two double convex lenses represent a compound microscope;
actually, they should be fixed at either end of a tube, blackened on
the inside. The lens AA, nearest to the object, would then be known
as the objective and the lens BB nearest to the observer’s eye would
be known as the ocular or, more commonly the eyepiece. There are, of
course, very many refinements, designed to make the instrument capable
of performing the most accurate work, and needless to say these simple
lenses would neither give very great magnification nor any clear
images. Let us describe a more refined compound microscope than the
one we constructed in our darkened room. The optical parts, that is
to say the lenses, are the most important parts of every microscope,
upon their qualities depend the degree of efficiency of the instrument;
the metal portions, known collectively as the stand, contribute to the
easier, smoother working of the microscope.
[Illustration:
_By the courtesy of Messrs. F. Davidson & Co._
THE HEAD OF A DOG FLEA
No wonder the flea is an annoying creature. As the plate shows, it is
armed with knives, lances and saws, all designed to injure the skin
of its victim.
]
The stand must claim our attention first. The base of the instrument,
called the foot, is usually either three-legged or horse-shoe shaped;
whatever its form it should be heavy, for only thus can the microscope
be steady, and steadiness is essential in all microscopic work. At
the top of the foot there is a joint, in order that all the other parts
of the stand may be inclined at any angle, from the vertical to the
horizontal. Just above the joint is a bent arm of brass, to the forward
end of which a brass tube is affixed. This tube is designed to hold
the lenses, the objective at its lower end, the eyepiece at its upper
end. The tube is always blackened inside; were this not the case, light
passing through the objective would be reflected in all directions from
the sides of the tube and a clear image of the object could never be
obtained. The tubes of microscopes vary in length according to their
country of origin; English and American tubes are ten inches long,
those of continental make vary from a little more than six inches to
rather more than seven inches in length.
Affixed to the lower end of the bent arm of brass, mentioned above,
is a flat metal plate, known as the stage; at its centre, there is a
circular hole through which rays of light pass to illuminate objects
placed upon it. Below the stage, at the edge nearest to the foot, there
is a metal peg, over which fits a tube to which a mirror is attached by
a moveable joint. The mirror reflects light rays through the opening
in the stage. The tube, holding it, can be slipped up and down the peg
under the stage, thereby bringing it nearer to or further from the
object and so altering the intensity of the reflected light, as we
shall explain in a moment. Owing to its moveable joint, it is possible
to swing the mirror to the right or left, so that the reflected light
rays do not pass directly through the object on the stage, but strike
it on one side or the other, thereby giving what is known as oblique
illumination.
The cheapest forms of compound microscopes have all the parts we have
mentioned, and focussing is carried out by sliding the tube, with its
objective and eyepiece, up and down within its holder, in order to
bring the objective further from or nearer to the object.
In more expensive instruments there are further refinements, in fact,
on some of the very costly present-day instruments, there are so many
appendages and appurtenances that it is doubtful whether some of them
are not more of a hindrance than a help, at any rate they increase the
possibility of trouble by their liability to get out of order. Such
microscopes are only of use to very expert workers; there are, however,
a good many additional features to be found on quite moderate-priced
instruments, features which are a great help to the microscopist.
It is obvious that we cannot attain any degree of accuracy in
focussing, especially with high magnifications, when we must perforce
raise or lower the tube by hand. To obviate this difficulty, most
microscopes are provided with mechanism known as a coarse adjustment;
it consists of milled screws at either end of a metal rod; in the
centre of the rod there is a little cog-wheel which engages with a row
of notches on the tube. By turning the milled screws slightly in either
direction, we can impart a considerable upward or downward movement
to the tube carrying the objective and focussing at once becomes a
more simple matter. The coarse adjustment is only useful for examining
objects with a low magnification; if we use it when objects are being
highly magnified we run the risk of screwing our objective down upon
our object, to the certain destruction of the latter and the probable
injury of the former. To obviate such a catastrophe, most of the
better class microscopes are also provided with a fine adjustment. By
means of this adjustment, which externally takes the form of a single
milled screw, a considerable turn of the screw in either direction only
imparts a very slight upward or downward movement to the microscope
tube. In the best instruments, movements of as little as one hundredth
part of a millimetre may be imparted to the tube by the fine adjustment
and, seeing that there are about twenty-five and a half millimetres to
the inch, it is obvious that a good fine adjustment is very delicate
and, being so, must be treated with care. The fine adjustment is used
to supplement coarse adjustment in the final focussing, when using high
magnifications.
A few words may be devoted to the mirror, for on its intelligent use
much depends. Usually we shall find that it is plano-concave, that is
to say, flat on one side and hollowed out on the other. The use of
the mirror, as we have mentioned already, is to reflect rays of light
through the opening of the stage on to the object we desire to examine.
Both mirrors will reflect parallel rays of light to a point, just as a
double convex lens will so direct them from their course that they meet
at a point. The concave mirror gives the more powerful illumination,
because it reflects more light rays than a flat mirror of the same
diameter.
We have mentioned that, to obtain full advantage from the mirror it
should be capable of movement to and from the stage. When we desire
strong illumination we arrange the mirror so that its reflected rays
meet at a point coinciding with our object. Should less intense
illumination be required, we slide the mirror nearer to the stage, and
of course nearer to our object, so that the reflected rays meet at a
point above the object.
[Illustration]
The two diagrams, given below, show the path of the rays of light,
where O is the object, and a trial with our microscope will soon show
which position gives the more powerful illumination.
For high-power work, such as bacteriology or even the examination of
sections of plants, etc., even the best concave mirror will not give a
sufficiently powerful illumination; accordingly an instrument, known
as the condenser, is fixed below the stage, between the mirror and the
object. The condenser, as its name implies, condenses the rays of light
reflected to it by the mirror. It consists of a series of lenses so
arranged that they will throw a very powerful cone of light. Provision
is made for focussing the rays from the condenser on to the object.
Sometimes, for special forms of illumination, it is necessary to cut
off some of the rays of light passing through the condenser. It may be
that we desire to dispense with the outer rays of the cone of light
or, when delicate details are being studied, we may wish to impede the
central rays. In either case diaphragms, popularly called “stops” are
used. Our diagrams show A the outer rays of a cone of light cut off and
B the central rays similarly treated.
[Illustration]
In old pattern microscopes and in many instruments not provided with
condensers, the diaphragm used for the purpose of cutting off the outer
rays of the cone of light, consists of a blackened circular metal
plate, perforated with a number of different sized circular holes.
This plate is fixed below the stage in such a manner that, as it is
revolved, holes of various diameters are brought one by one within the
cone of light. It need hardy be remarked that the smaller the hole
in the diaphragm the more light is cut off and the less reaches the
object. In more modern instruments and in practically all which are
fitted with a condenser, an Iris diaphragm is fitted. A diaphragm of
this nature consists of a number of thin, blackened, metal leaves,
fastened to a metal ring in such a manner that, when the ring is
revolved, the leaves close together, making the opening in the centre
smaller and smaller. The Iris diaphragm has many advantages over the
old perforated metal plate. At will, we can have any opening from full
to the merest pin-point or we can cut off the light rays altogether,
should we wish to do so; we are not confined to a definite number of
stops. As we cut off these outer rays of light we shall find that, up
to a certain point, though the illumination becomes less and less the
object becomes more and more clear, or, to use the correct expression,
its definition is improved.
When it is necessary to cut off some of the central rays of the light
cone, either a circle of glass with an opaque centre is dropped into a
metal holder below the stage, or a circular metal plate, held in the
centre of a metal ring by three arms, is used in the same manner.
The effect of cutting off the central rays of the light cone is, of
course, to reduce the illumination and to show up delicate detail to
advantage. No direct rays of light reach the objective, such as do
pass into the microscope are all diffused from the edges of the object.
We have already mentioned that the optical parts of the compound
microscope are of greater importance than what may be termed the
mechanical portions and the objectives are more important than
the eyepieces. Better results can always be obtained with a good,
high-power objective and a low-power eyepiece, than with an inferior
objective and a good quality eyepiece. The merits of the eyepiece,
however great, will not be adequate compensation for the failings of
the objective. Modern objectives are composed of several lenses and of
a combination of flint and crown glass, as we explained in our last
chapter. They are so designed that they can be screwed into the lower
part of the microscope tube. The focal length of each objective is,
or should be, marked upon it; as a general rule, however, it may be
taken that the smaller the lower lens, the shorter its focal length and
therefore the greater its magnifying power.
The form of eyepiece most usually met with is known as Huyghen’s.
It consists of two plano-convex lenses, with their flat or plane
surfaces directed away from the objective. The smaller of the two
lenses is situated nearer to the eye of the observer and is known
as the eyeglass; its function is to magnify the image formed by
the objective. The larger, lower lens is known as the field or
collecting glass; it renders the image clearer though, in so doing,
it reduces the magnification of the eyeglass. In instruments provided
with more than one eyepiece we shall wish to know which gives the
greater magnification; this is or should be marked upon the metal rim
surrounding the eyeglass but, in general, it may be stated that the
shorter the eyepiece the greater its magnification. We repeat again,
increase your magnification always, when possible, by using higher
power objectives rather than eyepieces with greater magnifying powers.
Sometimes it is necessary to use a greater magnification than our most
powerful objective will give us; then we must fit our most powerful
eyepiece and draw out the upper part of the microscope tube--in the
best instruments they are made to pull out, after the manner of the
telescope. The effect of so doing will be to increase the magnification
considerably but, at the same time, the definition or clearness is
seriously impaired.
For the examination of practically all our microscopic objects we
require a number of slides, little glass slips of good, thin, clear
glass. They may be used over and over again unless we make permanent
preparations, but we are hardly likely to do so in our early days. The
slides are held in place on the microscope stage, either by a pair of
clips attached thereto or by resting against a bar running across the
stage. We may here remark that it is essential always to keep one’s
microscope slides absolutely clean. Dirty slides denote the careless
worker; moreover, dirt when magnified is misleading. Objects which
are being examined in water or any other liquid should be covered
with a cover-slip, an exceedingly thin circle or square of glass. The
cover-slip is as much a protection for the objective as for the object
and its cleanliness also, is all important.
We have not mentioned any refinements such as the mechanical stage, by
means of which slides on the stage may be rotated, moved to the front
and to the back of the stage or from side to side. We have omitted
these because they are not essential even for the very best work; they
lend additional comfort to the use of the microscope but, again, they
are not essential. The microscopist who requires such luxuries may
learn about them in the larger text-books on the microscope.
CHAPTER V
ANIMAL LIFE IN THE PONDS AND STREAMS
The enthusiastic microscopist will probably never lack material for his
instrument, whatever branch of microscopical work he may decide to make
his own. To the student of Pond Life, either animal or vegetable, there
is granted a never-ending store of beautiful and interesting objects.
Because one pond has been thoroughly searched and all that it can offer
has been carefully examined, we must not conclude that no other pond
will be worth our attention. Though indeed many little animals occur
over and over again in practically every pond, there are other equally
interesting animals which only occur in certain localities and for
these we must keep a sharp look-out.
The apparatus needed for the collection of the denizens of ponds and
streams, need consist of no more than a net with a very fine mesh and a
jar in which to bring our captures home; for, of course, animals which
dwell in water need not be dried on the journey home. Various useful
accessories for the student of pond life are sold at very reasonable
rates by most scientific instrument makers.
We shall find many representatives of the animal world in our pond
and exceedingly interesting most of them will prove. From the mud we
may obtain the “protean animalcule,” known to scientists as _Amœba
Proteus_, the most lowly of all animals. Though this creature is
plentiful and just visible to the naked eye, he is not easy to separate
from his surroundings. He is almost colourless and therefore paler than
the mud. Having secured him on the end of a glass rod, let us examine
him in a drop of water on a slide. At first he will remain motionless,
as a protest against being disturbed; we shall not have to wait long,
however, for soon one part of his body will be seen to protrude and
then grow larger and larger till it forms a false foot; other parts may
follow suit, till he is more elongate than oval, and he moves in the
direction of his false foot with a curious gliding motion. His pace is
not great and has been calculated at a twenty-fifth of an inch in an
hour. Really the “protean animalcule” is little more than an animated
drop of jelly, a fact we can substantiate by watching him feed. His
food consists of minute water plants such as diatoms, and when one of
these plants comes within his line of march he simply surrounds it with
his false feet and, as it were, flows around it. When he has digested
all he can he flows away from the undigested portions; he has no mouth
or any of the organs usually associated with animal anatomy.
While hunting for our _Amœba_, it is highly probable that a very
active little slipper-shaped organism may have forced himself upon our
attention. From his shape he has earned the popular name of the slipper
animalcule. He is rather more highly organised than the _Amœba_ for he
possesses a mouth, as we shall see when we are able to examine him. So
rapidly does he swim, however, that something must be done to curb his
activity; he may either be killed with a drop of weak acid or we may
put a little tuft of cotton wool on our drop of water and a coverslip
lightly over that. The threads of cotton wool will form a network, in
the meshes of which the active little animalcule will be confined.
Careful observation will show that he is, like a slipper, more pointed
at one end than at the other; that there is a funnel-shaped orifice,
his mouth, at one side of his body; and that he is covered with little
threads which lash the water with rhythmic movement and propel him with
considerable rapidity. These little threads also send currents of water
to his mouth, and in the water is his food.
Having examined these two free swimming denizens of the pond, we may
advantageously turn our attention to some of the weeds growing therein.
Careful examination with our pocket lens will almost certainly reveal a
number of minute living creatures attached to the submerged stems and
leaves. It is impossible to describe all the interesting creatures we
might find: we must content ourselves with two, because they are common
inhabitants of many ponds. One interesting little creature we are
almost certain to find, the _Hydra_. He may be green or he may be brown
but in structure he will remind us of a small sea anemone. When we
put him under the microscope he has the appearance of a mass of jelly
attached to the water plant on which we found him. He will soon open
himself out, however, and we shall see that his free end is provided
with a number of tentacles; these he waves about in the water to draw
small swimming creatures to his mouth which is situated in the centre
of the group of tentacles. Any luckless creature, coming within reach
of the _Hydra_, is at once stung with one of the barb-shaped stings
which stud his tentacles and then passed to his mouth. The _Hydra_
is wonderfully tenacious of life; it is said that he has been turned
inside out, like a glove finger, without suffering any inconvenience.
Probably the specimen we are examining will have a swelling on the side
of its body, which might be mistaken for the result of some injury; it
is nothing of the kind; it is merely a bud which will grow into a young
_Hydra_ and, when old enough, become detached from its parent and float
away to another plant. Under a fairly high magnification, we shall
almost certainly see something gliding rapidly over the body of the
_Hydra_; its movements are too quick to allow of careful examination;
the creature which is almost as elusive as the slipper animalcule is a
parasite of the _Hydra_.
Not quite so common as our last object but still common enough to be
mentioned here is the beautiful “bell animalcule.” Like the _Hydra_,
this creature, except in its very young stages, remains affixed to a
water plant. In shape the “bell animalcule” resembles a wineglass on
a long delicate stem; round the part corresponding to the rim of the
glass, there is a fringe of the hair-like, water-lashing structures
with which so many of these lowly creatures are provided, and these
structures also surround the entrance to the funnel-shaped mouth. When
undisturbed, the bell animalcule has its slender stalk fully extended
and its little threads lash the water vigorously, causing currents,
containing food material, to travel towards its mouth. A sharp tap on
the microscope slide will cause the creature to contract, the threads
cease their lashing and the stalk contracts spirally, so that the body
of the animalcule is drawn close to the object to which it is attached.
By degrees the spiral uncoils and the little threads resume their
lashing.
Sometimes, as we examine our bell animalcule, we may be fortunate
enough to see it splitting into two parts to form two separate
individuals. This curious process should be watched carefully. The
upper part of the bell splits first and, by degrees the whole bell
divides into two equal parts so that we have a pair of bells on a
single stalk. The next stage consists of the formation of a ring of
whip-like structures round the base of one of the bells; both bells,
by the way, have the circlet of whips round their upper edges. Soon
after these additional little whips are formed, the owner of them
breaks away from the stalk and swims about in the water for a time,
finally coming to rest on a suitable water weed. Then the lower ring
of whips has served its purpose and in its place a long stalk grows;
from this time forward the new bell animalcule will never move from the
position it has chosen. This form of increase, this simple splitting
takes place over and over again but by degrees the little animal
appears to become exhausted and the process slows down or stops.
The partially exhausted _Vorticella_ may gain increased vitality by
fusion with another individual and this process also we may have the
luck to see though it is less frequent than the simple splitting.
Sometimes a bell may be observed to divide, not into halves, but
into two unequal parts. The smaller of these parts may divide again
into from two to eight parts, each one of which, having developed a
fringe of little whips, swims off on its own account. These little
barrel-shaped swimming forms, instead of settling down and forming
stalked bells, seek an exhausted creature, fuse with it near the base
of its bell and finally become absorbed by it. The result of this
fusion is that the bell animalcule takes on a new lease of life and
once more begins to divide actively.
In our search for specimens for our microscope we may come across a
very common pond dweller, closely related to the bell animalcule,
known by the name of _Carchesium Spectabile_. We cannot fail to
recognise its family likeness to the form we have already studied, for
it consists of a large number of stalked bells growing on a single
parent stem. It is really a little colony of bells. When one of the
young individuals of _Carchesium_ settles down in the spot it has
selected for its dwelling-place, it grows a stalk just as _Vorticella_
did, and it divides later into two individuals. Now in _Vorticella_
only the bell divides, in _Carchesium_ part of the stalk divides also
and, instead of swimming away to find a new home it remains attached to
the parent stalk. When this has happened several times a goodly colony
is formed.
There are few fresh-water animals more commonplace and apparently
uninteresting when observed casually than the pond sponge and the river
sponge. Yet, if we take either of them home and examine them with the
aid of our microscope, we shall be delighted with our specimens. In
reality they are of absorbing interest and, at certain times of the
year we may easily obtain young sponges, and capital objects they make
for the microscope.
[Illustration:
_Photos by Flatters & Garnett_
1. STARCH GRAINS OF POTATO
Starch is largely used as an adulterant of various foods. The
Potato starch grain resembles a miniature oyster shell.
2. PHOSPHORESCENCE ANIMALCULAE
These minute animals, occurring in millions, render sea water
beautifully phosphorescent.
3. PROTEUS ANIMALCULE
The lowliest of all animals. It is a jelly-like animal which
changes its shape from hour to hour.
4. CYCLOPS
A common one-eyed water animal. The female, illustrated, carries
her eggs in a pair of relatively large bladder-like sacs.
]
Before we describe our two specimens let us try to explain what manner
of creature a sponge really is. If we examine any bath sponge, we
notice that it is perforated with many small holes and some larger
ones. Some sponges show this better than others. The small holes
are pores, the large ones mouths, but, as we shall see in a moment
we must not run away with the idea that they in any way resemble
our familiar idea of a mouth. These large holes are called oscula by
scientists, but we wish to avoid scientific words as much as possible.
The simplest sponge of all, consists of a little bag, which remains
affixed by its base to a seaweed. All over its sides there are many
pores and at its tip there is a single osculum; it is known as the
Purse Sponge and is common round our coasts. The inside of the purse
sponge is lined with cells, each one of which is tipped with a little
whip which waves about unceasingly. The waving of the whip causes water
to flow through all the pores into the hollow bag and out again by way
of the osculum. Although most sponges, including our fresh-water forms,
are much more complicated than the purse sponge, the same thing happens
in them all, water is drawn in by way of the pores and forced out by
way of the oscula.
The best place to seek for the fresh water sponges is on the
under sides of floating wood, broken tree branches and the like.
Their appearance depends upon whether they have been growing in a
well-lighted spot or in darkness; they contain chlorophyl as do the
higher plants, and sponges grown in the light are green, those which
the light has not reached are buff-coloured or corn-yellow. The pond
sponge is brighter green than its river frequenting relative and is
a coarser creature altogether. It often forms little finger-like
outgrowths, whereas the river sponge is more leaflike.
If we examine one of these sponges in a watch glass of water to which
we have added a very little carmine powder, we can easily see the water
currents entering the pores and coming from the oscula. Towards autumn,
if we open a river sponge we shall see little yellow bodies about the
size of a pin’s head; they are the buds from which the young sponges
arise. One of these must be removed very carefully and placed in a
watch glass of water; if our specimen be placed in the sun, we shall
not have many days to wait before we find that it has given rise to
an active transparent little creature, to a young sponge in fact. By
repeating our experiment with carmine, by the aid of the microscope, we
can see the water currents passing through its body.
Wherever we collect our pond water we are certain to find some of the
wheel animalculæ or rotifers. There are such a number of different
kinds that we might describe several, and yet not mention the one that
any of our readers had happened upon. They may be recognised, because
they are always so transparent that all their internal organs may be
plainly seen and they always have two or more discs or lobes, at their
forward end, fringed with fine whip-like threads. These little threads
are constantly in motion, so that they appear like little revolving
wheels hence the name, wheel animalcule. Nearly all these creatures
have a sucker or false foot at their tail end by means of which they
attach themselves to some support or they may swim freely in the water.
The study of rotifers has been the life-work of some scientists and
they will afford any microscopist, who is interested in them with
abundant occupation.
Amongst the water weeds we shall probably find another small but
striking creature, known as the sun animalcule, _Actinophrys Sol_. Why
exactly it is called the sun animalcule we cannot say, probably it has
earned its title from the fact that it resembles the conventional idea
of the sun, with light rays radiating all round its circumference.
The creature is whitish-grey in colour, spherical in shape and just
visible to the naked eye. All over the surface of its body there are
a number of apparently empty spaces, known as vacuoles--they probably
account for its peculiar colour. Everywhere, it is studded with long,
slender-pointed rod-like outgrowths. But rarely, the sun animalcule
exhibits any movement and for long periods the only signs that it is
living occur at feeding time. Its food consists of water animals and
plants, of varying size. When a small animal comes into contact with
one of the pointed rods, which radiate from the animalcule, it appears
to be held there in some unaccountable manner and, after a pause, it
begins a fateful journey by sliding down the rod to the spherical
body of its captor. Then it is passed into one of the vacuoles and
digestion very soon takes place. That it does not do so immediately
is shown by the fact that the wheels of a wheel animalcule, which has
been passed to the vacuole of our subject, continue their movements
for a considerable period. When larger animal food is partaken of,
a different method is pursued by the sun animalcule. A water-flea,
for instance, coming into contact with one of the rods will struggle
violently in its efforts to escape. Then the sun animalcule shows
real signs of life, for some of the other rods bend over and hold the
captive so that it, eventually, is passed to a vacuole.
In our chapter on agriculture we mention a peculiar flat worm known as
the liver fluke. This unpleasant creature has one or two relatives who
make their home in ponds and, though they are not more beautiful than
the liver fluke to look upon, they are quite harmless. One of these
flat worms is about 3/4-inch long and not unlike an indian club in
shape. It moves about, with some speed either by the aid of a sucker on
its head or by a curious gliding movement. The other pond-frequenting
flatworm is more like the liver fluke, its leaflike oval body, about
1/2-inch long, is pointed fore and aft. It is a common sight to see it
gliding here and there in search of still smaller animals off which it
may make a meal.
The sea mat and bird’s head, two common animal colonies of the
sea-shore, possess pond-dwelling relatives of the greatest interest
to the microscopist. Like the seaside forms they dwell in colonies.
One of the commonest is known as _Lophopus Crystallinus_ and it may
be found attached to duckweed, the curious little plant whose tiny
leaves float upon the surface of the water. _Lophopus_, when at rest,
resembles a little piece of jelly. If we are patient and watch it
under our microscope we shall see it expand, sending forth a number of
stalks, each one tipped with a horse-shoe shaped feathery tentacle.
Each of these tentacles belongs to a separate animal which with its
fellows forms a colony. Not so interesting are the branched, threadlike
colonies of _Plumatella Repens_ which may be sought upon the leaves of
water plants. Our ponds can furnish no more extraordinary object for
our microscope than a colony of _Cristatella Mucedo_; it is curious in
appearance and still more curious from the fact that though a colony
of animals it acts like a single individual in crawling over the weeds
and stones in shallow, sun-kissed water. The _Cristatella_ colony is
jelly-like and greenish in colour, in length it may grow to a couple
of inches. Its under surface is flat, whilst from its upper, convex
surface the little animals forming the colony wave their brush-like
tentacles in the water.
In searching the various pond weeds for specimens we are sure to
meet with various jelly-like masses; these must always be examined
carefully. They may be the egg-masses of interesting water creatures;
various water-snails, for instance, protect their eggs with a
jelly-like covering.
If we meet with any large fresh-water mussels, sometimes called swan
mussels, we shall probably find the fleshy parts of the molluscs
swarming with minute creatures, which we may conclude are parasites.
Mussels, like nearly all living creatures, have their parasites it is
true, but what we have discovered will almost certainly prove to be
young mussels. We must examine them under the microscope to make sure.
Then we shall see, if young mussels they be, very minute and very
thin-shelled little creatures; the edges of their shells are armed with
teeth and are quite unlike the highly polished and smooth shells of the
parent mussel. We shall also see a long thread issuing from the animal
within the shell. If we are examining the young mussel in water we
shall notice that it is constantly snapping its two shells together.
Had we left these young mussels undisturbed, a very curious life they
would have led. They would have remained attached to their parent for
some little time probably, or some of them might have fallen to the
muddy bed of the pond. Their behaviour in either case would be the
same. The long threads that we have already examined would have floated
in the water and, directly they were touched by a passing fish, the
little shells would begin to snap violently. The lucky ones would not
snap in vain for they would close upon the fin or tail of the fish and
then their snapping would cease. Like the bulldog, these little mussels
may take a long time in getting hold but when once they have managed
to fasten their teeth into anything it is well-nigh impossible to make
them let go. Usually they never let go, but are carried away by the
fish. In most cases, the irritation they set up in the flesh of their
new-found foster parent causes a swelling to occur with the result that
the little mussels are engulfed. Within the flesh of the fish they go
through a series of changes; they lose their teeth and their tell-tale
thread, they become in fact miniatures of the adult mussels, then they
manage to escape from the fish, settle down in the mud and fend for
themselves.
The crabs and lobsters which we know so well have fresh water relatives
in nearly every pond. Many of the creatures we have examined have
needed careful search to discover their whereabouts; not so the fresh
water crustacea, as they are called. Their activity, their curious
movements in the water compel attention.
The fresh water shrimp is a curious little creature, sometimes he
paddles his three pairs of hind legs and sometimes he jerks his body in
a ludicrous manner, in either case he manages to propel himself rapidly
through the water. He is about half an inch long, brown in colour and
with a curved body not unlike a shrimp. If we examine him under the
microscope we notice that his front legs are bent forwards, whilst his
hind legs are bent backwards. The male water flea is much larger than
the female, a fact which probably accounts for the fact that these
little animals often carry their wives about with them by seizing them
with their fore legs.
The water louse we may also encounter, he is not nearly so interesting
as the fresh water shrimp. He is closely related to the wood louse
which we all know, and has a similar flattened body.
Very much smaller though even more interesting is the common water
flea. By day these animals retire to the mud at the bottom of the pond
but, morning and evening, they swim actively with a curious jerky
motion. We must examine our specimen carefully for he is of more than
ordinary interest. We cannot fail to observe how transparent he is,
so much so that all his internal organs can be plainly seen, but let
us deal with his exterior first of all. His large eyes are plainly
visible, but his most conspicuous feature is the pair of large branched
feelers, by means of which he swims. If we examine several specimens,
one or more is certain to be a female, then we may probably observe
an egg in process of formation in the brood pouch, a large, elongated
cavity, just below the back of the animal. Immediately above the brood
pouch, the heart is situated and, if we can induce its owner to keep
still for a moment or two, the heart-beats may be plainly seen.
Not unlike the water flea as it swims about in the water of our
collecting jar is the curious, transparent little creature known as
_Cypris_. Although so transparent its body is contained in a pair of
shells, very similar to those of the mussel; a fact which formerly
led to its being classed with the shell-fish. We may well examine
this little fellow under the microscope for much of his structure may
be made out through his shell. His very conspicuous eye is sure to
attract our attention; he possesses but a single eye and seems to make
up for the lack of a second by having a very large one. Two pairs of
feelers project beyond the shell in front. Of his two pairs of legs the
foremost, or at least their tips, hang down below the shell, but the
last pair, as we can see through the shell, are turned upwards. At the
hinder end of the body, there are two long bristles, they may best be
seen when _Cypris_ is swimming. Of what use exactly these bristles may
be to their owner is not definitely known, but it is thought that they
are of service in keeping his shell clean.
Another active little animal, quite as common as the water flea, is
sure to attract our attention. It is no larger than the water flea
but much more elongated; some specimens are bigger than others, and
the bigger ones are the females. Its name is _Cyclops_ and, though so
common, it has no popular name.
_Cyclops_ is so named on account of the fact that it possesses but a
single eye; it is, however, rather an interesting creature in other
respects, so we will study it more closely. Looking down upon the
creature, we see that the front part of its body is composed of an
undivided shield, behind which there are four plates and behind these
again there are in the male five and in the female four segments or
rings, at the extreme tip there is a forked tail, each fork being
furnished with a number of bristles. On the head are two pairs of
organs, one pair long the other pair short and, if we observe _Cyclops_
in the act of swimming, we shall see that the long pair of organs
play the chief part. In the centre of the front of the head there
is a black or red patch--the eye. Very frequently we may meet with a
specimen carrying a relatively large bladder-like body on each side of
its abdomen. These bladders which are each about one-third as long as
the creature which carries them, are egg-sacs.
If we are able to secure one or two specimens with egg-sacs attached,
we can study the young _Cyclops_ without much difficulty. Take
a small glass tube--a test-tube as used by chemists will serve
admirably--partly fill it with pond water and add a little water weed,
then introduce the egg-bearing females and place in the light. We must
watch the tube from day to day, and before long it will be evident
that the young ones have arrived in the world, for we shall have no
difficulty in seeing dozens of little white specks swimming about in
the water and settling on the sides of the tube. We must remove one
carefully on the end of a glass rod or on a paint brush and examine
it in a drop of water under the microscope. This young creature is
totally unlike its parent, it is oval and possesses three pairs of
stiff bristles, of which the first pair are simple and the other two
pairs are branched. Although the bristles are used solely for swimming
at this stage, it may be of interest to mention that in the adult
_Cyclops_ they become transformed into jaws and the two pairs of organs
we have already examined. At the front end of the oval body we can
closely distinguish the single eye, which persists throughout life.
CHAPTER VI
PLANT LIFE IN PONDS AND STREAMS
In this chapter we shall confine ourselves to the true water-dwelling
plants, as distinct from those, such as the water lilies, which though
never found growing on dry land, appear undecided whether they will
be water plants or land plants. Looking at the matter from a more
scientific point of view, all our pond plants will be much lower in the
scale of development than the water lilies and other flowering plants.
Pond life is rich in subjects for the microscopist. Any stagnant
pool may contain organisms which will delight the naturalist who has
always depended upon his unaided vision. Curiously enough, amongst
the most wonderful of all these pond-dwelling plants are the Diatoms,
which consist of but a single cell. They are so numerous, they exist
in so many different forms and in so many different situations that
were we able to describe them all, we should require the whole of a
large volume, much larger than this. In colour, Diatoms are usually
brown or brownish, although they contain chlorophyll, the green
colouring matter of higher plants. In shape they may be rod-shaped,
crescent-shaped, circular, wedge-shaped, oblong or oval. Some float
about freely in the water, some are attached to supports by means
of stalks. Some lead a solitary life and others dwell together in
colonies. One feature they have in common, a curious flinty cell wall,
and this is their most interesting point to the microscopist. This
natural armour is in two parts which fit one within the other like the
two halves of a Japanese basket. All manner of beautiful sculpturing
marks these beautiful frustules as they are called; in some cases they
are perforated and the living matter from within passes through the
pores and forms a jelly-like covering for the little plant.
We must make a point of collecting all the Diatoms we can find, for
they are always interesting; moreover, they are easily preserved and
made into permanent slides, for the little plants may be boiled in
acid to destroy their living parts and the frustules will survive the
boiling undamaged.
One might wonder how such humble plants, surrounded as they are with
flinty walls, could increase. They frequently do so in a simple manner.
The living matter of which the plant is composed pushes the frustules
apart and divides across the middle. The result of this event is the
formation of two plants, each with a single frustule. In a very short
time each plant grows a new frustule, but it is always much smaller
than the one with which it started.
The movements of some of the free swimming, that is to say non-attached
Diatoms, are worthy of study. The scientific name of one kind,
translated into everyday language, means little boats, and indeed they
are well named for their beautiful aquatic manœuvres rival those of any
ship.
Somewhat similar in habit to the brown Diatoms are the green Desmids,
but, whereas, the former also occur in the sea, the latter are all
confined to fresh water. Sometimes Desmids are so numerous that
they make the pond water as green as green-pea soup. It would be as
impossible to describe all these plants as was the case with the
Diatoms, but generally they may be recognised by the fact that they are
composed of two similar halves, separated by more or less of a waist.
Although some of the Desmids exhibit a certain amount of movement they
are not active like the Diatoms.
Late spring and autumn are the best seasons to hunt the ponds
for our next object, which rejoices in the name _Chlamydomonas
Angulosa_, a good example of the extraordinary fact that some of
the smallest animals and plants have the longest names. This little
plant is interesting in itself and doubly so, because it was for
long thought to be an animal; it is wonderfully animal-like in its
movements. _Chlamydomonas_ is very minute, so we must use our highest
magnification when we examine it. It is an oval, one-celled plant
enclosed in a clear membrane. The green colouring matter is arranged in
the form of a cup, within the hollow of which is a mass of granular
living substance. At the forward end of the plant, there is a clear
space and near the tip a brownish dot, known as the eye spot; which,
though incapable of seeing as we understand it, is sensitive to light,
as shown by the fact that the little plant will swim towards moderately
intense light and away from strong light. If we stain the plant with
iodine we can plainly see a pair of little whips arising from the clear
portion at the forward end; it is by the lashing of these that the
plant is enabled to swim.
In the mud of our pond we may find a little colony of plants which
might forgivably be mistaken for a collection of the individuals we
have just studied. Each member of the colony is very much smaller
than _Chlamydomonas_ to be sure, but each one has the outer membrane,
the brownish eye spot and the pair of little whips. On the other hand
the chlorophyll fills the whole of each cell and is not arranged in
the form of a cup. Sixteen cells form a colony, and the whole mass is
a flat plate; the little whips move in unison and the whole colony
revolves after the manner of a wheel.
Another little colony we may encounter also, consists likewise of
sixteen cells very like the ones we have described, but somewhat
wedge-shaped instead of oval. A jelly-like mantle encloses the colony
and, in outline, it is spherical, so that when the little whips, which
project through the mantle, lash the water the whole colony revolves.
The most remarkable of these colonies of cells is known as _Volvox
Globator_, it may be recognised by its perfectly spherical shape and
its characteristic movements in water. _Volvox_ is about 1/25 inch in
diameter, and although to the uninitiated it appears to be a single
minute plant, in reality it is a colony of upwards of twenty thousand
cells. The colony may be considered as being made up of thousands upon
thousands of cells, very similar to those of _Chlamydomonas_, and each
one arranged with its pair of little whips directed outwards.
Within the _Volvox_ sphere we may observe a number, usually one to
eight, of smaller spheres. These are so-called daughter colonies which
have arisen from the continued division of special cells. They develop,
fairly rapidly, into young _Volvox_ colonies, then they burst through
the cells of the parent colony, swim out into the water and quickly
grow to the size of the _Volvox_ from which they were formed.
There is another kind of _Volvox_ of a yellowish colour and much
smaller than _Globator_, the big one is the one we must procure; it is
much more easily studied.
Certain of the plants we shall find in our pond are so animal-like in
their movements, that the microscopist who sees them for the first
time may wonder whether we are not mistaken in calling them plants. We
have already described the common _Chlamydomonas_, with its curious
jerky method of propelling itself through the water. There is,
however, an equally common one-celled, pond-frequenting plant which
has puzzled naturalists even more, for it certainly possesses many
very animal-like characteristics. Its name is _Euglena Viridis_ and
we require our highest magnification to examine it for it does not
exceed one-two hundred and fiftieth part of an inch in length. Usually,
_Euglena_ is cigar-shaped but, as it possesses the very unplant-like
characteristic of not having a firm cell wall it can change its shape
to a considerable extent and it often assumes curious forms. At the
forward end of this minute plant there is a single whip-like thread, by
means of which it swims; a little below the base of the whip, there is
a red eye spot. Elsewhere we have described how the protean animalcule
feeds by flowing round its food-material, _Euglena_ feeds in a similar
manner, but it also feeds after the manner of a plant. When this active
little plant is about to increase it either divides into two lengthways
or becomes surrounded with a firm wall within which it breaks up into a
number of young forms which are released later by the bursting of the
wall.
[Illustration:
_Photos by Flatters & Garnett_
1. BLADDERWORT
A British water plant which entraps small animals in its bladders
and digests its captives.
2. SPORES OF HORSE TAIL
These spores with their thread-like outgrowths vary in appearance
according to the moisture in the air.
3. HAIRS ON A POTATO LEAF
The star-shaped hairs on a potato leaf make beautiful objects for
the microscope. Leaves of many plants are clothed with curious
hairs.
4. SPIROGYRA
A green thread-like water weed. Two threads are shown fusing
together; from each part of these fused threads a new plant will
arise.
]
Small wonder that scientists were in doubt concerning the true nature
of _Euglena_, its non-walled cell and peculiar mode of feeding are
indeed puzzling. There are many similar cases amongst these lowly
organisms, in fact there are some creatures which may best be described
as partly plant and partly animal. Those of our readers who are
interested in such problems may read an excellent account given much
more fully than we could give it here, in Professor F. W. Keeble’s
_Plant Animals_, one of the excellent Cambridge Manuals of Science.
The fact is that the demarcation between plants and animals, low in
the scale of development, is not nearly so pronounced as it is amongst
the higher forms of life. Amongst the plants of our pond we shall find
that, like the land plants, most of them are green, and many of them
are thread-like, so that the task of distinguishing one from another
may appear difficult. Examined with the naked eye, many of them appear
remarkably similar to one another; under the microscope the differences
are obvious.
One of the most remarkable of the commoner pond plants is known as
_Oscillatoria_; it does not boast of a popular name but its scientific
name is not very difficult to remember after we have witnessed its
oscillations. _Oscillatoria_ is a plant with particularly animal-like
movements. It is merely a thread, usually green-blue in colour, but
sometimes red or violet. The threads are never branched and, except
when in motion, are straight. Under a moderately high magnification we
can see that this thread-like plant is not composed of a single cell
but that it consists of a number of cells, placed end to end. Sometimes
a few of these cells will break away and start life on their own
account.
Whatever interest the structure of _Oscillatoria_ may have for us, we
cannot help being struck with its movements, and we must make a point
of observing them. The movement is peculiar and not easy to describe
nor, so far as we know, has it ever been explained. A thread will be
seen to glide backwards and forwards, becoming somewhat curved and,
at the same time, revolving on its axis. Eel-like is perhaps a good
description of the movement. It is possible to distinguish this plant
from other pond dwellers by its slimy “feel” which arises from the fact
that each thread is enclosed in a jelly-like sheath.
The silk weeds, _Cladophora Glomerata_ are somewhat similar in
appearance to the plant we have just described, but they are denizens
of running streams rather than of ponds. They are the green thread-like
plants we so often see attached by one end to rocks and stones beneath
running water. Each plant consists of a long, cylindrical structure
composed of several cells. We mention the silk weeds here, because they
are best of all plants for showing cell growth. This growth takes place
at or near the unattached end of the plant and is easily observed. The
end cell may be watched for the process. Its green contents will be
observed to contract in the middle so that it assumes an hour-glass
shape. Then, where the contraction has taken place, we can watch the
formation of a wall right across the cell, so that when the process is
completed we have two cells where formerly there was one. More rarely,
the contents of a cell will be observed to bulge out a side wall, then
a new wall is formed to divide it off from the main cell and thus the
beginning of a branch is formed. The silk weeds exhibit no movements,
except such as are imparted to them by the running water.
A very beautiful little pond plant is known to science as _Draparnalda
Glomerata_. We shall probably forget its name, but we can never forget
the plant itself when once we have been fortunate enough to see it.
A single row of large, transparent cells, containing very little
green colouring matter, forms the main part of the plant. At regular
intervals from these transparent cells, there arise rings of deep green
branches, each one tipped with an extraordinarily long, colourless
hair. _Draparnalda_ is indeed a plant worth looking for.
Two common little green plants grow so near to the edges of ponds that
they may well be included amongst our pond plants. The simpler of the
two, known as _Vaucheria Sessilis_ thrives on almost any damp soil and
may even form a covering on soil in pots. In structure, _Vaucheria_ is
very simple for it consists of a single, frequently branched, tubular
cell. The little attaching organ, by means of which the plant fixes
itself to some firm support, is colourless, so too is the tip of the
cell where growth takes place.
We must examine this little plant when we come across it and we must
not fail to notice that it is composed of but one cell. The only time
at which we can find any cross walls, is when _Vaucheria_ is about to
increase. Then the tip of the cell swells, like a little club, and a
cross wall separates it from the rest of the plant. The contents of
this cell rounds itself off, becomes fringed with innumerable little
lashing whips and escapes from a pore at the tip of the cell in which
it was formed. This little organism swims about for a time in the
water, for _Vaucheria_ only increases in this manner when it is under
water, at length it comes to rest and forms a new plant.
Closely related to _Vaucheria_, but not quite so common, is the curious
little plant known as _Botrydium Granulatum_. Like our previous
example, it is a one-celled plant and herein lies its interest. When
find a specimen growing by a pond, we shall notice a green bladderlike
portion, not more than 1/6 inch in diameter, which projects above the
ground. Below ground, if we pick the soil away carefully we may observe
a number of colourless, branched structures which do duty for roots.
The bladder and underground portions are hollow, being lined in the
case of the former, with a network of chlorophyll beautiful to observe
under the microscope.
_Botrydium_ usually increases by buds which form on the green bladder,
break away and grow into new plants. Should the level of the water in
the pond rise, and the little plant become submerged, it splits up into
a number of small bodies, each provided with a minute whip-like organ,
with which they swim rapidly ashore where they develop into new plants.
In dry weather the plant forms a number of little cells each one
surrounded by a firm cell wall. When moisture comes again these little
cells give rise to structures which will grow into new plants.
Surpassing even _Drapernalda Glomerata_ in point of beauty, is the
“water-net,” _Hydrodictyon Reticulatum_, but it is not nearly so
common, being confined to ponds in the South and Midlands. When full
grown it hardly comes under the heading of a microscopic object, for it
may measure as much as six inches in length. This remarkable pond plant
consists of an open network of green filaments. Apart from its striking
appearance the most remarkable thing about the “water net” is its rapid
growth. Carpenter, a noted microscopist says:--“The original cells of
which the net is composed measure one-two thousand five hundredth part
of an inch in length but in a few hours they grow to one-twelfth of an
inch or 1/3 of an inch in length. We often hear people remark that they
can see plants grow, but their statements are not literally true; in
the case of the ‘water net’ it is actually possible to see the growth.”
By the side of our pond we shall probably observe some masses of a
bluish-green jelly-like substance. It is uninteresting-looking material
for the microscope, but we must not pass it by. The blue-green jelly
encloses a plant called _Nostoc_, which resembles nothing so much as a
necklace of beads; this we can plainly see under a low magnification.
We shall observe that the plant is twisted spirally within its covering
and also that most of the cells, which we have compared to beads, are
similar to one another in size. At intervals there are larger cells
and as the plant increases in length they are emptied periodically, so
they are evidently used as food stores. From time to time portions of
the plant break away, worm their way out of the jelly and move about,
rather after the manner of a worm. Eventually, these wanderers come
to rest and become surrounded with more of the blue-green jelly, thus
forming a new _Nostoc_ colony.
These plants sometimes appear, apparently from nowhere, on garden
paths, walls and similar situations, during damp autumn weather.
On this account the plants have been called “fallen stars.” Their
appearance is not so mysterious as it might seem for the _Nostoc_
colony has probably been where it is found, all through the summer,
in a dried up, contracted state. Only when rain comes, does the jelly
envelope absorb water, swell up and assume its normal appearance. The
blue-green scum which floats on stagnant water is a closely related
plant.
Another plant which looks like scum on the water is known as
_Spirogyra_ and a very beautiful object it makes for the microscope.
It is bright green, without a bluish tinge, so it need not be confused
with the plant we have just mentioned. There are seventy or so
different kinds of _Spirogyra_, therefore our description must be the
one that will apply to all. The plant is thread-like and, even in the
larger kinds, the threads are not more than one-hundredth part of an
inch in diameter. Like _Oscillatoria_ the plants are not attached to
any support. Each thread is composed of many cells, arranged end to
end; we can distinguish the cell walls clearly, but what will chiefly
attract our attention are the beautiful bands of green colouring
matter, running spirally, round each cell. If we are fortunate, we
shall see the plant in the act of increasing; this is not the simple
operation we witnessed in the silk weeds. From two threads lying
parallel to one another, we shall see swellings arise on the adjacent
cell walls; the beautiful spiral bands will begin to break up at the
same time and to collect in a mass towards the centre of each cell.
The swellings of adjacent cells touch, their end walls break down so
that the two cells become connected, our figure shows the fusion taking
place, then the contents of one cell passes into the adjacent cell and
fuses with the contents of the latter. The new cell, which now contains
not only its original contents but that from a cell in another plant,
becomes detached from the thread, assumes an oval shape and sinks to
the bottom of the pond, where it rests awhile. At a later period the
cell wall bursts and a new thread of _Spirogyra_ develops from it.
When we are examining _Spirogyra_ we may notice some very minute
brick-red, spherical bodies adhering to the green threads. These are
little pond animals, known by the name of _Vampyrella Spirogyræ_. This
little creature passes through an interesting and easily observed
series of changes. Its life is very uneventful, consisting of a good
meal of _Spirogyra_, a period of rest followed by an increase, and
this is repeated over and over again. If we watch our little sphere
through the microscope, we may be lucky enough to see the contents
divide into four parts. Now we must watch carefully, for very
interesting events are about to take place. Each of the four parts into
which the contents of the sphere has split, escapes into the water
and swims about for a time; it then becomes spherical, but instead of
having a smooth outer surface, as it had when we first observed it,
we can see that it is now studded all over with very fine threads. It
then wanders along a _Spirogyra_ plant, attaches itself to one of the
cells, perforates its wall and sucks out the contents. This performance
it repeats several times then, evidently satiated, it loses its threads
and resumes the appearance it had when first observed and thus it
rests, till division of its contents into four parts takes place once
more and the little comedy is repeated.
Now we must tear ourselves away from our pond; there are very many
interesting objects for our microscope on the sea shore and if we delay
too long here, we shall not be able to give them the attention they
deserve.
CHAPTER VII
THE MICROSCOPE AND PLANT LIFE
The science of botany consists of many branches and, in most of
them, the microscope is the scientist’s constant aid. The study of
bacteria, really a branch of botany, we have dealt with in another
chapter, so here we will omit these interesting though lowly plants.
By far the number of botanical objects for the microscope consist
of sections--exceedingly thin slices of whatever portion of the
plant is being examined, cut either with a sharp razor or a special
instrument called a microtome. Section cutting, though not a difficult
accomplishment, requires a considerable amount of practice and cannot
be learned from a book; all our descriptions, therefore, will be
confined to objects from the plant world which may be studied without
the assistance of razor or microtome.
One cannot help being struck with the fact that green is the prevailing
colour among plants and the reason is not far to seek. If we take a
cabbage leaf and carefully tear off the skin, we shall find green
spongy matter below. A little of this green material may be examined
under the microscope and will show us rounded green bodies composed
of a substance called chlorophyll. Now chlorophyll is absolutely
necessary to all plants, except the fungi and to one or two parasitic
plants. It is necessary because, by its aid, plants can build up raw
food material into food which will be useful to them. It is not formed
in darkness; that is why a board, a roller or any similar object left
on a lawn, causes the grass below to turn yellow; it is the reason
also why certain parts of plants, not usually green, turn that colour
when exposed to the light. Chlorophyll does not always occur in round
globules, sometimes it is found in bands.
One of the most interesting botanical studies for the microscope is
furnished by the leaf of the American water-weed. This plant, which was
introduced into the country from North America some years ago, has now
spread far and wide and is easily obtained. A leaf which is slightly
yellowed with age is the best to take for the experiment. It should
be cut from the plant, placed at once in a small bottle of water and
kept warm for a few hours; this may be accomplished by keeping the
bottle in one’s pocket. After a sufficient interval, put a drop of
water in a clean slide, put the portion of leaf in the drop of water,
cover with a coverslip and examine with a moderately high power. If the
experiment has been properly carried out a wonderful sight will reward
us. We shall see that the leaf is divided into a number of divisions
called cells; this name has been handed down from the very early days
of the microscope, because of a supposed resemblance to the cells in
a bee’s honey comb. In each cell we shall see signs of activity, the
little round grains of chlorophyll are there, but instead of being
stationary, as in the cabbage leaf, they are slowly moving round the
walls of each cell. In reality they are carried along in the stream of
living matter within the cell. It is a wonderful sight and brings home
to the observer very forcibly a fact which is liable to be forgotten,
that the plant is just as much a living being as an animal. Perhaps
our experiment will not succeed at the first attempt, then we must try
again; maybe we have been too rough in detaching the leaf or we have
not kept it sufficiently warm. Sometimes the movement may be started by
slightly warming the slide over a flame; too much heat, of course, will
kill the leaf.
We shall see this green colouring matter over and over again in our
botanical studies, in fact it is found in all manner of situations,
in leaf and stem. Very often its colour is hidden by sap of another
colour, as for instance in copper beech leaves or in the brown
seaweeds. Chlorophyll dissolves in alcohol, however, and this affords
us a ready means of detecting its presence though we cannot see its
green colour. If we boil any leaf, suspected of containing chlorophyll,
in alcohol we shall obtain a solution with rather peculiar properties
because, when held up to the light it appears green, but when light is
reflected from it, it appears reddish.
From the under side of the cabbage leaf whence we obtained our first
specimen of chlorophyll, we must now take another piece of skin. If
we perform the operation properly the skin will be colourless, like
a piece of thin parchment; any green colour will show that we have
torn off more than the skin and we must make another attempt. Having
secured our piece of skin we place it in a drop of water on a clean
slide and examine it under the microscope. We first notice that the
skin is divided up into a number of small areas called cells and dotted
here and there amongst the cells are several oval bodies, containing
chlorophyll. These oval bodies are the pores through which the leaf
breathes, amongst other things. In the centre of each pore there is
a hole, at least there is if the pore is open, for the two cells
comprising the pore have the power of opening and closing.
It is interesting to try the same experiment with a fern leaf and to
notice that there are pores, very similar to those of the cabbage, but
that the walls bounding the cells of the leaf are irregular and that
they contain chlorophyll. We may try several other leaves and also the
upper and lower surfaces of leaves, then we shall soon learn that, in
leaves with distinct upper and lower surfaces, there are far more pores
on the lower than the upper surface; leaves like those of the iris have
almost the same number on each side, and floating leaves, like those of
water lily have all their pores on the upper side. There is a reason
for this; the pores are likely to become filled with dust, being on the
lower side they are protected somewhat; flat leaves, by their shape,
afford no protection and floating leaves must have their pores on the
upper surface to obtain air.
There are many other interesting things we may learn about leaves, with
the help of our microscope. The cabbage leaf is quite smooth, but if we
are observant we shall have noticed that sometimes each leaf appears
as though it had been powdered, it has a decided bloom. The bloom does
not appear on the leaf for ornament but for a purpose. It is a waxy
substance and it prevents the leaf from losing moisture too quickly
in dry weather. This is very important for the plant; if the moisture
taken up from the soil were lost in the air too quickly by the leaves,
the plants would wither and eventually die. It is not all plants which
can wear a protective covering when danger threatens, most plants have
either no protection or are permanently protected. There is a large
class of plants with folded or rolled leaves; heather and marram grass
belong to this class. We must examine some of these leaves and we shall
find that all the pores are on the inside of the leaf whether it be
folded or rolled. The reason for this is that moisture also escapes
through the pores and, when they are thus protected, it is not carried
off too quickly by drying winds.
Many plants are protected, as far as their leaves are concerned, at
anyrate, by hairs. They take the place of the bloom in such plants as
the cabbage. There are thousands of plants with hairy leaves and they
will provide as many interesting objects for our microscope. Let us
examine as many as we can for the hairy covering of each plant will
be a little different to the one we examined previously. There are
simple hairs, quite ordinary affairs, forked hairs, branched hairs,
T-shaped, star-shaped and club-shaped hairs. If we are clever with our
microscope we shall notice that, however complex each hair may be it is
really nothing but one cell of the skin of the leaf which has assumed a
peculiar shape.
The leaves of the nettle are armed with ordinary and stinging hairs;
the latter are worth examining and we shall notice that there is one
great difference between all the other hairs we have examined and
the stinging hairs of the nettle. The former are of one cell only,
the latter of several cells. A high magnification will show that the
stinging hair of the nettle is not quite so simple as it appears at
first sight.
There are plants whose leaves are protected by very thick skins and
others whose leaves become armed with hard flinty matter, so that they
resemble stones rather than leaves.
If we can find some quite young seedlings we must manage to secure one
or more for examination under our microscope. We must take one up very
carefully and wash the earth from its roots--if we pull the earth away
our specimen will be ruined. Near the tips of the root branches we
shall see something which might be mistaken for mould. Our microscope
will show us that they belong to the root; they are, in fact, root
hairs. We shall very likely be able to make out that, like the leaf
hairs, each root hair is made up of a single cell. The root hairs are
interesting because it is through them that water is taken up by the
plant from the soil.
In many plants a considerable space separates the root from the leaves.
When we have learned how to cut sections, we can make slides for our
microscope which will show us the whole course along which the water
travels, from its point of entry at a root hair to its exit at a leaf
pore. Although we have not yet reached that stage, it need not prevent
us from seeing some of the minute tubes through which the water passes.
Any fleshy stemmed plant will serve our purpose. We must tear it to
pieces lengthways with a needle and we shall find many threads--this is
not their correct name but it expresses our meaning--running the whole
length of the stem. They run, in fact, from the tips of the root to the
leaf, and may be seen as leaf veins. If we remove one of these threads
and tease it with needles on a slide we shall reduce it to still
finer threads. By the way, teasing in the sense we have used it here,
means separating the various parts. Let us examine some of these fine
threads, we shall see that some of them appear like coiled springs at
first glance. A more careful examination will show us long tubes with
spiral thickenings. We all know the garden hose-pipe with stout wire
coiled round it as a protection; these tubes may clearly be compared
with the familiar hose-pipe, where the rubber portion represents the
cell wall and the stout wire the thickened parts of the wall. There is,
however, this great difference, the wire is outside the hose-pipe, the
thickened portion of the plant tube is inside the wall. These tubes are
the ducts for water passing from root to leaf.
If the agricultural side of botany attracts us we shall not have much
difficulty in finding many more objects from the fungus world than are
mentioned in our chapter on Agriculture, whilst the study of bacteria
may truthfully be termed never ending. Ponds and rivers teem with
vegetation suitable for microscopic study. The testing of foods for
impurities is largely botanical work. The botanist, of all men, need
never allow his microscope to be idle.
Our British insectivorous plants are of great interest and they will
supply us with some objects for our microscope. We only possess
three different kinds of these curious plants in this country, the
Bladderworts which live in water and the Sundews and Butterwort, which
frequent moist, peaty land.
The pond-dwelling Bladderworts are not rare, indeed they occur in
plenty in certain localities, but they are not very evenly distributed
over the country and in some districts one may search for them in vain.
It is worth while making a special effort to obtain a specimen. Each
plant bears a number of hollow structures, the bladders. There is an
entrance to each bladder, edged with stiff hairs and closed by a trap
door, which opens inwards but will not open outwards. All these parts
may be seen under the microscope as may the interior of a bladder; its
walls are studded with short hairs. When small water animals enter a
bladder, it is said that they do so to escape from their enemies; they
are entrapped forever, they die and eventually decay. The juices which
arise from their decaying bodies are absorbed by the hairs lining the
bladder.
The Sundews are pretty plants, with rosettes of reddish leaves and
minute white flowers. With the naked eye we can usually see many drops
of clear liquid on the leaves, a number of substantial-looking hairs
and a few insects adhering thereto. If we examine one of these hairs
under the microscope, we shall see that it is club-shaped; it is, in
fact, a hair which gives off a sticky liquid with the power of holding
any luckless insect that settles thereon and absorbing its softer
parts for the nourishment of the Sundew. An examination of a complete
leaf with our pocket lens will show that where an insect has settled,
several of these hairs have curled over so that they touch their
victim. Because the hairs possess this power of movement they are often
wrongly called tentacles.
Butterwort, like Sundew has a rosette of leaves but they are greasy
looking and pale green. Their flowers are a pretty blue. Most probably
we shall notice that the edges of the leaves are curled inwards,
and if we look below the curled portion we shall surely find some
captured insects undergoing digestion. The leaf of this plant makes an
interesting object for the microscope. There are two kinds of hairs
on its surface, short stout ones and longer knobbed ones; the former
give off a sticky liquid which holds any small insects that touch it,
the latter give off digestive juices. While the hairs are used in
digestion, after the manner of those of Sundew, the leaf itself curls
so that more of the hairs are brought into contact with the victim and
thereby its digestion is hastened.
Many small flowers may be examined with low magnifications. When we
examine them thus, we shall probably realise for the first time how
beautiful are many of these seemingly inconspicuous blossoms. Grass
flowers are always interesting; they are not ornamental it is true, but
that does not detract from their interest. There is one part of each
flower known as the stigma; it is the part on which the pollen grain
must be placed in order that seeds may be formed. The pollen grains
are taken to the stigmas in many ways, but the most usual agencies
are insects and wind. In the case of grasses, wind is the agency and
for that reason the stigmas of grass flowers are feathery, so that
they can easily hold the pollen grains carried to them by the lightest
breeze. We shall probably see many pollen grains entangled in the
feathery stigma of the flower we are examining. In the flowers of other
plants we shall find, when we magnify them, that there are all manner
of contrivances on the stigmas, all designed for holding the pollen
grains; hairs, knobs, hooks and the like.
An interesting collection could be made of various pollen grains,
which are easily obtained by merely dusting the anthers of flowers
on to a clean, dry slide. They are varied in shape, colour and size;
some are smooth, some studded with spines, others again, those of the
_Mallow_ for example, have little lids which open when the pollen grain
germinates. The germination of pollen grains is easily observed under
the microscope, by putting a few of the grains in an exceedingly weak
solution of sugar and water. The vigil may be a long one, but if the
pollen grains are ripe and fresh, and the sugary solution sufficiently
weak, the patience of the microscopist will be rewarded by the
observance of the bursting of the pollen grain’s coat and the outgrowth
of the pollen tube.
Other pollen grains worthy of examination, are those of various lilies,
of _Eschscholtzia_ and of _Scotch Fir_; the last named have curious
little air-bladders, for the purpose of rendering them more buoyant.
Many lowly plants thrive in weak sugar solutions, after the manner of
pollen grains. The yeast plant is one of them. A very small portion of
yeast, in a drop of sugar solution, will show us one of the simplest
methods of vegetable reproduction. Yeast is a fungus and it is also a
plant composed of only one cell. Under the microscope, it appears as
a colourless oval body. The sugar solution causes it to multiply and,
after the lapse of a little time, most of the yeast plants will be seen
to bear outgrowths, called buds, which grow larger and larger till, at
length, they break away from the parent plants and start a separate
existence. Sometimes, when these plants are increasing very rapidly,
the buds will bear smaller buds and these again still smaller ones till
a fairly long chain of yeast plants is formed.
It is always interesting and also instructive to make comparisons as
we progress with our work. To illustrate our meaning let us compare
the budding of the yeast plant with the budding of the hydra, which is
described in our chapter on pond life. In the same chapter we described
the division of a proteus animalcule into two separate organisms, a
process which is also undergone by bacteria when circumstances are
favourable to their increase. We shall find many points of similarity
if we make careful comparisons, and several important differences.
Objects for the microscope we can find in plenty, without going far
afield. The white mould which we can probably find in the larder, on a
pot of jam or other food that has been allowed to stand for some time,
will provide a good subject to start upon. A little of this plant, for
such it is, carefully lifted on to a dry slide will show the threads of
the mould, terminated by round black knobs. Breathe on the specimen and
the moisture of your breath will cause these little balls to break and
set free a quantity of fine dust-like bodies called spores. The spores
will be carried about in the air, they are so light, and if they settle
on a suitable medium they will germinate and start another growth of
mould. The blue-green mould of cheese is constructed quite differently;
its spores are not contained in any hollow structures like those of our
first object, but grow in chains radiating from a central point, like
the outstretched fingers of the hand. It is this fungus, by the way,
which imparts the colour and flavour to gorgonzola cheese.
For some reason living organisms, possessing the power of movement,
be it ever so slight, are always more attractive than those which are
apparently motionless. Let us study two common objects from the plant
world which may easily be obtained by any nature student, objects
which owe their power of movement--not to be confused with locomotion,
by the way--to the presence or absence of moisture in the air. On
the under side of the fronds of many ferns there will be found more
or less rounded reddish-brown spots. These outgrowths, for such they
are, vary in position and shape according to the species of fern. An
examination, with a pocket lens, will show that these brownish spots
consist of minute tufts of knobbed structures, growing from the tissues
of the frond. Sometimes the structures are naked, sometimes covered
with a membrane. In either case, one or more of the knobbed structures
is worth examining under the microscope; we shall then see that it
consists of a stalk terminated by a thin walled portion, shaped like a
bi-convex lens. Round the edge of the greater part of this lens-shaped
portion there is a much more substantial-looking rim. Within the
lens-shaped part we can easily see brown spores. If we have chosen our
object at an opportune moment, any excessive moisture in the air will
cause the thick-walled rim to straighten itself out, tearing away the
thin-walled, lens-shaped part in so doing and setting free the spores.
Closely related to the ferns, the horsetails provide another
interesting object for the microscope. The fertile shoots of these
plants, somewhat resembling asparagus, though in reality belonging
to an entirely different family, will, when gently tapped on a clean
dry microscope slide, leave behind a pale yellow powder. The powder
consists of spores, and most interesting they are. When dry, each spore
will be seen to have four somewhat thread-like outgrowths, flattened at
the end; breathe on the spores and each of the outgrowths will coil up
so as to form a complete covering for the body of the spore. As drying
takes place, these outgrowths gradually uncoil again.
We have mentioned spores several times in this and other chapters.
Strictly speaking a spore cannot be compared with a seed, but for
our purpose it is sufficient to know that spores are more or less
seedlike in appearance and that they give rise to new plants when they
germinate. They are found in all ferns, on horsetails, these odd plants
with their creeping stems and rings of scale-like leaves, on club
mosses, mosses proper and fungi but not on flowering plants.
Should the student of plant life not yet be satiated with following the
suggestions we have made, he can turn his attention to fruits and seeds
and the contrivances designed for their distribution. The fruits of
goosegrass, popularly known as cleavers, are studded with little hooks
so that they may adhere to any passing animal. The fruits of Burdock
are similarly armed and if we make a study of fruits and seeds we
shall find that this is a very common method of ensuring distribution.
There are also a number of seeds covered with hairs which render them
buoyant; those of the willow herb are easily found, so too are the
fruits of dandelion, thistle and groundsel. These and many more will
give us many an interesting hour, towards autumn.
CHAPTER VIII
ANIMAL LIFE AND THE MICROSCOPE
There are few more interesting animals than spiders and we may spend
many an hour learning details of their structure, which only the
microscope can show, and studying their habits, for only by doing so is
it brought home to us how astonishingly clever they are. The spider,
of course, is not an insect; it has eight legs, whereas the insect has
only six, its head and thorax are fused, but in the case of insects
head and thorax are separate. There are many other, less evident,
points of difference as we shall see.
For the microscope, there are few better objects in animal land than
the feet of spiders. Their study will give us plenty of occupation for
they are modelled on various plans, according to the different kinds of
spider. Taking the common garden spider as our first example we shall
find that its foot is a most ingenious contrivance. Our microscope will
show us that the foot is armed with a pair of comb-like claws. A little
study of the habits of the spider will enlighten us concerning the
uses of these combs. At this point we may remark that the examination
of living creatures beneath the microscope should, whenever possible,
go hand in hand with a study of habits. Over and over again in our
microscopical investigations we shall come across structures which
appear to be useless as far as we can surmise. A careful observation
of the living owners of these puzzling structures will probably clear
up the whole matter. Well, let us watch a garden spider; if we do so
intelligently we shall see two uses of these combs and may guess the
third. The spider uses its combs as we do, to straighten its hair;
they also clean its body. It uses them to obtain a firm grasp of the
threads of which its web is composed and, though we cannot see this, so
quick are the movements of the creature, the combs serve a very useful
purpose in holding captured prey.
The garden spider and its relatives are distinguished by the fact that,
in addition to the two large comb-like claws, they possess a third
smaller claw and some toothed spines. The small claw and toothed spines
are movable and, when pressed against very firm grasp. With these
cleverly contrived feet she--it is always the lady spider who makes
the web and does all the work--hauls in the slack of the combs of the
larger claws afford their owner a her web and owing to their firm grasp
she can run readily over its meshes.
The house spider, which spins a web seemingly in a disordered tangle
and quite unlike the beautiful web of the garden spider, has feet of
a different pattern. The most interesting feature about the legs of
this creature is the wonderful double comb with which it teases out the
threads of its web as they are formed. This comb takes the form of a
double row of minute, curved spines on the last joint but one of the
hind legs; it must certainly be examined under our microscope and we
should try to see the combs being used by the spider.
We must also make a point of examining the feet of a wolf spider for
they are constructed on a different plan to those of the spiders we
have mentioned. Wolf spiders are the creatures which spin no proper web
but lurk in holes in walls or in the ground and dash out from their
hiding places to seize their prey. They usually line their lairs with
silk. We shall have more to say about wolf spiders in a moment.
The Zebra spider, which belongs to the family of jumping spiders, has
very curious feet, not so much on account of its claws as because of
the curious clubbed hairs which adorn them. This little spider is
black, with white stripes on body and legs; the peculiar habit, for
a spider, of leaping upon its remarkable hairs on its feet render it
exceedingly sure-footed and it has need to be, for it exhibits prey.
There are many other spiders which we may examine with the certainty
of finding some features of interest, the Drassid spiders which lurk
beneath bark and stones; the crab spiders usually brightly coloured
little fellows with the habit of living in flowers; the little
money-spinners and the harvest-men; these last are not true spiders but
they are none the less interesting, they are the small-bodied, very
long legged creatures which occasionally find their way into our houses.
Having taken our fill of the spiders’ feet we may well turn our
attention to their heads. If we have caught a spider in the act of
killing a struggling fly, it must have struck us that one bite from
the spider is sufficient to kill its victim. Let us see if we can find
the jaws which so quickly bring death even to large insects. We shall
require a steady hand and some little skill to examine them properly
but the task is not beyond our powers. Having killed our spider we
must snip off its head, place it on a slide and examine it with a low
magnification. Looking straight at the face, we can plainly see the
sharply pointed, hinged jaws; in nearly all spiders they work from side
to side and they can be closed on their hinges like pocket knives. With
a pair of mounted needles and two steady hands, let us dissect the head
of our spider, so that we obtain one of the jaws quite free from its
surroundings. At the base we shall find a little sac, the poison gland,
and if we now magnify the jaw much more highly we shall observe a tiny
hole very near the tip. When the spider has grasped her prey in her
jaws she causes the poison from the poison glands to pass into the body
of her victim, by way of the little hole in her jaw; the poison causes
paralysis and the victim struggles no more.
The only excuse we can make for spending so much time with the spiders
is that they are of the greatest interest to the microscopist.
Returning to our friend the garden spider we must examine the spinning
organs, known as spinnerets. These are to be found near the tip of
the abdomen on the under side. There are six pairs in all in the
garden spider but the middle pair are shorter than the others and, in
consequence, not easily seen. The tips of the fleshy little spinnerets
should be highly magnified and we shall notice that the structure of
the tips differs in each pair of spinnerets. On the foremost pair,
there is a fairly large projection and numerous small ones; on the
middle pair three large projections and many smaller ones, whilst on
the hind pair, in addition to the small projections there are five
large ones. The large projections are called spigots and the small ones
are known as spools; from the former is derived the strong silk of the
web, from the spools the fine threads issue.
On the underside of our garden spider there is a dark patch and, just
in front of this dark spot, are a pair of slits. These we must open up
very carefully, in a dead specimen of course, and within, if we have
succeeded in our dissection, we shall see from fifteen to twenty little
flaps resembling the leaves of a book, in fact they are known as lung
books and, by means of them, the spider breathes.
One more word and we must leave the spiders. The eyes must be examined
in every specimen. Most spiders have eight eyes, set like little gems
in the front part of the head; some have six eyes, some only two and
a few kinds are eyeless, but these last spend all their lives in dark
caves, so eyes would be useless to them. When we examine the eyes of
wolf spiders we shall observe that they are placed on the tops of
little projections so that their owners may better be enabled to see
all around them.
The hairs and scales of many spiders make beautiful objects for the
microscope. We must make a point of examining the hairs of the water
spider also the scales from the Zebra spider. The latter with their
feathery form and iridescent colouring, are particularly beautiful.
We may advantageously spend a moment or two in the examination of the
spider’s web and the threads of which it is made. The strands radiating
from the centre of the web differ from those which are arranged
spirally. The latter are covered with a sticky substance as may be seen
under the microscope. When these spiral threads are laid down by the
spider, the sticky substance covers their whole length in a thin film,
but the little architect adds a finishing touch, by pulling the thread
as a bowman pulls his bow and then releasing it suddenly. The result of
this performance is that the sticky substance forms a series of minute
globules over the whole length of the thread.
In order to be in a position thoroughly to master the details of
animal structure it is necessary to have acquired sufficient skill to
cut sections. They cannot, however, be cut so easily as is the case
with plant sections. The various parts of animals are either so hard,
_e.g._, bones and teeth, that they must be treated almost as pieces of
rock and rubbed down till they are transparent, or they are so soft
that they require soaking in various chemicals to make them harder and
even then it is usually necessary to imbed them, _i.e._, surround them
with some easily melted substance which sets moderately hard, such as
paraffin wax. Cutting sections of animal parts is beyond the average
amateur.
The feathers of birds make beautiful objects for the microscope. For
those microscopists who desire beauty of colour rather than details of
structure it is hard to beat the glorious shades of certain feathers
beneath the microscope. To obtain the best effect a fairly low
magnification should be used and all manner of lighting thrown upon
the object, for we have all seen the feather which appears drab at
one angle is of the greatest brilliance at another. Various feathers
and from various parts of birds should be examined, if we desire to
understand their structure. Each feather consists of a vast number
of cells but it is improbable that we shall be able to prove this
statement by the examination of any large feather. We must take a
down feather, notice carefully the arrangement of its various parts,
for it will be interesting to compare this soft, weak feather with a
comparatively strong flight feather from a wing. Now, under a higher
magnification, we can plainly see the little cells of which the down
feather is built up.
One of the strong wing feathers of such a bird as a pigeon is of the
greatest interest as a microscopic object. We must take a few of the
barbules, the slender, flattened portions of the feather which fringe
either side of the barb. A moderately high magnification will show how
ingeniously they are contrived. The hinder side of each barbule is a
moderately thick upwardly curved edge whilst, on the forward side,
there is a row of curved hooks. When the feather is neat and tidy, and
its owner when in good health usually sees to it that its feathers
are well kept, the hooks of one barbule engage with the curved edge
of the next barbule. The feather, by means of this ingenious locking
device, becomes much more nearly a solid structure than would be the
case if the barbules did not hook on to one another. The arrangement
for hooking together the fore and hind wings of bees and wasps is very
similar. We may examine a number of flight feathers but we shall not
find any very striking differences between those of various birds. All,
apparently, follow a common design.
From feathers to hair and from hair to horns and some scales is not a
very far cry. We have talked about the examination of hair in another
chapter, so we will not repeat ourselves here. Scales we shall most of
us have opportunities of examining in plenty. We have just mentioned
that some scales are comparable to hairs and feathers. Such scales
are to be found in snakes and lizards. The scales of fish are of a
different order but they are equally or even more interesting when
examined under the microscope.
If we live in a district where many and various fish are caught we
shall soon discover that their scales differ in a remarkable degree.
Some are of the texture of horn, some are gristly, some bony and some
covered with enamel, after the manner of teeth. Not only do they differ
in texture but in design as we shall see in a moment.
Certain fishes, the eel is one, the mackerel another, are said to be
scaleless. As a fact their scales are very thin and transparent and so
arranged that they are less evident than those of other fish. By taking
a little of the skin of one of these fish we can easily detach a few
scales for examination. Those of the eel we shall find are very thin
and delicate and quite transparent. These and all other fish-scales
may be made into permanent slides by mounting in Canada Balsam, as
described in our last chapter.
The carp, whiting, salmon, sprat, herring and many other fish have
scales called cycloid or circular; the term is rather a misnomer
because they are not truly circular, but the name is used to
distinguish them from other scales. The structure is easily made
out with a moderate magnification. Many of these scales, however,
exhibit portions more dense than the rest; these dense spots are
caused by little particles of lime which may be seen under a higher
magnification.
[Illustration:
1. A THORN INSECT.--A striking example of protective resemblance.
When resting on a thorny twig this little insect is safe from all its
enemies. 2. THE HEAD OF PALM WEEVIL.--The long snout distinguishes
the weavils from all other beetles. Its very long front legs are also
worthy of notice. 3. A LEAF INSECT.--Green in colour, this insect
bears a remarkable similarity to a leaf. Its sluggish habits heighten
the illusion. 4. THE HEAD OF STICK INSECT.--There are few more
curious insects than the stick insects. The specimen illustrated has
a very bird-like head.
]
Perch, pike, sole and some other fish have much more peculiar scales,
known as ctenoid or combed for the reason that their unattached, that
is to say their hinder margins, are toothed like a comb.
The scales of sharks, dog-fish and rays are called placoid for they
are toothed; not only so but their arrangement is frequently quite
dissimilar to the scales of ordinary fish. Taking the herring as our
example, but a salmon or any other fish would serve equally well, and
examining the arrangement of its scales with the help of our pocket
lens, we shall find that the scales are fixed to the fish by their
forward edges and that each scale partly overlaps its neighbour, as do
tiles on a roof. In the shark family, however, the scales are often
relatively wide apart, they do not overlap but are imbedded separately
in the skin. The scales of rays have each a hard spine projecting from
the centre, those of sharks and dog-fish have teeth, and they are teeth
not only in appearance but also in structure.
The ganoid scales of sturgeon we are hardly likely to meet with.
Sometimes these fish are on sale in London and other large towns and a
specimen of their scales may be procured. They are bony in structure
and, though interesting, require a considerable amount of preparation
to render them sufficiently transparent to be examined under the
microscope.
It is interesting to note that all the fossil fish which are
discovered from time to time have either ganoid or placoid scales,
a fact which shows that the sharks, rays and sturgeon are directly
descended from creatures which swam the seas thousands of years ago.
The shells of shell-fish are not easy to examine microscopically,
but frequently their plates may be detached from the edges of such
shells as oysters and mussels and these should be examined. If the
outer part of the shell be taken we can easily see its honeycomb
structure and, by adding a little acid and waiting till all action
has ceased, we shall have a structure remaining which is remarkably
like a number of plant cells. The inner layer of many of these shells
is composed of beautifully iridescent mother-of-pearl. Now such
iridescence is usually caused by surfaces furrowed with many very fine
lines and mother-of-pearl is no exception. Under the microscope, with
a moderately high magnification, we can see minute striations all
practically parallel to one another.
The cuttlefish is peculiar in having a skeleton which is a moderately
soft plate. These plates can often be found washed up by the tide,
may be cut out from a dead cuttlefish or bought from a chemist’s as
cuttlefish bone. However we secure the material we shall find that one
side of the “bone” is hollow and that across this hollow, delicate
plates run parallel to one another at intervals. Between these parallel
plates there appear to be a number of fibres but, if we cut a thin
slice of the structure and examine it under the microscope, we shall
see that the apparent fibres are really very thin plates of bone which
wind and double upon themselves in a beautiful manner. The structure of
these plates gives strength to the bone without adding to its weight.
Snails are sure to attract the microscopist sooner or later, so too are
slugs. Many of the latter have shells, small flat or ear-shaped shells,
quite different to the portable homes of snails. In many young snails,
which may be killed by dropping into boiling water, we can find the
shells so transparent that they form good objects for our microscope.
Sometimes they are composed of six-sided cells, sometimes of beautiful
star-shaped cells.
From the microscopist’s point of view the most interesting feature of
the snail is its rasping organ, often wrongly termed the tongue. To
find this organ it is necessary to open up the mouth of a dead snail,
and if we seek the assistance of our lens while doing so, we shall
have no difficulty in finding the rasp--it may be recognised by the
minute teeth with which it is furnished. The whole structure should be
carefully removed and mounted upon a slide. In some kinds of snails
there are but a hundred teeth, other kinds, however, possess as many
as twenty-six thousand eight hundred. The snail makes use of this
remarkable organ to procure its food. Vegetation is pressed against a
plate at the top of the creature’s mouth and literally filed into small
pieces by the rasping organ. Captive water snails may be watched while
using their rasp upon the water plants or upon the green slime which
soon accumulates in aquaria. The eggs of snails are easily found and
should be examined, the queer little inmates may be studied through the
transparent shells, in all stages of development.
The naturalist whose inclinations lead him towards the study of animal
life will find plenty to occupy his time and his microscope. All kinds
of eggs of small creatures may be watched as they develop. Frogs’
eggs are of interest in this respect, so too are tadpoles which hatch
from them. The whole blood circulation in a young tadpole may easily
be studied under the microscope, the structure of the external gills,
the gradual change to internal gills, the development of legs, the
absorption of the tail. The tadpole and its marvellous changes will
afford sufficient microscopic material to last for many weeks.
CHAPTER IX
THE STUDY OF THE ROCKS
The study of rocks and minerals by means of the microscope is apt
to be disappointing. In the first place, to study them seriously we
require a special microscope, the ordinary instrument, with which we
may poke into the deepest secrets of the animal and plant world cannot
translate for us half the story of the rocks. Again, to understand
rocks and minerals we must study them somewhat deeply. Geology, as
the science of rocks is called, is no more difficult than botany or
zoology, the sciences of plants and animals respectively. Botany and
zoology, however, appeal in some degree to nearly all of us; we may
learn a good deal concerning the structure of the cockroach with the
help of our microscope and be interested in the revelations of our
instrument, but to embark on a detailed course of the minute internal
anatomy of insects would appeal to few of us. Animal or plant life may
be studied piecemeal and enjoyed on account of its absorbing interest.
Geology must be studied from its very beginnings if we are really to
understand what we see beneath the microscope.
In the hand, a lump of rock, say of granite, may be of exceeding
beauty. The body of the rock is, perhaps, a delicate pink, scattered
here and there are the flat glittering plates of mica and brilliant
crystals of quartz. Other rocks, less common, vie with the rare gems
for beauty of colouring and lustre. As thin microscope sections these
once gorgeous specimens are colourless, dull and, unless we understand
them, uninteresting.
There are, however, many mineral substances which we may study with
advantage for, if our investigations do not take us very far towards
elucidating the story of the rocks, we shall at any rate discover
something that is new to us. We may well commence our studies with the
examination of ordinary sand. This is not a rock, you will probably
exclaim. You are right but one day it may be a rock, it all depends
upon circumstances.
Before we take out our microscope let us have a short talk about rocks
in general, then we may understand better where we are. Rocks of one
kind and another make up the crust of the earth, that is pretty obvious
anyway. Thousands and thousands of years ago, how many we are not
prepared to guess, this old earth of ours was a sphere of molten rock.
Needless to say it was far too hot for any plants or animals to dwell
upon it. Very, very gradually the outer crust cooled down and in time
it became sufficiently cool to support animal and vegetable life. Then
there were rivers and seas and then came, from time to time, rain and
wind and frost and even earthquakes. The earthquakes cracked the crust
of the earth, moisture entered the cracks and, when the frosts came,
pieces of rock were broken away, owing to the expansion of the moisture
in the cracks as it became converted into ice. The rain and wind helped
to carry the broken pieces of rock, ever downwards towards the sea,
but before the sea was reached the big boulders became broken up, by
their buffeting, into shingle and sand and mud. In the course of long
ages, longer than it is easy to imagine, these broken pieces of rock,
gathered together as we have seen from various districts, may have been
left high and dry, for the face of the earth has not always been as we
know it now.
In time all the little particles became welded together to form a new
kind of rock. Sometimes animals and plants were buried in the mud
destined to become a rock and their parts were so well preserved that
they may not only be recognised by present-day scientists, but in
many cases their structure may be made out so well with the aid of a
microscope that no one would guess they had been buried thousands of
years ago.
New rocks have been formed not only by the breaking up and welding
together of the original earth’s crust, but by animal and plant
remains. In some places, in past ages, billions and billions of little
shell-fish have lived in the waters, died there and their shells
have fallen to the bottom of their watery home. Now we know their
shell remains as chalk or if it has undergone great pressure, owing to
changes in the earth’s surface, as limestone. There are rock masses
also formed of the remains of countless Diatoms; sponge spicules, too,
have played their part in rock formation.
It is clear from what we have written that, in the first place, the
rocks of the earth form two great divisions, the rocks which formed
the original crust of the earth, Primary rocks they are called, and
the later-formed Secondary rocks. The Primary rocks are often glassy
in appearance, they show unmistakable signs of having once been molten
and they never contain animal or plant remains, fossils as they are
called, for the reason we have already explained. The secondary rocks
often betray their origin by occurring in layers, or strata to speak
more scientifically. Always when we think of the rocks we must think in
thousands of years, then it will be easier to understand the formation
of these strata, each one of which may represent the work of hundreds
and hundreds of years.
Now to return to our sand; we have explained briefly that sand is
really the remains of broken up rocks. First we have the solid rock,
boulders are broken from it as the boulders are acted upon by rain,
frost and wind: they become more and more broken up and at the same
time they are carried towards the sea where they take the form of
shingle. The shingle, in the course of time, becomes broken up into
sand, and the sand again becomes so finely divided that it forms mud.
From this it is clear that the composition of the sand depends largely
on the nature of the rocks in the neighbourhood.
[Illustration:
_By the courtesy of Messrs. F. Davidson & Co._
1. FORAMINIFERA
Small sea shells. The inmates die and the shells, falling to the
bottom of the sea, are gradually converted into chalk.
2. DIATOMS
Small water plants which make beautiful objects for the microscope
by reason of their remarkable shapes and sculpturing.
]
First of all, we must place a little of our sand upon a piece of black
paper and examine by reflected light. We shall soon notice that sand
is anything but the simple gritty looking substance it appears on the
sea shore. At least four different substances are certain to be seen;
if the sand be clean, we shall see bright glassy crystals of quartz;
angular, sharp-edged pieces of flint; minute, flat, glistening plates
of mica and the broken remains of shells. Should our sand be taken from
a spot in the vicinity of volcanic rocks, it will probably contain
opaque pieces of magnetite; we may not recognise these when we see
them, but they may be separated from the rest of the sand by means of
a magnet, to which they will adhere. Probably our sample of sand will
be dirty or stained with iron, in this event we may wash it with weak
acid, then after drying, we may examine by transmitted light and the
various crystals will show up well.
Let us take some clay as our next example. Clay is the substance from
which the rocks known as shale and slate have been formed. We cannot
see much of the structure of clay with our microscope so let us wash
it and, by so doing separate it into its components. We must put a
little clay into a tumbler of water and stir it vigorously for some
little time, then still stirring, if we pour off the muddy liquid into
another tumbler, we shall find that our old friend sand has settled to
the bottom. Clay then is merely mud and sand, but we must not throw
this sand away without examining it for, very frequently, beautiful
minute fossils are to be found amongst it. Little creatures dwelling in
the mud, become buried in it and, as more and more mud is formed above
them, and it becomes partly solidified in the form of clay, they become
fossilized.
If now we examine a piece of sandstone with our pocket lens we shall
find that it very closely resembles the sand we have already studied.
It may be so soft that we can break it up in our fingers, then if we
examine the powdered sandstone beneath the microscope we shall need
to be experts to tell whether we are examining sand or sandstone.
Should our rock be too hard to break up in our fingers the addition
of a little weak acid will reduce it to sand. We see now that a
large proportion of sand and a small proportion of mud result in the
formation of sandstone, whereas if the proportions are reversed and we
have more mud than sand the resulting rock is known as shale or slate.
In shale we very frequently find fossils, in sandstone rarely, for the
reason that structures buried in the mud, destined to become shale,
are protected from the atmosphere and the action of water; sandstone,
on the other hand, is porous, moisture trickles through it and any
delicate structures which may have been buried in it are more likely
to decompose than to become fossilized.
Limestone is easy to obtain and may occupy us for a few moments. We do
not wish to go into technical details, but we may say that the word
limestone includes a number of rocks which differ largely in appearance
and to a considerable extent in composition. It would, perhaps, be
more correct to say that there are several kinds of limestone. Some
kinds are made up almost entirely of shells and very interesting they
are as microscopic objects. One may wonder how a geologist can state
with certainty that some spot, may be many miles from the sea, was
once covered by salt water. One of these shell-formed limestones may
give him the information; he knows that certain shells comprising the
rock must have belonged to marine animals, he knows too that whole
mountains, which these rocks sometimes form, are not carried bodily on
to dry land, so the obvious inference is that the rock was formed below
the sea.
The softer limestones may easily be crumbled and powdered for
examination under the microscope; the harder kinds should be treated
with acid. When acid is added to limestone a considerable effervescence
takes place, for the acid decomposes a substance known as calcium
carbonate, which the limestone contains, and bubbles of gas are given
off. When the limestone has ceased to effervesce the portions of the
rock which remain, may be carefully washed in water, dried and examined
under the microscope. We shall find that the substance we examine
consists of sand containing a goodly number of plates of mica and there
may also be a number of sponge spicules. We must bear in mind when we
are examining the remnants of limestone after treatment with acid, that
shells are largely composed of calcium carbonate, so it is useless to
look for any shell remains, for the acid will have dissolved them.
Calcium carbonate, in a nearly pure state, will and does form rock-like
structures; we are most of us familiar with stalactites which are
formed on the roofs of caves. These structures are usually composed of
calcium carbonate though sometimes, notably in some of the Derbyshire
caves, Barium takes the place of Calcium. Should we have the chance
of examining a section of a stalactite we should certainly do so. We
can see the rings which are formed, as layer after layer of calcium
carbonate is deposited, in fact the section of a stalactite bears a
striking resemblance to a stem of a tree and has, before now, been
mistaken for a fossil stem.
If the study of rocks appeals to us we should make a point of examining
all the specimens we can lay hands upon. Many quite common specimens
may easily be obtained; rock-salt, for example, though in itself not of
great interest as an object for the microscope, will readily dissolve
in water, leaving behind an insoluble residue of iron which is well
worth examination. The majority of rocks, however, are not affected by
water and but little by acids. With such specimens the only course
open to the microscopist is to prepare sections.
The making of rock sections is certainly different to the cutting of
plant or animal sections. It is a laborious business as the enthusiast
will find to his cost. The professional makers of rock sections have
special grindstones or lathes for the purpose and, even so, the process
is not rapid. The amateur must needs do all his preparation by hand.
The requirements, in addition to the piece of rock of which we require
a section, are a small square of plate glass, some Canada Balsam,
emery powder of various grades and an unlimited stock of patience. If
possible we choose a piece of rock with one side as nearly as possible
flat; this is merely to save labour; the piece of rock should be
roughly about half-an-inch square. As a start we rub the flattest side
in a mixture, practically a paste, of coarse emery powder and water.
As a matter of fact, we may keep the piece of rock in our pocket and
grind it when occasion offers on a flat stone wall or on any surface
that will assist in producing a flat surface. When we have ground our
surface flat and smooth, we finish it off with fine emery powder and
may then polish it with jeweller’s rouge. So much for the first side
and, if we do not cry enough at this period, we may proceed to the
grinding of the other side. Taking our slab of plate glass, we fasten
the polished side of our piece of rock to it by means of Canada Balsam,
then we may rest for a few days while the Balsam sets. As soon as the
rock is firmly fixed to the glass we proceed as before, but in this
case the final grinding and polishing must be very carefully carried
out. Towards the end of the operation our original lump of rock will be
reduced to the thinness of a cover slip and to its fragility. Having
given the finishing touches with jeweller’s rouge, we put the glass,
with its attached rock section into a bottle of xylol (to be obtained
from any chemist). The xylol dissolves the Canada Balsam and the rock
section falls from its support. A further washing in clean xylol should
be given and then the section is ready for mounting, which is best done
in Canada Balsam. The section may be carefully fixed to the slide with
a drop of Balsam or it may be covered with a cover slip, in the usual
manner.
In our concluding chapter we give hints on slide making and addresses
of firms who supply slides. Our advice in that chapter is to prepare
one’s own slides where possible; in the case of rock sections, however,
we must change our advice, only the microscopist of unlimited leisure
can find time to make his own slides.
One of the most interesting branches of rock study deals with the
fossil remains of plants and animals. Fossils are interesting in
themselves: they are doubly interesting because they tell us more than
we could ever have discovered without them, concerning the living forms
which inhabited the earth at different periods. Geologists know the
order in which the various secondary rocks were formed and by studying
the fossils of these various rock formations, from the earliest to the
latest, can tell which animals and plants have been longest upon the
earth. Some of the living forms of to-day have existed from very early
times; we know, for example, that cockroaches were upon the earth long
ages ago, for their fossil remains are found in the very early rocks.
For the most part the fossils of animal forms do not make good objects
for the microscope. The _Foraminifera_, minute creatures dwelling in
shells, are the most suitable for microscopic examination and very
beautiful some of them are. They are best examined by reflected light,
for then their little shells show their delicate pearly sheen.
Sponge spicules, as may be guessed from their hardness, are common in
the fossil state; there are also fossils of sea-anemone ancestors and
fossils also of _Hydra_ colonies. The last named, known as graptolites,
look not unlike lead-pencil marks on the rock. They exist in many
forms; _Diplograptus_ has a stem with two rows of cups in which the
little creatures lived long ages ago; _Monograptus_ has but one row
of cups upon its stem, whilst _Didymograptus_ is a branched form.
These fossils are by no means rare and are worth studying under the
microscope.
There are curious fossils knows as _Trilobites_, not unlike the
present-day wood lice; they too may be studied, for some of them show
all their characteristic markings as plainly as they must have appeared
on the living animals.
In the plant world, many fossils in amazingly, good states of
preservation have been found. Some of the giant Club Mosses from the
coal measures, exhibit all the characteristic stem markings, leaf
scars and the like, so clearly that one might imagine one examined a
living specimen. Most of the plant remains are beyond the reach of
the amateur, but many of them may be viewed in museums, in different
parts of the country and the microscopist, whether he be a student of
rocks or a lover of plants, should make a point of examining them. From
quite fragmentary remains, scientists have been able to conjecture what
vegetation covered the earth at various ages. The present is the age
of flowering plants, but long ago the world was green with giant Club
Mosses and Horsetails, very humble plants at the present time.
CHAPTER X
THE MICROSCOPE AS DETECTIVE
It is an unfortunate fact that our food is not always absolutely pure.
It may be contaminated with foreign matter either by accident or by
design. However careful the manufacturer may be in, say the preparation
of cocoa, some dust, some waste vegetable matter, perhaps even a few
stray dried insects may occur as impurities. They are out of place
certainly but, at the worst, they are a sign of lack of care on the
part of the manufacturer. There is another, more serious side to the
question of food adulteration, where the foreign matter is added
purposely, either because it is cheap, because it weighs heavily,
imparts a pleasing colour or an agreeable aroma. Such adulteration is a
form of fraud and the microscope is an invaluable aid in its detection.
In many respects the microscope is a better informant than the tests
of the chemists; in some cases, however, it merely supplements and
confirms the chemical results. Let us consider, for a moment, the
advantages possessed by the microscope and also where chemistry scores.
Very frequently the results of costly law cases hang on the reports of
expert food examiners; every care, therefore, must be taken to avoid
error. This being the case, whenever possible, chemical tests should
be carried out to confirm the results of microscopic examination. When
both microscopist and chemist come to the same conclusion, there is
not likely to be any mistake. There are tests which the microscope
cannot perform, there are some, also, which are beyond the powers
of the chemist and many which are very difficult for him. A drop of
milk, for example, examined under the microscope shows a number of fat
globules floating in a watery liquid. However clever the microscopist
and however accurate his instrument, he cannot tell if there is an
excessive quantity of water, yet a simple chemical test will answer
the question. This is a case in which the microscope is of little use,
although it is only fair to add that microscopic examination would
reveal the presence of blood, hair and dirt, to mention three common
impurities, which the chemist in his test for watered milk would
quite overlook. With a little care and the use of suitable stains,
any bacteria which might be present would also show plainly under a
powerful microscope.
Now for an example or two where the microscopist has the advantage
of the chemist. Some jam makers have been known to be sufficiently
unscrupulous to sell “raspberry” jam contaminated with a large
percentage of some cheaper fruit, such as gooseberry. The seeds of the
two fruits differ so markedly that it is really not necessary to employ
a microscope to discover the fraud, but a case is on record where
wooden seeds were used, so like the true seeds of the raspberry, that
a very careful examination was necessary to show what had happened.
In our chapter on the Microscope in Agriculture we have referred to
this point in greater detail. Starch of various kinds is a very common
food adulterant and the experienced microscopist can estimate almost
precisely, the proportions of different starches in a mixture, a
feat which would sorely puzzle the chemist. So in certain cases the
microscope is indispensable.
Briefly the microscope is a time saver; chemical tests occupy a
considerable time; microscopic examination is quick, the experienced
microscopist at once recognises what he observes. Very small quantities
can be examined under the microscope, relatively large quantities are
required for chemical tests. Again, if only a small quantity of the
material is available for examination and it is necessary to carry out
chemical tests, they can be performed under the microscope and this
point is considered in another chapter.
We have mentioned that starch of various kinds is a common adulterant
of many foods and the budding food analyst might do worse than learn
to recognise the various starch grains under the microscope. They
are easily obtained and as easily observed. Each variety of starch
has grains which are remarkably constant in their characteristics. A
beginning may profitably be made with potato starch, for its grains
are large and they possess certain well-marked features, which may or
may not be present in the grains of other starches. By scraping the
newly cut surface of a potato we can obtain thousands of starch grains.
The surface of the potato must not be grated, just a gentle scraping
with a pocket knife and a mere speck of the cloudy liquid that is
obtained, added to a drop of clean water on our slide, will suffice.
Cover the object with a cover glass and examine under a fairly high
magnification. There are countless, oval, almost transparent bodies
in our field of view, they are potato starch grains. Each one, as we
shall see when we make a more careful examination, is not unlike a
miniature oyster-shell. In the shell, there is a point which is its
oldest part and the remainder has grown, layer by layer, round that
point till the shell is fully formed. Now we magnify the starch grains
as highly as possible and slowly rotate the fine adjustment to and fro,
for the reason that the object is not flat and by doing so, we obtain
all its parts in focus in turn. If the illumination is not too intense,
we shall notice a minute dark dot corresponding to the oldest part of
the oyster shell; it is, in fact, the oldest part of the starch grain.
Around this point we can see as we focus up and down, ring after ring
where the grain has grown larger and larger. The dark spot is called
the hilum and the rings are known as striations. In the potato starch
grain the hilum is not central and the striations are not circular.
Wheat has large and almost round grains without a hilum or striations,
those of Barley are very similar but smaller and not so uniformly
round. Rye grains are frequently cracked and often have ragged edges.
A very large number of these objects may be examined, for it is useful
to know their structure if one’s object be to examine various foods;
from the point of view of beauty, when examined with a polariscope,
they have few rivals. Maize starch, which is to be found in most houses
under the name of corn flour consists of two kinds of grain. Some
are many sided and angular, all of one size and without striations,
they are also split at the centre; the other grains are rounded, of
various sizes and are never like the angular grains grouped together.
The former come from the horny part of the maize, the latter from the
floury portion.
Rice starch is also many sided and angular, almost crystal like; there
are, however, never any rounded forms and this serves to distinguish
it from maize starch. The shape of Arrowroot starch grains varies
according to the plant from which it is derived, for this substance
does not all come from one kind of plant but, whether the grains be
pear shaped, hammer shaped, triangular or dumbell shaped they all
show striations and an x-shaped split in place of a hilum. Tapioca
starch grains are usually grouped together in twos or threes; when
they rest on their flat surfaces they appear circular and each hilum
is surrounded by a dark ring, when on their sides they are seen to be
sugar-loaf shaped.
Many more starches can be found without going far afield, Sago, Peas,
Beans, Lentils and Bananas are a few common commodities containing
starch. An effort should be made to study the very curious dumbell
shaped starch grains of the Spurge and its relations. All these plants
contain a white milky juice in which the starch grains float; by
squeezing a little of this milky fluid into a drop of water on a clean
slide the grains can easily be observed.
It is sometimes difficult to observe starch grains till a fair amount
of experience has been gained in the use of the microscope. Should
this difficulty arise, it may be overcome by adding a drop of a weak
solution of iodine. This will stain the starch grains a deep blue
colour and render them very easy of observation. The iodine solution
must be weak, however, or the staining will be excessive and the
objects rendered black and non-transparent.
Having examined many or all of the specimens we have mentioned let us
turn our attention to some of the common foods, and learn some of the
methods used in testing for impurities. Ordinary household bread, it
is hardly necessary to state, is rich in starch and, by trying the
iodine test, mentioned above its presence is easily shown. With a
weak solution the deeper the blue colour produced, the greater the
quantity of starch. Some parts of the bread will be stained yellow,
this indicates the presence of another nourishing component of bread.
Certain kinds of bread are supposed to contain no free starch, because
this substance is not beneficial to some people. Iodine again will
reveal whether the bread is as it is described, for, if there be
no free starch there will be no blue colouration. Brown bread will
show much more of the yellow colouration and less of the blue than
white bread. Good, well-baked bread should keep for a considerable
period without turning sour; we can easily see whether our sample is
satisfactory by running a drop of litmus on to it and watching the
effect under the microscope; if the litmus remains unchanged in colour
the sample is not sour; if, on the other hand, the litmus turns red it
shows us that acid is present and that our bread is not as it should be.
Tea is difficult to prepare for microscopic examination and most of
the tests call for expert knowledge, not only in the management of the
microscope but of the plant itself. The structure of the leaves can
be made out clearly in specimens which have been soaked for a time in
water, but this is of little interest to the ordinary microscopist.
One very pretty test may, however, easily be performed. We all know
that it is not good to drink tea which has been standing for a long
time. Some tea-drinkers are so particular that they cannot bear to see
the teapot shaken before they have poured out their cup. All this
trouble arises because tea contains a poison called “theine”; it is an
alkaloid, one of a large class of chemical substances which are nearly
all deadly poisons--cocaine and nicotine are alkaloids. Although theine
is poisonous, tea which contained none of this substance would be
tasteless and the absence of this substance shows that the tea leaves
have been badly prepared. Tea after being gathered should be dried at
once, sometimes it is re-dried and this process drives off the theine.
For our test we require, in addition to our microscope, two watch
glasses, a piece of copper wire gauze and a gas burner or a spirit
lamp. Place a little tea in one of the watch glasses and cover with
the other watch glass; then heat gently on the wire gauze. In a few
minutes drops of moisture will appear on the upper watch glass; after
about ten minutes’ heating beautiful, long, needle-shaped crystals
will begin to appear, with a little further heating we shall obtain a
good crop of lovely crystals on the upper watch glass and they make a
splendid object for examination under a low magnification. The crystals
are of theine, the poisonous component of tea, and the test is used
to discover whether the tea has been redried during its preparation;
redried tea gives no crystals.
[Illustration:
_Photos by Flatters & Garnett_
THE STINGING HAIRS OF A NETTLE
These hairs are much longer than ordinary plant hairs. Sharply
pointed at one end, there are sacs at their bases containing acid.
BUTTERFLY WING SCALES
Scales from the wing of a butterfly. Each scale is a hollow sac,
affixed by its notched end to a pit in the insect’s wing.
]
The examination of cocoa for impurities is a matter rather for the
chemist than for the microscopist. It contains a vast number of starch
grains, not unlike those of rice, except that they are rounded. Coffee
often contains a number of impurities, the chief being chicory,
various starches, ground acorns and date stones. Chicory is really an
impurity, though it is one often asked for by coffee-drinkers. It is
easy to detect the amount of chicory present in a sample of ground
coffee, by throwing a little of the mixture on to water. The chicory
sinks at once, whereas the coffee floats for a while because it is
oily. In pure coffee there should be no starch and the iodine test will
readily show whether we are dealing with a sample free from starch or
not.
Mustard is very rarely purposely mixed with any impurities, in fact it
is probably the least likely to be adulterated of any article of food.
Under the microscope a large number of small objects, very similar to
starch grains, can be seen. They are the cells containing mustard oil
and they are not stained blue by iodine. A specimen of pure mustard
contains no starch. Pepper is by no means easy to test for impurities.
It contains minute starch grains, which can be recognised under the
microscope after staining. It is mentioned here because of a very
interesting and easily performed experiment that will appeal to every
microscopist. Place a little pepper on a clean slide and moisten it
with a drop of alcohol, allow it to stand for a minute or so then add
a little dilute glycerine, cover the specimen with a cover glass and
examine it under the microscope after the lapse of about five minutes.
The sight of wonderful prismatic crystals forming one by one in rapid
succession will be ample reward for the trouble taken. A drop of
strong nitric acid, which must not be allowed to come in contact with
any part of the microscope or with one’s hands or clothes, will turn
the crystals a rich orange colour. The crystals are composed of a
substance called piperine.
Everyone knows of the importance of pure water for drinking purposes
but the word pure in this case is used in a very wide sense, for the
only really pure water is distilled water and it would not form a very
good beverage. Although ordinary tap water may contain a number of
impurities it is not easy to see them without taking a little trouble.
If our tap water is so contaminated that a drop, examined at random,
shows us all manner of solid matter floating in the water there must be
something seriously wrong. Those who are engaged in testing water under
the microscope, first take a big jug full of the water and allow it to
stand for twenty-four hours, covering it the while to keep out dust. At
the end of this time most of the water is carefully drawn off, from the
top, with a siphon and the remainder, after stirring is poured into a
conical glass and allowed to stand for a further twelve hours. Then the
upper portion of the water is again siphoned off and a little of the
remainder, which is left in the point of the conical glass is drawn by
suction into a special kind of glass tube, called a pipette. This final
sample contains all the solid matter which settles to the bottom of the
water after standing for thirty-six hours.
The impurities likely to be present in water make such a formidable
list that we can only mention a small number. There may be various
mineral substances, such as lime, sand or clay; vegetable substances,
starch grains, fragments of wood, pieces of decayed leaves and the
like or there may be hair, scales of fish, etc. The impurities may
be living plants, of which the most likely to be found are bacteria,
diatoms, desmids and Volvox, amongst plants, and rotifers, Vorticella,
Paramœcium, Amœba, also portions of tapeworms and their eggs amongst
animals. These creatures are all described elsewhere so we need not
dwell on their peculiarities here.
In addition to all of the above and many more not mentioned there are
four metals commonly found in impure water, either in small solid
particles, or in the form of one of their compounds soluble in water.
The metals are iron, copper, zinc and lead. Very simple chemical tests
will show whether they are present or not. To a drop of the water add a
very minute quantity of hydrochloric acid and of potassium ferrocyanide
solution. When iron is present the solution will turn blue; in the
presence of copper it will become bronze coloured, whilst zinc turns
it a milky white. To detect lead, take another drop of water and add a
very small quantity of potassium chromate solution. If the suspected
impurity is present the solution turns yellow. All the chemicals for
these simple tests may be obtained at any chemists and there is this
great advantage in testing under the microscope--only very small
quantities are required.
Butter can hardly be described as an interesting object for the
microscope, nevertheless, it may be of use to explain the methods of
its examination. A small sample should be placed upon a clean slide, a
drop of olive oil added and the whole covered with a cover slip which
may be pressed firmly till the specimen forms but a thin layer. Of
course the most important impurities likely to be present are bacteria
but these we cannot see without special preparation and we are not
dealing with bacteria in the present chapter.
If our specimen is in a film sufficiently thin to be transparent, and
we should have made it so by pressure on the cover slip, we may first
of all examine it carefully for starch grains which, by the way, should
be absent. The amount and size of the drops of water, which every
butter sample contains, are of importance in deciding its quality.
In good butter there are a few scattered drops of various sizes; in
milk-blended butter the water globules are all very small and all
of the same size, or as nearly so as the eye can judge; in butter
containing an excessive amount of water the drops appear large, much
larger than in a normal sample.
If we examine various samples, we shall find that some are much more
transparent than others, the transparent samples being fresh butter.
The curd also in fresh samples is much more finely and evenly scattered
in the field of view than in older samples. Renovated butter, that
is to say rancid butter which has been melted and made palatable by
forcing steam through it, should be examined by oblique light--easily
arranged by tilting the mirror at an angle--when the curd appears as
white patches on a dark background.
It is curious that one article of food, honey, is more likely to be
pure when it contains impurities. This sounds like a bull but a great
deal of honey is manufactured from various sugars but not by bees. This
artificial honey contains no pollen grains, in fact any honey found to
be free of pollen should be looked upon with suspicion. Starch often
occurs in artificial honey, never in real bee-made honey.
To many foods adulterants are added as preservatives, the nature and
quantity of such additions is settled by Act of Parliament. Many foods
are preserved with small quantities of Borax or Boric Acid. The use of
Formaldehyde, formerly sold under the German trade name of Formalin,
is not unknown but it is very injurious. Salicylic Acid which was
formerly much used is being supplanted by Benzoic Acid, for the reason
that the latter is not so easily detected and therefore prosecution for
excessive quantities is not so likely to follow. These preservatives
are not easily detected by the microscopist unless he be a chemist also.
As we have already remarked adulterants are added for the sake of
colour, either because the public demand certain colours, or to hide
fraud. Milk for instance, when watered, assumes a characteristic
blue colour; to hide the blue shade various dyes, anetto, turmeric or
carrot juice are added or one of the aniline dyes, products of coal
tar. This form of deception became so common that now the public demand
yellow milk and butter. Jams made from inferior or over-ripe fruit
are frequently coloured with coal tar dyes, so also are cheap sweets.
Oxide of iron is added to potted meats, sauces, etc., to improve
their appearance. Bottled peas, which if untreated would be of a
yellowish-green colour, are made to appear bright green by the addition
of the poisonous blue vitriol; fortunately this chemical unites with
the chlorophyll of the peas to form a compound which is insoluble
in the human body and so no great harm is done. We may compare the
chlorophyll in a healthy, undoctored green pea with that in a pea which
has been treated with blue vitriol; under the microscope we shall
notice the striking difference between the two.
When we admire the beautiful crystals which go to the making of a piece
of lump sugar we little dream that, if those crystals were pure they
would be yellow. The housewife, however, demands her lump sugar white
so ultramarine is added to mask the yellow colour and give the sugar
its white appearance.
Most of these impurities are difficult or impossible of detection
under the microscope; they are added to give the food a more pleasing
appearance in the first place, for there are undoubtedly certain
people who prefer to consume food which appeals to the eye, though of
doubtful purity, rather than unadulterated though perfectly pure fare.
When added solely for the sake of appearance it matters little, but the
habit of making these additions is frequently cultivated to hide bad
material and imperfections in manufacture.
Sometimes adulterants find their way by accident into our food. A good
many years ago numbers of people were poisoned by drinking beer, in
some cases with fatal results. Tests were made and the beer was found
to contain arsenic but how it got there remained a mystery. At length
the glucose, a kind of sugar used in making beer and added also to a
good many of our foods, was found to contain the substance. Now in
the making of glucose, sulphuric acid is used and in this particular
case the impure or commercial acid had been taken. This impure acid
frequently contains arsenic and, in the case we mention, the results of
its use were disastrous.
CHAPTER XI
BACTERIA
There is probably no scientific work more wedded to the microscope than
the study of bacteria. We may learn a great deal about birds or insects
or rocks or minerals, without any instrument but we can learn little of
the bacteria unless they are highly magnified.
There is such an extraordinary amount of misconception concerning
bacteria that, it will be time well spent if we attempt to clear up
all misunderstanding at the start. Bacteria, often called microbes
or germs, are looked upon with considerable awe by most people, who
associate them in some vague way with disease. There is no denying
that many bacteria are responsible for certain diseases; many more are
perfectly harmless and a goodly number are exceedingly useful.
To enumerate all the bacterial activities would require a large book
but briefly, apart from the disease-causing bacteria, they enter into
the manufacture of cheese and butter, of wine and vinegar; they are
essential to brewing and tanning; they act as scavengers over the face
of the earth, breaking up a mass of decaying animal and vegetable
matter into simple chemical substances which can then be used again as
food for plants; some of them also can take a gas called nitrogen, from
the air, and pass it on to green plants.
What are these active little creatures? The question is a natural one.
They are merely very minute, one-celled plants. Each one possesses
a firm cell-wall, filled with living matter; in an earlier chapter,
we described the one called protean animalcule and, although it was
composed of but a single cell it had no definite wall. This is one of
the essential differences between plants and animals, both of them are
made up of one or more, maybe millions of cells, but each plant cell
is surrounded by a well-defined wall, animal cells have no such walls.
The exact position of the bacteria in the plant world is still open to
doubt. Most scientists place them amongst the fungi; for, with very few
exceptions they possess no chlorophyll. One or two of them, however,
do possess a green colouring matter which, if not chlorophyll is very
near to it; on this account other scientists are of the opinion that
they are related to the seaweeds. It is a matter, however, that does
not concern us very deeply, for our purpose it is sufficient to know
that they are plants. When they were discovered, nearly three hundred
and twenty-five years ago, they were looked upon as minute animals
and it is curious that the belief has survived this long period of
time in the popular mind. Long before the activities of bacteria were
connected with various phenomena, such as infectious diseases, souring
of milk, etc., it was thought that these changes were brought about by
chemical action. Like many of the early theories, this one contained a
half truth, for a great many of the changes brought about by bacteria
are really due to chemical action initiated by the organisms. In other
words, the bacteria set free certain substances which actually cause
the changes to take place.
Let us make our statement clear by a simple experiment. To a little
fresh milk we add a weak acid, the milk curdles at once and by dipping
a piece of litmus paper (obtained at any chemist’s) into the mixture,
it will turn red, showing the presence of acid. Litmus, by the way, is
obtained from a lichen; in the presence of acid it is red, an alkali,
the opposite of an acid, turns it blue. In a neutral solution, that is
to say one that is neither acid nor alkaline, litmus is of a purplish
hue.
To continue our experiment, we allow another sample of the same milk
to stand for a day or two in a warm, dark place and again the milk
will be curdled. A test with the litmus will show that the solution is
acid. The bacteria themselves have not curdled the milk but they have
liberated a substance, called a ferment, which has split up part of
the milk into an acid, amongst other things and that acid has actually
done the curdling. For this reason, weak alkalies are sometimes added
to milk. Acids and alkalies, of equal strength form neutral solutions,
so that, when the milk bacteria begin their activities which result
in the formation of acid, it is at once made neutral by the alkali.
By this means, curdling is postponed for a little while, though there
comes a time, of course, when all the alkali is used up, then the acid
gains the upper hand and curdling takes place. We could if we wished
continue adding more and more alkali to keep pace with the formation of
acid, but too much alkali would be as unpalatable as too much acid, so
nothing would be gained.
Before we bring out our microscope to examine these lowly plants, we
will first of all kill a myth which has survived, in the non-scientific
mind, since the eighteenth century and then describe briefly the life
history of a typical bacterium. Now for the myth. Bacteria are so
minute, their activities so great and the results of their activities
so far reaching, that it is hardly surprising to learn that, even at
the present day, bacteria are supposed simply “to happen.” We talk of
thunder turning milk sour, but thunder can no more sour milk than can
a passing train or an air raid or a burst in a water main. True, milk
turns sour more quickly in thundery weather than in frosty weather,
because, when thunder threatens, the air is warm and the milk-souring
bacteria increase more rapidly in warm weather than in cold. We must
remember always that bacteria are living beings and in common with all
other living things they must have parents. What probably took place
at the beginning of the world we cannot consider here but one thing is
certain that, at the present day, no living matter is produced from
non-living matter; “life from life” is the only theory that will stand
scientific tests and this has been the case ever since the simplest
microscopes were thought of and thousands of years before that. Any
substance, however liable to decay, if rendered germ free and kept germ
free, will retain its fresh condition indefinitely. Could bacteria
or germs, call them what you will, simply happen it would be useless
attempting to fight against them.
Bacteria are everywhere. In the water we drink, in the milk, butter,
cheese and in dust. We cannot avoid them, try as we will; it is
fortunate, therefore, that the majority are harmless. You may be
surprised that, with this ubiquity, you have never seen one. When,
however, you learn that most of them are only about twenty-five
thousandths of an inch long and that a thousand million of them could
be packed comfortably into a little box, whose sides measured but a
twenty-fifth of an inch in length, it is not really so surprising
after all. Being so small, the activities of a single bacterium are
insignificant; that “union is strength” was never better exemplified
than amongst these lowly plants. There are no male and female bacteria,
in the majority of cases they increase by splitting, in fact they
are often called splitting plants. The change may be watched under
the microscope. The plant elongates somewhat, it becomes narrower
and narrower in the middle, it develops a waist in fact; finally the
two halves part company and each one leads a separate existence as
a bacterium. This splitting progresses at an extraordinary rate. A
celebrated scientist once wrote: “Let us assume that a microbe divides
into two within an hour, these two into four in the next hour, these
again into eight in the third hour and so on. The number of microbes
thus produced in 24 hours would exceed 16-1/2 millions; in two
days they would increase to 47 trillions, and in a week the number
expressing them would be made up of 51 figures. At the end of a day (24
hours) the microbes descended from a single individual would occupy one
fortieth of a hollow cube with edges one twenty-fifth of an inch long,
but at the end of the following day would fill a space of twenty-seven
cubic inches, and in less than five days their volume would equal
that of the ocean.” It is hardly necessary to add that these alarming
figures represent what would happen if no accident befell the bacteria,
they show the enormous vitality possessed by the smallest of all
plants. Even allowing for misadventure their increase is alarming;
actual tests, with a sample of milk containing originally 153,000
bacteria per cubic inch, show that the cubic inch contained after one
hour, 539,750; after two hours, 616,250; after seven hours, 1,020,000;
after nine hours 2,040,000 and after 25 hours 85,000,000 individuals.
The writer whom we have just quoted calculated that a single bacterium
weighs about 0.000,000,000,024,243,672 of a grain, that forty thousand
millions weigh one grain and that two hundred and eighty-nine billions
weigh a pound. The descendants of one bacterium weigh 1/2666 of a
grain, after twenty-four hours; more than a pound after two days, and
sixteen and a half million pounds after three days. The assumption in
this case, also, is that no harm comes to any of them; the mortality
amongst bacteria is, clearly, very great.
Sometimes, owing to external conditions, such as lack of food certain
bacteria produce spores. The power of spore formation is not possessed
by all bacteria and those which are able to bring it about are
difficult to kill for the spores, which contain the living material
of the bacterium are surrounded with walls which will resist boiling,
drying, freezing and all manner of ill treatment. The spore formation
of bacteria is very simple, all or part of the living contents of the
bacterium becomes surrounded by a tough wall and remains so surrounded
till circumstances are favourable, when the wall bursts, its contents
escapes and becomes a bacterium, capable of founding a new colony by
the method of splitting we have already described.
Now let us try to find out what sort of plants we are to look for, when
we are searching for bacteria, under our microscope. They exist in many
forms, to which special names have been applied, and it is unfortunate
that, very often, their external form varies according to their state,
thus a bacterium may be spherical when young and rod shaped when older.
Some bacteria are spherical and are known as _Cocci_ or _Micrococci_,
from Greek words meaning a berry or a little berry respectively;
sometimes these spherical bacteria occur in pairs, then they are called
_Diplococci_ (double berries); or in chains, _Streptococci_ (chain
berries); or in bunches, _Staphylococci_ (grape berries). They may
resemble short rods, when they are called _Bacteria_, a name, by the
way, which is also applied generally to all microbes; they may, on the
other hand, have the appearance of longer rods and then they are called
_Bacilli_. Some of these longer rods may be curved or even corkscrew
shaped when they are known by the name of _Spirilla_. Rather fearsome
names some of these we fear and we wished to avoid long names, but they
appear over and over again in books and papers relating to bacteria so
we are compelled to introduce them to our pages. Many bacteria possess
no power of movement, others swim rapidly, by the aid of the lashing
movement of little whip-like structures with which they are furnished.
After all this preamble, which we hope has cleared up certain
misconceptions regarding bacteria and has given the reader some insight
into their habits, we may proceed to the examination of some of the
plants themselves. At the outset we have a confession to make. Bacteria
can only be studied seriously, by those who possess very expensive
and elaborate apparatus; considerable technical skill is required
to prepare the plants for examination--many of them indeed can only
be seen after they have been stained and lastly, to trifle with the
disease-causing members of the family may lead to dangerous if not
fatal results.
Having issued our warning let us see what we can do in the way of
microscopic investigation. The easiest subject with which to make a
start is the Hay Bacillus, _Bacillus Subtilis_, not because it is the
largest of the bacteria by any means, but because it is very easily
obtained. Each plant measures about five thousandths of an inch in
length, so we shall require a high magnification to examine it. Having
obtained a small quantity of hay, we must boil it in water for about
three-quarters of an hour and then set it aside for some hours. In due
course the water will contain hundreds upon hundreds of bacteria or,
speaking more correctly, of bacilli. For our work, we shall require a
special kind of microscope slide; instead of the piece of plain glass
we have been accustomed to use we must obtain one with a circular
portion, hollowed out from the centre. Having done so, we take a clean
glass rod and, with it, transfer a drop of the water, containing the
bacilli, to the centre of a clean coverslip. Invert the coverslip so
that the drop is on the lower surface and place it over the hollow
portion of the slide, in such a manner that the drop still remains
suspended from the coverslip; this is known as the hanging-drop method
and requires some little skill to accomplish satisfactorily. When
our slide is prepared, with a magnification of at least one thousand
diameters, we may reasonably hope that our trouble will be rewarded.
At first we shall probably see nothing. We recall that we had some
difficulty in examining starch grains, on account of the fact that they
were colourless. This time we are dealing with a far more difficult
subject. When our eyes become accustomed to the light, however, we
shall be conscious that there is something moving in our drop of
water. The Hay Bacillus is one of the moving forms, each individual
is furnished with a number of little whips whose lashings enable it
to travel through the water. The whips cannot be seen in unstained
bacilli; experience, however, tells us that they are there, for all
these lowly plants which show movement are seen when stained, to
possess the little whips. The process of staining kills the plants so
that we cannot see the little whips in action.
Having detected that movement is taking place, a little adjustment of
focus and a further search will reveal the bacilli to us, as little
rod-like, colourless individuals. We shall see their cell contents if
they are sufficiently highly magnified and also their cell walls. We
may even observe them splitting, each one into two individuals. We must
keep our sample of water for later examination. In fact, we may examine
drops from day to day, in exactly the same manner. After a short
lapse of time we shall notice that the bacteria have increased to an
alarming extent and also that they no longer swim about. At this period
they tend to arrange themselves in chains lengthwise, their cell walls
also lose their clear cut appearance and become jelly like, yet withal
they may still continue to split up.
If we now examine the water from which we have taken our drop we
shall probably find a scum floating on the surface; it consists of
millions upon millions of hay bacilli, huddled together so to speak.
It is the beginning of the end for them, nourishment is becoming
scarce and important changes are about to take place. Let us continue
our examination day by day, that we may discover what is happening.
Before long, we shall find that within each cell wall, which is no
longer jelly like, there is a darker cell contents than we saw before.
The bacilli have, in fact, formed spores. Now we may plug our bottle
containing the remainder of the water, with cotton wool and set it
aside for some months if we wish. At the end of that time, by pouring a
fresh supply of water upon the spores, we may start them all into a new
vitality and the whole process will be repeated.
We have mentioned that bacteria should be stained, in order to make
their presence more easily detected. This is hardly the place to enter
into a lengthy discussion concerning the methods of staining but, for
the benefit of our readers, who wish to pursue the subject further,
we will state as concisely as possible how simple staining may be
accomplished. Our requirements are, a pair of Cornet’s forceps, two
small bottles of stain, say Carbol-Fuchsin and Methylene Blue and a
larger bottle of 1/2 per cent. Acetic acid; these may be obtained from
the firm who supplied our microscope and, for the beginner at anyrate,
it is cheaper to buy the solutions ready made, than to attempt to make
them up at home. Slides and cover slips, we require, of course, and
they must be absolutely grease proof; it may be necessary to boil them
in a strong solution of caustic soda to effect this result. A small
bottle of Canada Balsam completes our requirements.
Should we wish to examine a drop of milk for bacteria, we proceed in
this manner. With the aid of the Cornet’s forceps pick up two cover
slips, place a drop of milk on one and cover with the other. With
thumb and finger bring the glasses into close contact, so that the
milk forms a thin film. Slide one glass from the other and set aside,
milk side upwards, till dry. Next take each cover slip, separately, in
the forceps and pass rapidly two or three times through the flame of a
spirit lamp, this fixes the bacteria, if any be present, to the glass.
Now having poured a little of the stain, say methylene blue, into a
shallow vessel, a saucer will do, we place our cover slips therein for
two minutes or so. Then, remove them with the forceps, wash in water
till no more stain comes away and set aside to dry. When dry, take a
clean slide, place a small drop of Canada Balsam at its centre and
gently lower the cover slip thereon, stained side downwards. If we now
examine our slide under a high magnification, we shall easily be able
to see whether bacteria are present or not. Should our preparation be
too deeply stained, a good slide will show the bacteria stained blue
against an almost colourless background, we must immerse our second
preparation for a few moments in a little of the 1/2 per cent. acetic
acid which will have the effect of removing the excess of stain; then,
after washing and drying, we proceed as before.
Beautiful double staining may be performed by the following method.
In addition to the chemicals we already possess we shall require
some 5 per cent. acetic acid. Double staining is especially useful
for spore-forming bacteria, so we may take some of the Hay Bacillus
at sporing time. Proceed exactly as described above substituting, of
course, a drop of water known to contain the bacilli for the drop of
milk. When the two cover slips are ready for staining, warm some of
the Carbol-Fuchsin in a saucer and leave the cover slip therein for
five minutes, then transfer to a 5 per cent. solution of Acetic Acid
till all the stain appears to be removed, afterwards wash in water. The
cover slips must next be immersed for a few minutes, two should be long
enough, in Methylene blue solution, then washed and, when dry, mounted
on a slide with Canada Balsam, as described above. If the staining has
been properly carried out, we shall have a most beautiful preparation,
showing spores stained red and the rest of the bacilli blue.
The work that may be done with bacteria is limitless but, to advance
very far, we shall need facilities for obtaining what are known as
“pure cultures.” Let us make the term clear. Suppose we take milk,
water, butter, anything in fact upon which bacteria will grow and
examine them carefully. If we have the requisite knowledge and
recognise what we see, we shall find not one kind of bacterium but
a number of different bacteria. Now by certain manipulations, which
need not be described here, all the different kinds of bacteria may
be sorted out, so that we have colonies consisting of one kind of
bacterium only and such a colony is known as a “pure culture.”
In practice, bacteriologists do not use the rough and ready methods
that we used in dealing with the Hay Bacillus. They prepare pure
cultures and cultivate the bacteria on various substances, differing
markedly from those on which they originally lived. For example, a
jelly-like substance, mainly composed of beef broth and gelatine is one
of the favourite substances on which to grow bacteria, milk is also
used in some cases and also slices of potato. All this may seem to have
little to do with the microscope, but indeed the bacteriologist relies
as much on the behaviour of his pure cultures, growing on gelatine,
etc., as on their appearance under the microscope. Some bacteria will
not grow on the surface of the gelatine but only in the body of the
substance, where air cannot reach them; others cause the gelatine
to become liquid; others give off a characteristic smell or impart
a well-marked colour to their food material; a few even cause the
gelatine to become luminous. These easily seen characters are quite
as typical of the various bacteria which bring them about as are the
microscopic characters; in fact it sometimes happens that only by a
combination of the two is it possible to be certain of what one has
obtained.
CHAPTER XII
MEDICAL WORK WITH THE MICROSCOPE
To the medical man, a microscope is all important. In the first place
it is absolutely necessary for him to have an accurate knowledge of
the human body; he must be able to recognise healthy blood, he must
know the varied cells composing muscle and bone, etc. Then again the
medical aspect of bacteriology is all important; we have devoted a
separate chapter to bacteria and there we warned our readers not to
look upon all bacteria as harmful, nevertheless, the harmful ones are
all important in medical work.
Of recent years a knowledge of the structure and habits of certain
insects has become an important branch of medicine for many of these
creatures carry diseases from one patient to another. This branch of
medicine is more important in the tropics than in this country, for
most of the really harmful insects live in warm countries.
A number of lowly animals claim attention from the medical man for
they are parasites of the human body; many fungi, too, cause disease. A
certain amount of chemical work with the microscope also falls to the
lot of the medical man.
We have stated many times in our pages that, both plants and animals,
are made up of one or more cells and that the cells of plants are each
surrounded by a more or less rigid cell wall, whilst those of animals
have no definite cell wall. As a start in our medical investigations we
may examine some cells from our own person. There is no need for alarm,
the operation is quite painless. After having prepared our microscope
and got ready a clean slide, on which we place a drop of clean water,
we must take some clean, blunt instrument such as a tooth-brush handle
and gently scrape the inside of the cheek. Having done so, dip the part
of the tooth-brush with which the scraping has been done, in the drop
of water, then cover the water with a cover slip.
A careful examination of our object with a fairly high magnification
will show that, though the scraping was very gentle and quite painless,
something has been removed from the mouth. We shall see a number of
cells, somewhat overlapping like the tiles of a roof. They are almost
colourless, and have ill-defined margins, certainly no cell wall, and
each cell contains a dark spot near its centre. These cells from our
own person will give us a good idea of the appearance of animal cells,
for we are animals, much as we may dislike the idea. The dark spot
in the middle of each cell is called a nucleus, it is the most active
part of the cell and when the latter is about to divide, as it does
when growth takes place, the nucleus always divides first.
[Illustration:
1. CRYSTALS FROM HUMAN BLOOD
It is often possible to identify the source of blood by means of
crystals which can be obtained from it, for each race of animal has
its own special form of blood crystal. The large crystal at the
margin exhibits the typical shape.
2. CRYSTALS FROM THE BLOOD OF THE BABOON
These show some resemblance to human blood.
]
Now we may perform another operation upon ourselves, rather more
painful than the last one but not very serious. We wish to examine some
human blood, so we tie a handkerchief tightly round one of our fingers
and with a clean needle--be sure that it is clean--make a puncture in
the finger tip. The handkerchief bandage will prevent our feeling any
pain. We must put the drop of blood we have obtained in the centre of
a clean slide and examine it under the microscope. While the blood is
still liquid, we shall see a number of circular discs floating about,
they are very small being only 1/3200 inch in diameter. The centre of
each disc appears darker than the rim, but this darker shade is only
apparent. Using a term we introduced in our chapter on the lens, we
may call the discs double concave. Now let us watch our objects for a
moment and we shall notice that they begin to arrange themselves in
chains, they appear like a number of draughtsmen placed one upon the
other, our drop of blood is now beginning to clot. A simple experiment
may be attempted at this stage; we must run a drop of water on to our
little blood discs and watch carefully what happens. We shall see that
they change their shape gradually, from being double concave they
become flat sided, and this is not all, for they continue to change
till both sides bulge outwards and they may be described as double
convex.
These little discs are known as red blood corpuscles. Their shape is
some indication of the animal to which they belong; those of man, as we
have seen, are circular and so are those of most of the higher animals,
except the camel tribe, which has oval, red blood corpuscles. Birds,
reptiles and fishes have corpuscles agreeing in shape with those of
the camel. In size there is a great deal of difference between the
corpuscles of various animals; a member of the deer family has the
smallest and a creature related to our Newts, called Proteus, has the
largest. The size of the blood corpuscles bears no relation to the size
of the animal to which they belong, those of the frog measure 1/1108
inch, or nearly three times the size of the red blood corpuscles of
man. Without much difficulty it should be possible to obtain other
samples of blood and the red corpuscles should always be examined,
needless to say the blood should always be in a fresh condition. In
addition to the red corpuscles we may notice a few smaller circular
bodies, they are the white corpuscles. If we have any difficulty
in finding them in our own blood, we can examine another specimen
of frog’s blood, in which they are more easily seen. It has been
calculated that in a cubic inch of blood from a healthy human being
there are eighty millions of red and a quarter of a million of white
blood corpuscles. Although there is such an enormous difference in the
sizes of red corpuscles from various animals, the white corpuscles
are remarkably constant, measuring about 1/3000 inch in warm-blooded
animals and 1/2500 inch in reptiles. Sometimes, as we are examining
the white blood corpuscles under the microscope, we shall notice that
they behave in a similar manner to the Proteus Animalcule, which we
described in our chapter on Pond Life. A kind of creeping motion takes
place and the corpuscle loses its circular shape. In our own blood this
movement lasts but a very few moments, in frogs’ blood, by keeping our
slide moderately warm, we may witness the movement for some time.
It is perfectly easy to watch the circulation of blood in the foot of a
frog. For this purpose we require a piece of apparatus known as a frog
plate, this is merely a flat brass plate perforated with a number of
holes, through which tape is passed to bind the frog down and having
also a hole the size of the opening in the microscope stage, into which
a circle of glass is usually fitted. Such a plate may be made of wood
and will serve our purpose quite as well as the more expensive brass
plate. We must bind our frog firmly, but not too tightly with wet rag,
leaving one leg exposed. This free leg must now be fastened in such a
manner that one of the webs between its toes comes over the opening
in the plate, finally the toes must be carefully tied with string so
that they remain apart and the web is fully expanded. Having fixed
our frog in a satisfactory position let us examine his blood-vessels
under a fairly high power. We shall find that there are large and small
vessels in his webbed foot; we will devote our observation to the
small vessels, called capillaries, from the Latin _Capillus_, a hair,
because of their small size. Probably the blood will not flow when we
make our first examination and this points to one of two things, either
the frog is bound too tightly or he has not recovered from the alarm
he experienced at his treatment. In the latter event he will not be
long recovering and his blood circulation will soon be in full working
order; in the former case, we must loosen his bandages. In some of the
very small blood-vessels we shall notice that the blood always flows
in one direction, in others it does not appear to have any definite
direction. In either case, however, we can see the red corpuscles
flowing rapidly along the central stream of the blood-vessel whilst the
white corpuscles travel much more slowly along the sides.
The medical man may be called upon to decide two questions concerning
human blood; he may wish to know whether it is healthy and, in the case
of certain crimes, he must be able to state positively whether certain
stains are caused by human blood or not. Dealing with these questions
in order, let us see how we would proceed. There are many people, far
too many in towns who are described as bloodless, the expression is of
course an exaggeration for no bloodless person could continue to exist.
What really happens is that such people are deficient in red blood
corpuscles. If we examine two samples of blood, one from a perfectly
healthy subject and one from a so-called bloodless subject we shall
probably not be able to detect any difference between the two, but the
experienced medical man will soon see that one sample has too few red
blood corpuscles.
An unhealthy state of the blood is often indicated by the shape
of the blood crystals. Let us see how we may obtain some of these
crystals. If we add a drop of ether to our drop of blood upon the
slide, and wait a few moments till the ether has evaporated, we shall
notice when we examine our object again, under a high magnification,
that a number of prismatic crystals have formed, especially towards
the edges of the slide. Now in certain diseases of the blood, these
crystals are no longer formed in their usual shape, thus pointing
out to the medical man that something is amiss. Again there are many
blood parasites just as there are external parasites of man. In the
disease known as malaria, very small parasites are introduced into
the blood by mosquitoes. Each of these little parasites enters a red
blood corpuscle, divides up into many smaller individuals, causes the
corpuscles to burst, then each little parasite attacks more corpuscles.
Some of these little bodies are sucked up, along with the blood of
malaria patients, by other mosquitoes and, if the insects are of the
particular kind which spread malaria, the parasites complete their
development within the mosquito. Patients suffering from that terrible
African malady known as sleeping sickness, have blood parasites
resembling minute eels with long threadlike tails. These parasites are
the cause of the malady and are introduced into the blood by flies,
closely related to house flies. There are a very large number of blood
parasites of one kind and another, so it is clear that a knowledge of
blood is very important to the medical man.
From our remarks concerning the sizes of the red corpuscles in the
blood of man and various animals, one might be excused from thinking
that it would be quite easy for anyone with a little experience to
recognise human blood. As a matter of fact it is very difficult, it is
always a doubtful matter to rely upon size alone. We have mentioned
blood crystals and these give us a slightly better clue to the origin
of the blood, for the blood crystals of different animals vary in
shape, far more than their corpuscles. Those of the guinea-pig,
for example, are little four sided pyramids; those of the mouse,
eight sided and so on. Some apes, however, have blood crystals very
similar to those of man, so similar that the two may be confused. The
microscopist who is compelled to give an opinion concerning the origin
of a sample of blood, especially blood which is some days old, is faced
with no light task.
Without the assistance of the microscope, medical men would never have
discovered the cause of many of these insect-borne diseases, they could
never have traced the development of the blood parasites in the bodies
of insects. For many years it was thought that the disease malaria
was caused by the damp air of low-lying land, in fact the word malaria
is derived from two Italian words meaning bad air. It was not till the
advent of the microscope that the true cause of the disease was learned
and the discovery was made that the only connection between the malady
and dampness is that the mosquitoes which carry the disease can only
thrive in damp situations.
Medical work with the microscope also entails a study of external and
internal parasites of man other than those which infest the blood.
Some of the larger human parasites we know only too well, they force
their unpleasant attentions upon us from time to time, but fortunately
they are not as a rule serious and are soon got rid of. The cleanest
of us, in these days of universal travel, cannot avoid the visitations
of the lively flea, he is at worst an annoying companion, but he has a
cousin who is responsible for the passing of the germs of the dreaded
plague from rats to man. Plague long remained a mystery, which, without
the help of the microscope, would probably have remained unsolved to
this day. Many of these apparently harmless blood-sucking insects may
prove to be disease carriers, as medical knowledge and microscopic
investigation is brought to bear upon them.
The majority of the smaller external parasites are beyond the reach of
the amateur microscopist. A number of course are available to medical
students, but we write for the ordinary enthusiast and not for the
specialist. Many interesting little mites, closely allied to the
cheese-mites, cause skin diseases and the creatures themselves as well
as their tunnels in the skin are interesting.
There is one little parasite which we nearly all of us carry without
knowing it, and it makes quite an interesting object for the
microscope. Its name is _Demodex Folliculorum_ and it dwells in the
sweat glands, especially those round about the nose. It is very minute
and requires a high magnification for its examination. The adult is
worm-like and tapering in the hinder two-thirds of its body, whilst the
front third, to which is attached four pairs of short, fleshy legs, is
stouter. From the eggs which may be heart or spindle shaped, little
six-legged grubs arise; later these grubs change into the eight-legged
adults. All the changes take place in the sweat glands, and the
creatures live with their heads turned away from the gore of the gland.
To observe these strange little creatures it is only necessary to
squeeze some of the white congealed perspiration from one of these
glands on to a slide, coyer it with a drop of oil or xylol and spread
out the object with a needle. By cutting down the light considerably by
means of the diaphragm, we shall be able to distinguish all the stages
of these little creatures which, unbidden, share our lives.
Other parasites come to us in our food and, of these, one of the
most dangerous is known as _Trichinella Spiralis_ and gives rise to
a disease known as trichinosis. We need not give a precise account
of the life history of this interesting worm, it will suffice for
our purpose to know that it finds its way into the bodies of human
beings, in badly cooked pork or in improperly cured ham or bacon. The
little worm is only about 1/25 inch long and, at first, dwells in the
intestines of the pig. Each female produces upwards of fifteen thousand
young and these pass into the blood of their host and thence to its
muscles. Snugly coiled into a spiral between the muscle fibres, each
young worm becomes surrounded with a lemon-shaped covering, which is
exceedingly resistant. Suppose now a human being should happen to eat
such pork, the juices of the stomach would dissolve the pork and also
the little case enclosing the parasite. Once set free in the human
body, the whole proceeding is repeated and, at last, the result is very
severe muscular pains for the victim. The medical man is called upon to
study such parasites as _Trichinella_, and they are not so uncommon as
one might think.
In addition to animal parasites medical science demands a knowledge
of many disease bringing fungi. One of the commonest is the fungus
responsible for the unpleasant malady called ringworm. But we have
written sufficient to show that the general practitioner needs his
microscope always by his side. Not only so but very many instruments he
uses in his work, though called by other names, are really microscopes
adapted for special purposes.
CHAPTER XIII
THE MICROSCOPE AND AGRICULTURE
Probably there are not many farmers who use a microscope and fewer
still who use one to help them in their business, yet there are few
people to whom one of these instruments would be more useful. Their
seeds are often far from pure and the microscope will reveal the
impurities which may consist of dirt and dust, or of other seeds, seeds
which will grow into weeds and make the crop less valuable or, if
present in large quantities, render it valueless. Agricultural plants
become attacked by varied diseases which can only be studied under the
microscope; insects also do their share of destruction and much may
be learned about them when they are magnified. Fungi and insects not
only attack crops but domestic animals as well. The microscope is an
invaluable aid in studying the soil, in dairy work and in many other
ways closely connected with agriculture.
That the testing of agricultural seeds is very important is shown by
the fact that not very long ago a deputation urged the Government to
establish a National Seed Testing Station; no further plans have been
made, however. Seed testing is very interesting work, every seed has
its particular shape and markings and the student soon becomes absorbed
in seeking for weed seeds among the collections he examines. A weed in
the sense we use it here is not necessarily a harmful plant, it is a
plant in the wrong place. For example a carrot growing in a field of
turnips, though a useful plant would be a weed. When the farmer sowed
turnip seed he did not do so with the object of raising carrots.
The only apparatus necessary for the study of most farm seeds is
a powerful magnifying glass, one that will enlarge the seeds ten
diameters or more. When beginning this work, a difficulty occurs at
once for, without assistance from an expert, it is by no means easy
to learn the names of the seeds one examines. The difficulty can be
overcome to a certain extent if we know the names of flowers, for then
we can collect the seeds from these flowers and we shall have properly
named specimens as a guide. Beginning in this way, we shall soon find
that the seeds can be arranged in groups and there will then be no
difficulty in recognising say clover seed or grass seed, though much
more experience will be necessary before we can say to which kind of
clover or grass the seed belongs.
Many of these seeds are well worth studying, whether we are interested
in seed testing or not. The corn buttercup and the wild carrot have
curious spined seeds; those of the larkspur when magnified appear to be
studded with little shreds of paper. White and red campion, have kidney
shaped seeds studded with warts and so similar to one another that
the microscopist who can distinguish one from the other may consider
himself something of an expert. Spurrey has lens shaped seeds with
raised equator-like rims. Evening primrose seeds are curious because
they are found in all sorts of shapes. The seeds of rib-wort plantain
resemble miniature date seeds, others resemble minute bananas, some
are perfectly round, others almost square; some have smooth shining
surfaces which look as though they had been artificially polished,
others again are wrinkled and deeply furrowed; but, most curious of all
the common seeds are those of the cornflower, they resemble nothing
so much as little shaving-brushes, with bright yellow bristles. Many
profitable hours may be spent in studying the seeds of our common
native plants, both wild and cultivated.
There are two specially obnoxious plants whose seeds may be mixed with
agricultural seed, to the dismay of the farmer. We refer to Broomrape
and Dodder, both of them unable to earn their own living and depending
for their existence on the robbing of other plants. Broomrape usually
attaches itself to the roots of Hazel, Poplar or Beech and steals its
food therefrom, but its fleshy pink stems and flowers may sometimes be
seen in clover fields, then clover is its victim.
More common and more destructive is the little Dodder, a member of the
Convolvulus family. Its seeds are very minute and when they germinate
they give rise to a seedling not unlike a piece of wire. With one end
fixed in the ground, the other waves about till it finds a clover plant
round which it twines and not only so but it sends out suckers which
microscopic examination shows, penetrate the stem of the clover to rob
its food. By pulling a Dodder stem away from the clover we can clearly
see a number of holes where the suckers have entered.
Fungus diseases and insects wage constant warfare on the farmer’s
belongings. That we may better understand the structure of the
disease-causing fungi we are about to examine, let us refresh our
memories concerning that very common fungus, known as white mould and
mentioned in an earlier chapter. The reason fungi cause damage to
other plants, the one invariable reason, is that they, being unable
to manufacture food for themselves, steal it from the plants on which
they grow. Some of them are parasites and steal their food from living
animals or plants; others live upon dead animal or vegetable matter and
the white mould is one of the latter fungi.
In most of the fungi which concern us we shall find that there is a
mass of minute, thread-like structures forming the main body of the
fungus and that, here and there, portions grow erect and bear spores.
The spores, it is important to remember, serve the same purpose to the
fungus as seeds to the flowering plants. They are blown or carried
by insects or other agencies to suitable situations for growth, they
germinate and form new fungi. They are smaller and lighter than the
most minute and dust-like seeds, so that the slightest breeze scatters
them far and wide. Let us compare the white mould with a mushroom; at
first sight the two plants appear very dissimilar, in reality they
are very similar to one another. The mould forms a thick felt of its
threads over the substance on which it grows and mushroom spawn if
carefully examined under a microscope will be found to consist of very
similar threads, sealed together to form thicker root-like structures.
The fungi, however, have no roots and these threads are strictly
comparable with those of the mould. Here and there the mould sends a
single thread into the air and each of these threads is terminated by a
little ball which bursts eventually and sets free its contained spores.
The same thing occurs with the mushroom; we have the upright growth,
not of a single thread it is true but of a number, welded together to
form a fleshy stalk; at the top there is, not a ball containing spores
but an umbrella-shaped structure whose under surface is composed of
a number of “gills” on which the spores grow at the ends of little
stalks. If a piece of the mushroom stem be torn into its separate
components and examined under the microscope, its similarity to the
more simple fungus is evident. One of the gills also may be carefully
cut away and examined; the spores will be seen at the end of small
forked stalks.
Having progressed thus far in our study of fungus structure, we may
examine a few of those which cause damage in farm and garden. For the
most part, the thread-like portion of the fungus grows within the plant
attacked and only the spore bearing portions appear on the surface.
There is one class of fungus, however, the Mildews in which practically
all the structure grows on the surface, only a few small, unbranched
suckers penetrate the plant attacked, for the purpose of obtaining
nourishment.
Though of great interest to the microscopist, the potato disease is
often the cause of serious loss to the farmer. Not only potatoes but
also tomatoes are attacked. A potato plant suffering from the disease
has irregular yellowish or brownish spots upon its leaves in the
summertime. An examination of the lower surface of one of these spotted
leaves will reveal a silvery white margin to each spot. This portion
should be magnified with a fairly high power and care must be taken
not to injure the diseased part of the leaf before it is examined. In
cases of serious disease, from nearly every pore on the surface of the
leaf fungus threads will be seen to issue. The threads are branched
and, at the end of each branch, they have a special kind of spore. They
look not unlike miniature leafless trees and they give the typical
silvery white appearance to the margins of the diseased spots. When the
spores of potato disease germinate, the young fungus threads enter the
leaf through a pore and for sometime afterwards there is no sign that
the plant is diseased. On this account potato disease and many other
fungus diseases are rendered more serious in that the farmer is not and
cannot be aware that his crop is attacked till the disease has taken a
firm hold. Eventually the potatoes themselves become brown, rotten and
breeding grounds for bacteria.
A very common plant disease which makes a good study for the
microscope, may be found in quantity upon shepherd’s purse, and as
it also attacks cultivated plants of the same family, cabbages,
cauliflowers and the like, it is of no little importance. In its early
stages, the fungus looks like patches of thick white paint upon the
plant and where the fungus grows the plant is invariably contorted. As
the fungus matures, the skin of the diseased plant splits and a white
powder issues. If some of this powder be highly magnified, it will be
found to consist of chains of spores, six or seven in a chain. The
spores break off singly and each one may start the disease in another
plant.
The microscopist who hunts in garden and farm for fungus diseases, will
assuredly meet with some examples of that large class known as “smuts.”
They are so called on account of the black powder with which the
attacked portions of the plants become filled. The smuts are very
important but are not of much interest as objects for the microscope,
so we will pass them over for subjects of greater interest if of less
importance.
Every farmer knows the familiar and destructive fungus known as “rust
of wheat,” it is one of a large class of most interesting plants.
The “rusts” are interesting to the microscopist on account of their
structure and to the botanist because they cannot, like other fungi,
complete their lives upon one plant. They derive their popular name
from the fact that they look like patches of rust upon the plants on
which they live. Some of the greatest living agricultural botanists
have spent many years on producing races of wheat upon which rust
fungus will not grow. Wonderful success has rewarded their efforts and
conferred immense benefits upon farmers. In spite of this, however, we
need not despair that we shall be unable to find a specimen for our
microscope, though it is happily an undoubted fact that the disease is
not so common now as a few years ago.
[Illustration:
_Photos by Flatters & Garnett_
1. CLUSTER CUPS
The spring stage of Rust of Wheat. Little orange cup-shaped growths
on the under side of a Barberry leaf. They germinate on Wheat to
form the summer stage of “Rust.”
2. RUST OF WHEAT
These little stalked spires are the winter stage of a serious
disease of Wheat. In the spring they germinate on Barberry.
3. POLLEN GRAINS ON A GRASS FLOWER
The feathery stigmas of grass flowers are beautifully adapted for
catching and holding pollen grains.
4. THE LOWER SIDE OF A FERN FROND
One of the brown outgrowths on the under side of a fern frond.
The stalked spore cases are seen, protected by an umbrella shaped
covering.
]
Rust of wheat fungus grows part of its time on barberry leaves and
part on wheat. In the summer, if we examine one of the rust-like
patches on stem or leaf of wheat we shall see that it consists of a
dense bunch of small, short stalks each one of which is terminated
by an oblong red-brown spore. If we keep another patch of the fungus
under observation, we shall find as the season advances, that instead
of the red-brown patch it has grown darker and darker till it has
become almost black. The microscope will show us that the structure
of the spores has altered considerably. There is still the same bunch
of stalks but they have lengthened somewhat and now each spore which
terminates each stalk is divided into two parts by a wall across its
narrow part. The walls surrounding the spores also appear thicker, as
indeed they are. These are the winter spores, they fall to the ground
eventually and there they remain, unharmed by frost or snow or rain,
till the spring. In the spring they germinate and give rise, not
immediately to another fungus, as might be expected, but to another
kind of spore. Curiously enough these new spring-formed spores cannot
grow upon wheat and unless they are carried by wind or some other
agency to a barberry plant their existence is ended. Should they reach
a barberry leaf, however, they will germinate, penetrate the leaf and
grow for a period. Eventually the fungus appears on the lower surface
of the leaf in beautiful structures called cluster cups. Under the
microscope, one of these cluster cups forms a lovely object. The leaf
skin is split and below the ruptured skin may be seen a flask-shaped
hollow filled with chains of minute golden-yellow spores. The spores
break away, one by one and favoured by fortune, are carried to a
wheat plant where they germinate and give rise to the familiar rust.
Any microscopist anxious for research has a life’s work before him
in tracing the histories of this one class of fungi, should he feel
inclined to shoulder the burden. Very many cluster cups are known and
very many rusts and all that is required is an enthusiastic mycologist,
as the student of fungi is called, to put the pieces of the puzzle
together, so to speak. It is not so very many years ago that the
connection between the cluster cups of barberry and the rust of wheat
was quite unthought of.
We cannot afford much more space to plant diseases, the farmer has
other troubles and we must mention some of them. We cannot leave the
subject, however, without a word concerning the mildews. As we have
mentioned, they are curious because they dwell outside the plants they
attack. Rose mildew is unfortunately all too common in every garden, it
may be recognised as a white powder covering leaves and buds. Under the
microscope, in the summer we shall find that it consists of a number of
thread-like structures, not unlike those of the common white mould and
that there are a number of erect chains of spores. Towards autumn, a
further examination will show us many round dark-brown structures from
which project a number of minute threads. These brown spheres are the
winter stage of the fungus, designed to withstand inclement weather.
In the spring, the spheres burst and set free a number of minute sacs,
each one containing eight spores. The spores germinate on rose leaves
and start the disease anew.
There will be no difficulty in finding mildews; they are all very
similar to the rose mildew in general but they all differ in detail.
The gooseberry mildew for example, has a large number of threads
running from its winter spheres and each thread is terminated by a
little group of branches. The sacs which fall from the opened sphere in
the spring, only contain four spores in this case.
The animal enemies of the farmer, so far as they concern the
microscopist are more difficult to study. Many of them are internal
parasites and to gain a real knowledge of their habits and life
histories necessitates a good deal of rather unpleasant work for which
the ordinary microscopist has neither the time nor the inclination.
In order to give our readers some idea of this work, let us take one of
the commonest of all agricultural parasites and trace its life history
whilst giving hints for its examination under the microscope. The
common liver fluke is a worm which, in the adult stage, frequents the
liver of some domestic animal, usually the sheep. A friendly butcher
will probably be able to supply us with a specimen and, when we receive
it, we shall probably dub it a very unwormlike creature. The worms form
a large class in the animal kingdom and they do not all resemble the
earthworm by any manner of means.
The liver fluke is a flat, almost leaf-like creature, it is not ringed
like the earthworm and, under the microscope, we can plainly see all
its internal organs. The fluke lays its eggs, each one enclosed in a
little capsule, in the liver of the sheep. They are carried to the
intestines and finally set free along with the animal’s excrement.
If then the eggs are blown, or carried by some means to water they
will continue their development, on dry soil they cannot long survive.
Each egg gives rise to a little organism which swims freely in the
water; it is shaded like a blunt-ended cone when extended and is
roughly oval when contracted. Its body is covered with little whip-like
structures similar to those of the slipper animalculæ, and it is due
to the lashing of these little whips that the creature moves through
the water. If we found one of these young flukes in some pond water
we might be forgiven for thinking it to be some near relative of the
slipper animalcule. When our subject finds a living water snail it
enters its breathing organs, becomes affixed to their walls and loses
its covering of little whips. It becomes transformed into a shapeless
mass which later develops into an elongated structure, quite unlike
the free swimming creature which took shelter within the snail. Next,
a migration is made to the liver of the snail where birth is given to,
from fifteen to twenty, curious little heart-shaped organisms each with
a tail about twice as long as its body. These little creatures escape
from the snail and swim freely in the water for a time. Eventually they
make their way to herbage growing by the waterside, affix themselves
thereto and become surrounded with a hard coat capable of resisting the
effects of hot sun or drying winds. Should this herbage be eaten by
cattle, the apparently lifeless young fluke bestirs itself, loses its
tail and wends its way to the liver of its host, then the story begins
again.
Having examined the adult liver fluke under the microscope, we shall
probably wish to find both the free swimming young forms, and if we
search carefully in ponds to which sheep have access we are likely to
be rewarded. It is obvious that the life of a parasite such as the
liver fluke is, of necessity, precarious. It is only chance or luck,
or whatever one’s favourite term may be, that brings the egg to water,
the young fluke to a snail, and the last free swimming form to herbage
that will be eaten by a suitable animal. As usual in such cases, nature
makes provision for emergencies by providing a large number of young,
in order to insure that some at least may be able to complete their
development. Owing to a series of changes, which we have omitted to
describe for the sake of simplicity, each liver fluke egg may give rise
to no less than three hundred and twenty of the final free swimming
forms.
As we have remarked, the study of parasites is difficult but it is
interesting. Very few of these creatures can complete their lives
without living at the expense of two different animals. The liver fluke
needs the water snail and some herb-feeding animal; there is another
parasite which spends part of its life in the pig and another part in
the grub of the cockchafer; a third parasite dwells for a time within
the thrush, and for the rest of its time within the garden snail, and
so on. Apart from the interest of the subject in itself, it brings us
face to face with the fact that many quite unrelated forms of animal
life are essential to the well being of a number of parasites. To the
farmer the subject is all important.
Insects of various kinds are all important in agriculture; most of
them are harmful, some few are useful. They have, however, been dealt
with in another chapter, so we will dismiss them here. The ticks are
closely related, and anyone with access to a farm should be able to
obtain some specimens. Whatever species we are able to obtain should
be examined under the microscope. Their feet are always interesting,
being furnished with powerful claws beautifully adapted to grasping the
hairy coats of their hosts. Their mouth parts are quite unlike those of
insects, and are always furnished with a number of backwardly directed
teeth, which are useful for tearing flesh sufficiently to draw blood on
which they feed.
CHAPTER XIV
THE MICROSCOPE AND INSECT LIFE
Some of our readers will probably remark that entomology, or the
natural history of insects, is really a branch of zoology and should
be treated as such. We cannot pretend that they are wrong, but it is
such a specialized branch that it merits separate treatment. Not many
years ago insects, with few exceptions, were looked upon as harmless
and often beautiful dwellers upon the earth. They afforded endless
amusement to certain enthusiasts who collected them for their colouring
or their odd forms. Recent developments of our scientific knowledge
have shown us that the insect is, other human beings excepted, man’s
most serious rival for the mastery of the world.
This state of affairs has been beautifully depicted by an American
naturalist; his words described a by no means unlikely final scene on
this earth of ours. He wrote: “When the moon shall have faded from the
sky and the sun shall shine at noonday, a dull cherry red, and the seas
shall be frozen over, and the ice-cap shall have crept downward to the
Equator from either pole, and no keel shall cut the waters, nor wheels
turn in mills, when all cities shall have long been dead and crumbled
into dust and all life shall be on the very last verge of extinction on
this globe, then, on a bit of lichen, growing on the bald rocks beside
the eternal snows of Panama, shall be seated a tiny insect, preening
its antennæ in the glow of the worn-out sun, representing the sole
survival of animal life on this our earth--a melancholy ‘bug.’”
There is probably no field more interesting for the microscopist than
that provided by the insect world. Unlimited explorations may be made
with the certainty of finding something new at every turn. Most people
begin their studies of insect life with butterflies and moths; some
folk to their loss never proceed further. We may well follow the usual
course, and make a butterfly our first study.
Any butterfly or moth will do for our purpose, any one with coloured
wings, for some have clear wings like those of the bees and wasps, but
they are not very common, so that we shall probably find a suitable
specimen at the first attempt. The more highly coloured the specimen,
the more attractive it will appear under the microscope. After killing
the insect, and not before, we may proceed to study it. Killing may
best be accomplished by means of a killing bottle, failing this a hard
nip on the body, between thumb and finger, will do, but it must be no
half-hearted proceeding or the insect will be injured without being
killed.
Having removed a wing and placed it on the stage of our microscope, we
must examine it by reflected light, for it is not transparent. This
may be accomplished, if we are using artificial light, by raising the
source of illumination well above the object, so that the light strikes
it at an angle of about forty-five degrees; by daylight reflected light
is easily managed. If we have never previously examined a similar
object we will be surprised at its appearance. All the beautiful reds
and blues, yellows and greens which comprise the brilliant livery of
these insects are seen, under the microscope, as hundreds of minute
scales which overlap one another like tiles on a roof. A higher
magnification will show that each scale is roughly flask-shaped, and
that its narrow end fits into a little socket in the wing proper. When
the scales are rubbed from the wing, nothing remains but a transparent
substance traversed by veins--to the microscopist the scaleless wing
is of little interest; to the entomologist it is important, for the
moths, at anyrate, are arranged into families largely according to the
arrangement of the veins of their wings.
Many other wings may be examined with advantage; gnats, for example,
are clothed with scales of varied shape, some hair-like, some forked,
some resembling a sickle, and some disc-shaped; these forms, by the
way, do not all occur upon the wings, but are found upon the head
and other parts of the body as well. The wonderful gauzy, iridescent
wings of dragon flies are interesting; those of various flies worth
examination also those of bees on account of the clever device for
uniting the front and hind wings during flight. On the front edge of
the hind wings the microscope will show us a row of minute hooks. When
the bee makes a flight, it hooks its hind wings to a ridge on the
hinder edge of the fore wings, so that for flying purposes it has, to
all intents, two wings instead of four.
Having examined all the wings we can find for the time being, we may
turn our attention to mouths. The mouth parts of insects are not
only interesting but important; one of the first things an economic
entomologist does with a new specimen is to examine its mouth parts.
The mouth will tell us what manner of feeder its owner may be. Some
insects have sucking mouths, and they must feed perforce on liquids;
others have biting mouths, and they are likely to do damage to crops
by eating them. Then there are lancet-like mouths and mouths which are
a compromise between biting and sucking ones. The subject, however,
is somewhat complicated, and entails a knowledge of insect anatomy,
so we will merely deal with a few easily understood examples. Our
butterfly has a sucking mouth; it is known as a proboscis, and may be
found, coiled like a watch spring, beneath its head. There is no trace
of anything in this mouth capable of biting or even piercing the most
delicate structure. The house-fly is also possessed of a proboscis
though of different design. Though a dangerous, disease-carrying
insect, it can do no harm with its mouth. The partiality of the
house-fly for sugar is well known, and it is interesting to learn
how, with its soft fleshy mouth, it can satisfy its cravings. Let
us watch one at work on a lump of sugar through our pocket lens. If
we look carefully we shall notice that the fly, as he thrusts his
proboscis here and there, emits from it a little drop of liquid; after
a momentary pause the liquid is sucked up again, it has dissolved a
little of the sugar, and the fly enjoys the sugar-laden liquid.
People frequently state that they have been bitten by a house-fly--a
sheer impossibility. What really happens is that they mistake the
very similar stable fly for the house-fly. If one of these insects be
captured and examined, we shall find not the soft fleshy proboscis
of the house-fly, but a cruel looking, awl-like mouth easily able to
penetrate the human skin. Certain tropical flies, known as Pangonias,
have such formidable and lengthy piercing mouths that they can
penetrate thick clothing and puncture the skin below.
Microscopists who care to follow up the study of insects’ mouths will
know that they are accumulating really useful knowledge. Those who do
not desire to go so deeply into the matter may well spare a few moments
for the examination of the green fly mouth, a needle-like piercing
organ which is thrust into plants for the purpose of sucking their sap.
The mouth of the gnat is a more difficult subject for the microscopist,
though no less interesting; it may be compared with the same organ of
the green fly, for it is used somewhat similarly, with the difference
that it sucks blood and not plant juices. It may be well to mention
here that the females alone suck blood, but it is easy to distinguish
the sexes, for the antennæ or feelers of the females are thread-like
whilst those of the males are feathered. A few adult insects have no
mouths, for they never feed during their short lives.
Caterpillars of all kinds and also beetles, grasshoppers, cockroaches
and the like have biting-mouth parts, and the student, who is not well
versed in insect anatomy, will learn more by watching one of these
insects partaking of a meal than by trying to discover the uses of the
various parts with the aid of a microscope. Caterpillars as a rule are
not shy feeders, and a pocket lens will show their sickle-like jaws
in full play. The grubs of house-flies are worth examining; they are
soft and fleshy except for a pair of horny hooks which are used to tear
up the food material. There are, however, so many different mouths we
cannot describe even the typical ones, but the microscopist will soon
discover those of special interest.
The feet of insects do not show so much variation as their mouths,
nevertheless they will afford ample material for many hours of study.
Our butterfly, which is now but a remnant, will provide our first
object. The design of its feet will depend, to some extent, on the
species of insect, but they will certainly be clawed. Other insects
with clawed feet--beetles, bees, and wasps--may be examined, and we
shall see that there are minor differences amongst them though their
general plan is similar. Sometimes we find a simple pair of claws on
each foot, in other cases each claw has a little spur, whilst spiders,
which, by the way, are not insects, have comb-like claws. The foot of
the house-fly is not only provided with a pair of claws, but also with
a soft fleshy pad, by means of which it is enabled to climb window
panes and similar smooth surfaces. If we are fortunate enough to obtain
a specimen of a louse, human or otherwise, we must not fail to notice
its strong grasping claw, used for taking a firm hold of the hair of
the creature on which it lives. Such objects are better examined under
a high magnification, along with a hair, then the actual method of
grasping may be observed.
The feelers of some insects are interesting; those of gnats we
have already mentioned, but they may be examined in detail. Those
of beetles are of very diverse form; some are thread-like, some
clubbed, some fan-shaped. Moths, too, have many curious designs to
show. Some of these feelers, when highly magnified, may be seen to
be pitted--hundreds of little sunken areas are scattered over their
surfaces, and it is probable that they are connected with a sense of
smell. In that case the feeler is a more important organ than one might
surmise from its popular name.
The hairy clothing of insects need not delay us long. Most interesting
of all are the feathered hairs of certain bees. In our chapter on
botanical work with the microscope we mentioned the feathery stigmas
of grass flowers and we also stated that they took that form, so that
pollen grains blown to them would be entangled in their branches. The
hairs of many bees are feathered for a similar reason, they gather
pollen and the pollen adheres to these “feathers” much more readily and
in much greater quantity, than it would to simple, unbranched hairs.
Some bees collect no pollen and, from them, feathered hairs are wanting.
Any microscopist who has followed us thus far, will have a fair idea of
the structure of a number of insects. In every case, where possible,
comparisons should be made between similar organs of different insects
and the investigations may be made more interesting by observing
the habits of the insects and trying to discover reasons for the
differences in structure. It is safe to surmise that there is a reason
in every case. There are many other interesting subjects which we have
not mentioned, the legs of insects--running legs of ground beetles,
digging legs of mole crickets, swimming legs of water beetles and the
wonderful pollen-carrying legs of bees. Then again, the eggs of many
insects are of surpassing beauty in shape, they may be round, oval,
oblong, nearly square and almost needle shaped; some are smooth and
shining like burnished metal, others beautifully sculptured; some
resemble miniature birds’ eggs, others are not unlike the seeds of
plants.
Many insects are too small to be cut up into their various parts,
legs, feet, wings, etc., by unskilled hands and they must be examined
whole. Perhaps you may think that an insect will be too big an object
for your microscope. Indeed there are some insects which measure nearly
a foot in length, but there are, on the other hand, beetles no longer
than one hundredth part of an inch. When we examined the house-fly
it is not unlikely that we found some minute creatures living upon
it. None of these is likely to be an insect, but as they are closely
related we may mention them here. Beneath the wings of the house fly
there are often minute, red, six legged young mites, all eagerly
sucking the juices of their host. Because they are six legged we may
be led to think that they are insects, for the entomologist knows that
the true insect, in its adult form, never has more than six legs. These
mites, however, later in life, drop from the fly and by developing
another pair of legs appear in their true colours. Various other
mites, including cheesemites, may be found clinging to the house-fly,
in fact it is by the aid of these insects that cheesemites are often
carried from cheese to cheese. One of the most curious parasites of the
house-fly must be sought upon its legs. If the search is successful,
curious, reddish-brown creatures, armed with formidable pincers, and
strangely reminiscent of miniature lobsters, will be found clinging
thereto. They are called chelifers and their home is the manure heap,
so that their presence shows us only too well where our friend the fly
has recently disported himself. His next visit was probably to our
food.
[Illustration:
1. THE HEAD OF A BEETLE
A remarkable beetle, with enormously developed fore-legs. The
object of these long legs is not known.
2. THE HEAD OF HERCULES BEETLE
This beetle, whose head resembles a lobster’s claw, is said to
carry his wife from place to place.
3. A CICADA
One of the noted singing insects. Kept as a pet in some countries,
voted a nuisance in others, the cicada is the medium of much
romance.
4. THE HEAD OF MANTIS
This illustration clearly shows the cruel rat-trap-like front legs
of the rapacious mantis.
]
If we can find a swiftly running stream within easy reach a little
time may be spent in searching the submerged rocks and plants for the
miniature stages of the buffalo gnat. This insect, which is known
to scientists as _Simulium_, has a most interesting life history.
Its popular name is derived from its hump-backed appearance and its
supposed resemblance to a buffalo. The female lays her eggs on a rock
or reed, just covered by running water, she never lays them in still
water. The greenish-brown, club-shaped grubs which come from the eggs
are curious and they will repay examination under a low magnification.
At the more blunt end of the creature there is a large sucker; it uses
this as a foot to support itself in an upright position. If we examine
our specimen under water we shall see that its horny head is decorated
with a pair of fans, each one composed of about fifty threads. These
fans open and close with a rhythmic movement and, in doing so, attract
small floating water plants to the mouth. Just below the head there
is a single leg with a sucker foot. When the grub walks, it does so
by a looping movement, holding fast to its support with fore and hind
suckers alternately.
The next stage in the life of the young buffalo gnat is even more
curious. On the surface of some submerged leaf we shall probably be
able to find a number of the slipper-shaped pockets made of closely
woven silk, in which the insect spends the final portion of its life
before turning into the perfect fly. Within some of the pockets we
shall find the creature itself and it must be studied. Its head is
still ornamented with a pair of fans, but in this case they are gills
by means of which the insect breathes and not food scoops, for it has
reached a stage when food is not taken. On its tail end there is a
hook, by means of which it anchors itself to its slipper-shaped pocket.
Probably we shall find bubbles of air collecting round some of the
insects, within their pockets; as the time approaches nearer and nearer
for the change to the perfect insect to take place the bubbles grow
larger and larger. Eventually the fly emerges within a bubble, shakes
it free from the pocket and floats to the surface of the water without
wetting its wings.
The pond will supply insects quite as interesting as the running
stream. Here we may find the eggs of Caddis flies, enclosed in
jelly-like envelopes, rope-like, horse-shoe shaped or simple masses.
The caterpillar cases of these insects should be slightly magnified
and examined, for they are marvellously constructed of shells or sand,
pieces of stick, leaves or other vegetation.
We may come across the young stages of the common gnat. Its eggs are
very small and when the mother gnat lays them on the surface of ponds
she glues them together so that they form what is known as an “egg
raft.” The “raft” we shall notice, if we examine it carefully, is
composed of a large number of eggs; each egg is elongated, pointed
at one end and blunt at the other. Every egg is arranged with its
blunt end downwards for, from that end, the larvæ gnats make their way
into the water. A larva, it may be explained, is the stage in insect
development which follows the egg; the next stage is known as the
pupa. When the eggs are first laid they are white but, before they
hatch, they become darker and darker, till they are almost black. The
larvæ which hatch from the eggs we must examine under our microscope
and also in the water--they may easily be kept in a small jar of pond
water. If we study their habits carefully, we shall observe that they
float almost at right angles to the surface of the water and that,
while doing so, the tip of a little peg-like outgrowth on the tail
end of their bodies is thrust out of the water. The little peg is the
breathing organ of the gnat larva and the flaps which open and close
at its tip are worth examination. In a few days, the denizens of our
pond water will change their appearance and become comma shaped. On
their heads we shall see a pair of curious horns, which project out of
the water as the creatures, which have now become pupæ, float at the
surface. The horns are breathing organs. As we examine these pupæ, day
by day, we shall see the various parts of the complete gnat as they
develop within the body of the comma. Finally, under the microscope,
we can trace all the parts of the gnat. Darker and darker the little
creatures become, as development proceeds; at the same time they become
less active and less comma shaped. At length the time comes when they
straighten their tails somewhat violently, their skin splits along
the back and out comes the perfect gnat. We can use him for further
microscopic work, so we will not let him go. If he be a male, his
beautiful feathered feelers or, if a female, her thread-like feelers
will make good objects for us, the scales from head and body, the wings
and feet are all worth the time we may spend in examining them. The
mouth parts, too, are interesting but rather complicated.
Many insects are capable of emitting more or less musical sounds. In
some countries these so-called singing insects are kept as pets, in
other countries the same insects are voted a nuisance; it is all a
matter of taste. Of all musical insects, the most noted is the Cicada
and its sound organs are easily seen; they occur only in the male, for
the female never sings. The Cicada belongs to the same great order of
insects as the green fly, which we have already mentioned. There is one
British species, and our readers who visit the New Forest may come upon
it. On the under side of one of these insects the beak, very similar to
the beak of the green fly, may be plainly seen. On either side of the
insect, just below the bases of the wings there are two nearly round
discs. These discs cover the sound organs, which are two ear shaped
membranes. By means of muscles the insect can cause the membranes to
vibrate and thus produce the sound which once heard, can never be
forgotten.
More easily found insects, in this country, at any rate, are the
cricket and the grasshopper. The cricket we all know is a persistent
songster. Let us examine him closely. We shall find, that the house
cricket has two pairs of wings; the fore wings are leathery, the hind
wings membranous. If we watch a male in the act of chirping, the male
crickets like the male Cicadas are the songsters, we shall observe
that he moves his wings slightly. If now we examine a dead male, we
shall find, on the under surface of the forewing, a rough patch. Let us
examine this patch under the microscope, it reminds us of nothing so
much as a file: it is a file in fact, and sound is produced by rubbing
this rough file against a ridge, which we can easily see, on the upper
surface of the hind wing.
It would be a useless accomplishment for the cricket to be able to
sing, if there were no ears to hear its song. Nature has arranged that
his song shall not go unheard and if we examine a female cricket, that
is to say a cricket which has no sound-producing apparatus, we shall
find an oval depression, covered by a membrane, on each of her front
legs; these are her ears and they enable her to hear her mate calling
to her.
We have often heard the song of the grasshopper as we have walked
through the fields and he too will occupy our time for a few moments.
When he sings, he kicks his legs rather violently and this gives us a
clue to the situation of his vocal organs. The inside of each of his
hind thighs is ridged and the edge of each ridge is, as we can see
if we magnify it, rough like a file. This file-like ridge is rubbed
against a smooth ridge on the edge of the fore wing and the result is
the familiar note of the grasshopper.
This insect also has ears, but they are not easy to find unless they
are pointed out to us. If we examine a grasshopper we shall see that
its body is divided into three parts: a head, a solid portion from
which the wings and legs arise, the thorax and a portion (the abdomen)
made up of a number of rings or segments. On the sides of the first
of these rings, counting from the forward end, we shall find small
depressions covered with membrane, these are the ears. It is curious
that although many of the grasshoppers cannot give out a note, so far
as human ears can detect, they nearly all have ears; maybe there is a
grasshopper song which only grasshoppers can hear.
Very many other insects have sound organs, but they are nearly all
constructed on the same plan. It may seem surprising that sounds can be
produced by these simple means. Sound is really caused by waves in the
air and these insect vocal organs set up rapid sound waves, by their
vibration.
The microscopist should never be at a loss for objects derived from the
insect world: it is impossible to walk without treading upon some six
legged wayfarer. The wing cases of beetles are often of rare beauty,
some on account of their sculpturing, some because of a mantle of
scales.
In our greenhouse and garden we can find mealy bugs, curious little
powdered insects which do an enormous amount of damage. The fringe
wings, or Thrips, are common and destructive; examine their curious
feet and their beautifully fringed wings, the sight will repay you. And
lastly, if you number an insect enthusiast amongst your friends, enlist
his aid in gathering objects for your microscope.
CHAPTER XV
THE MICROSCOPE BY THE SEASIDE--ANIMAL LIFE
It is always surprising that the majority of microscopists never dream
of examining any of the hundreds of beautiful objects which can be
found by the seaside, in the course of an afternoon’s ramble. That
every pond will contain ample material for study, the microscopist
knows instinctively; insect life and plant life also he studies, but
the microscope is generally left at home when a visit is paid to the
seaside. A rocky coast is better than a sandy one, for rock pools yield
many objects, and the warmer southern waters of our coasts are better
than the colder northern seas, but the microscopist who finds himself
on a northern sandy coast need not despair, if he search diligently he
will find material enough to occupy him for many a day.
Nearly every rock pool will provide one or more Sea Anemones; it is
hardly necessary to describe these “flowers of the sea” as they have
been called, they are such familiar objects and the brilliant colouring
of some of them makes them highly attractive. In many respects Sea
Anemones resemble the _Hydra_, one of the pond dwellers, they are
rather more highly developed, however. Any Sea Anemone will serve our
purpose because we are about to examine the little darts with which its
tentacles, and even its body, are armed. If we find several different
kinds of Anemone, we must take the most transparent we can find and
also a small specimen; we can examine the larger, more opaque ones
later. Having transferred our specimen to a small jar, containing but
a small quantity of sea water, we wait till it has recovered from its
transfer and spread its tentacles, then it must be killed by one of the
methods suggested in our concluding chapter (see p. 306). One of the
tentacles must then be snipped off with scissors--some people cut off
the tentacle without killing the Anemone and the animal does not appear
to suffer a great amount of inconvenience, in fact a new tentacle soon
grows to take the place of the old one. We do not recommend promiscuous
vivisection. The tentacle is placed on a clean slide, a cover slip
placed over it and pressure is applied. An enormous number of little
thread-like darts are pressed from all parts of the tentacle. In some
cases, little oval capsules are squeezed out and, in the capsules,
the darts may be plainly seen, coiled up. On applying pressure to a
capsule, the contained dart will shoot forth, much as does a glove
finger turned inside out, when we blow violently into the glove. These
little darts are of the greatest interest to the microscopist; they
vary in shape according to the kind of Anemone, as we shall find if we
try this experiment with various Anemones. Some of them are straight
with stiff bristles at their bases; some have backwardly directed barbs
at their tips; others are apparently jointed, forming a zigzag, with
a short length of the dart going from left to right, the next short
length from right to left, and so on to the tip. It is marvellous how
the darts can be accommodated within the capsule, for the average
length of the latter is but 1/300 inch whereas its dart may measure 1/8
inch. These little threads contain a poison capable of paralyzing any
moderate-sized fishes which they touch.
Have you ever seen a “comb bearer” or as it is often called, a “marble
bleb?” Probably you have though you may not know its name. Sometimes it
occurs in rock pools, though more often it is found in one’s shrimping
net and occasionally it is washed up by the tide, but it does not live
long out of water. The “marble bleb,” as its name denotes, is an almost
globular mass of soft, transparent jelly. It is practically colourless,
with the exception of eight bright coloured bands which run from end to
end of the animal. To the naked eye, this little denizen of the sea is
of rare beauty: as an object for low-power microscopy it is entrancing.
When magnified, the bright bands are seen to be composed of rows of
flattened outgrowths. If our specimen is small enough to be examined
in water, its real beauty can only be seen in this manner, we shall
observe that the flattened outgrowths act like paddles, sometimes they
work all together, sometimes independently of one another and this fact
explains the marvellous evolutions of the “marble bleb” in water. Now
it shoots forward in a straight line with some rapidity, now it rolls
over and over and swims onward while doing so.
In the sun, it displays glorious iridescent colouring. At the hinder
end of the “bleb” we notice a pair of hollows: from these, as we watch
it swim, we shall see it suddenly shoot out a pair of long feathery
tendrils and they may be withdrawn into the hollows as suddenly. We
must make a point of examining the “marble bleb,” it is one of the gems
of our coasts.
Superficially the common, sponge-like “Dead man’s toes” or, to give it
its more pleasant title, “Mermaid’s fingers,” is a very drab affair. It
is a dirty-brown lobed, spongy mass with a leathery skin; when removed
from the water it loses all semblance of shape. In sea water, however,
if we examine it carefully, we shall see that it is studded with
beautiful little flower-like creatures, each one resembling a miniature
Sea Anemone. Examined, in water, under a low power of our microscope
we can see the water current flowing through the channels with which
it is perforated, after the manner of a sponge. If, now, we take a
dead specimen and cut it up, placing a small portion on a slide and
shredding it with a pair of needles, we shall find, when we magnify
the result of our work, that there are a number of minute mineral
spikes, called spicules, and very beautiful objects they make for the
microscope. There are many sea-side creatures, which we may find,
closely related to “Mermaid’s fingers,” they may all be treated in the
same way for they will all yield spicules which will repay us for our
trouble. All these spongy organisms are not provided with spicules as
ornaments, though one might be forgiven for thinking so, seeing how
decorative many of them are. Their presence is necessary to strengthen
the spongy material.
The specimen we have just examined is not one animal but a colony of
very minute animals. These colonies are very common, not only in salt
but also in fresh water. They serve a useful purpose, for the creatures
composing them are so minute that they would fare badly did they
dwell alone. Dwelling together as they do and each one contributing
its share to the building of the home they appear to thrive to a
wonderful degree. The coral islands are built, as to their foundations
at anyrate, by millions of very minute animals, living together in
colonies.
We mentioned the sponges a moment ago and many of them may be found
around our coasts; not the household sponges we know so well, but
much smaller, though equally interesting, colonies. Like the better
known sponges, those which we find on our shores are perforated with a
number of holes through which the water is driven by means of little
whip-like structures which line the cavities. Professor Grant has
graphically described his impressions at witnessing this water current
for the first time. “I put a small branch of the _Spongia Coalita_,” he
writes, “with some sea water in a watch glass, under the microscope,
and, on reflecting the light of a candle up through the fluid, I soon
perceived that there was some internal motion in the opaque particles
floating through the water. On moving the watch glass, so as to bring
one of the apertures on the side of the sponge fully into view, I
beheld, for the first time, the splendid spectacle of this living
fountain vomiting forth from a circular cavity an impetuous torrent
of liquid matter, and hurling along, in rapid succession, opaque
masses, which it strewed everywhere around. The beauty and novelty of
such a scene in the Animal Kingdom long arrested my attention; but
after twenty-five minutes of constant observation, I was obliged to
withdraw my eye from fatigue, without having seen the torrent for one
instant change its direction, or diminish, in the slightest degree, the
rapidity of its course. I continued to watch the same orifice, at short
intervals, for five hours, sometimes observing it for a quarter of an
hour at a time, but still the stream rolled on with a constant and even
velocity. About the end of this time, however, I observed the current
to become perceptibly languid ... and in one hour more the current had
entirely ceased.”
Frequently the sponge we examine may be found to be studded with many
yellowish spots; closer examination will show that these spots are
composed of very small jelly covered eggs. Later these eggs find their
way into the cavities of the sponge and are forced therefrom in the
currents of water. Each of the young sponges thus expelled is furnished
with a covering of little whips, by means of which it swims about till
it can find a suitable spot on which to anchor and complete its growth.
The Sea Anemone, which has already provided us with objects for our
microscope, has many near relatives which we must make a point of
examining, while we have the opportunity. Many of these creatures, or
rather their colonies, for they do not live singly, are to the naked
eye, strangely like seaweeds. A number of them are moss-like and may
be found on wooden breakwaters and similar situations when the tide is
low; they should be collected and examined and, to see them at their
best they should be examined under water. It is hardly necessary to
describe any one of these colonies in detail, for they are so numerous
that the one we described might not come into the hands of our readers
for a long time. In general characters they are all somewhat similar
so we will confine ourselves to generalities. For the most part, the
stems and branches of these colonies are of the thickness of thread. As
we watch them under the microscope we shall see that they are studded
with little cups and, presently, from each little cup there appears a
little tuft of tentacles which is waved about in the water. Each member
of the colony is similar to its neighbour and each one, again, is very
like the fresh water _Hydra_ with which we are familiar.
Of all the common objects of the sea shore one of the commonest
everywhere is the sea-mat. Nine people out of ten or, we might safely
say that everyone who had not learned its true nature, would guess it
to be a seaweed. As we find it washed up on the beach it is almost the
colour of sand, somewhat rough to the touch, whilst its whole surface
is pitted with minute holes. The sea-mat, when dry as we usually find
it, is a remnant of a colony of sea dwellers very similar to those
we have just described. From each little hole, in a living specimen,
which we can find without much difficulty, there appear the familiar
tentacles; each hole is the home of a minute hydra-like animal.
Hooke, whom we mentioned in our chapter on the History of the
Microscope, though a careful observer, was quite misled by the sea-mat;
he thought it was a seaweed, for he wrote: “I have not, among all
plants and vegetables I have yet observed, seen any one comparable to
this seaweed. It is a plant which grows upon the rocks under water
and increases and spreads itself into a great tuft, which is not only
handsomely branched into several leaves, but the whole surface is
covered over with a most curious knot of carved work, which consists of
a texture much resembling a honeycomb, for the whole surface on both
sides is covered over with a multitude of very small holes, being no
larger than so many holes made with a pin, and ranged in the neatest
and most delicate order imaginable, they being placed in the manner of
a quincunx, or very much like the rows of eyes of a fly, the rows or
orders being very regular which way soever they are observed. These
little holes, which to the eye look round, when magnified, appear very
regularly shaped holes, representing almost the shape of a round-toed
shoe, the hinder part of each being, as it were, turned in, or covered
by the toe of the next below it. These holes seemed walled about with
very thin and transparent substance, looking of a pale straw colour,
from the edge of which, against the middle of each hole, were sprouted
out four small, transparent, straw-coloured thorns, which seemed to
protect and cover those cavities.”
As a well-known author has remarked: “This is really a wonderfully
faithful description of the common sea-mat, and one cannot help
picturing the surprise and delight of old author Hooke, could he have
seen a portion of a living colony under a modern microscope.”
One of our finds may be the “Bird’s head.” It is a branched form,
quite unlike the sea-mat but it is of even greater interest. Under the
microscope, we shall see the many waving tentacles, but another feature
is sure to attract our attention, a feature which is responsible for
the popular name of the colony. On the outside of each cavity
containing a member of the colony there is a structure which resembles
nothing so much as a hawk-like bird’s head atop of a long neck. While
the tentacles wave in the water, the bird’s head snaps vigorously,
moved here and moved there. The birds’ heads, which might be mistaken
for parasites stealing food from the waving tentacles, really perform
the useful function of keeping them clean and warding off creatures
which might do them harm.
[Illustration:
_By the courtesy of Messrs. F. Davidson & Co._
1. A SECTION OF HUMAN SKIN
The corkscrew-like pores, leading from the sweat glands to the
surface, are plainly seen in the section.
2. THE FACE OF A FLY
A wonderful photograph, at 15 inches, taken by the micro-telescope.
Notice the very large size of the eyes relative to the rest of the
head.
]
Now let us pass to quite different though equally common sea shore
animals, the star fish. There are very many kinds but the common star
fish will serve our purpose well. We may make a beginning by examining
his back under a low magnification and observing that it is protected
by a number of hard plates which form a very efficient armour. At the
point where two of the rays (the finger like structures) arise we shall
notice a small flat plate, this too is worth a moment’s inspection, for
it is the water pore through which the star fish takes in water.
The under surface of the star fish shows us of course its mouth in
the centre of its body, the soft fleshy suckers which cover the rays,
with the exception of a narrow line down the centre of each one. At
the tip of each ray there is an eye; it may easily be distinguished by
its bright red colour and microscopic examination will show us that it
is quite unlike any of the other eyes we have examined. Over each eye
there is a little tentacle; these little tentacles may be called the
noses of the star fish, for, by means of them it is able to smell. They
are as unlike our idea of a nose as are the little pits on the feeler
of the cockchafer which we examined, yet these noses and ours all
perform the same duties. Our time will be well spent if we devote some
of it to a search for others less common star fish. Some of them are
really beautiful and whatever specimens we come across can be compared
with the common variety which is everywhere.
Closely related to the star fish are the sea-urchins. The relationship
may not be apparent at first sight but a careful study of an urchin
and a star will reveal many points of similarity. Our object in these
pages, however, is to find material for our microscope and not to
unfold the relationships of various members of the Animal Kingdom. When
we have learned to cut sections, we may try our hand at the spine of
a sea-urchin, it is an object well worthy of study. The hard shell of
the urchin may be examined under a low magnification, we shall see then
that his armour is far more highly developed than that of the star fish.
Another near relative of the two animals we have just described is
the sea slug or sea cucumber. Though an article of commerce of some
importance in the far East, the sea slug is not so common on our coasts
as its relations. We must make a point of finding a specimen, however,
for it provides us with one of the most remarkable objects for our
microscope that could possibly be imagined. One species, which goes by
the name of _Synapta Inhærens_, is the one most worthy of examination.
We must describe the creature first of all so that we may know what
to look for. It is aptly named sea cucumber for it is not unlike that
fruit--yes! fruit is correct, though the cucumber is more often called
a vegetable. The animal’s skin is tough and leathery and at the head
end there is a fringe of feathery tentacles. The sea cucumber must be
looked for amongst sea weeds or, maybe, he lies buried in the sand,
with only his fringe of tentacles on view. A friendly fisherman will
probably aid us in our search.
Having found our creature we must examine his leathery skin under the
microscope. To the touch it is evident that it is studded with some
flinty matter, but the microscope alone can show us the amazing beauty
of this armour. Under a low magnification, we can see, dotted over the
leathery skin, some nearly circular plates to each of which is attached
a little anchor. Now, from a dead animal of course, we must scrape away
some of these objects and examine them with a higher magnification.
Even the hardened microscopist will be delighted when he sees the
armour of the sea cucumber for the first time. Each anchor is hinged
to a little plate, each little plate, nearly circular in outline, is
perforated with seven holes, six round the circumference and one in the
centre and every perforation has a toothed margin. So perfect are these
minute plates and anchors, that the most intricate man-made machinery
could not have turned them out more perfectly to pattern. They are
precisely similar to one another in size and shape; as objects for the
microscope, even the sea with all its store of wonders, can offer us
nothing more marvellous.
We may number the sea mouse amongst our treasures of the sea-side.
Though called a mouse, on account of its curious movements and partly
perhaps because of its appearance, it is really a worm. It does not
appear to have the slightest resemblance to the common earth worm, nor
to the liver fluke which is described on another page, nevertheless,
it is related to both these creatures. The raiment of the sea mouse
is gorgeous in the extreme; on its back is soft brownish hair, its
sides are clothed with yellow and green hair, displaying a wonderful
iridescence and amongst the hair on the sides there are many stiff
brown bristles. Of the covering of the sea mouse it has been said:
“It is as if all the hues of the rainbow were collected there, making
this remarkable animal a living jewel, and truly worthy of the name of
Aphrodite, the Queen of Beauty.” The bristles of this creature we must
examine microscopically, they vary in structure according to the kind
of sea mouse, for there are several kinds, but in some of them they are
formidably barbed, in all of them they act as a protection.
Many other worms, a number of them un-wormlike in appearance will claim
our attention but we cannot devote more space to them here. There is
the common shell-binder, a curious worm, which builds for itself a
still more curious shelter of broken shells. Another worm, _Serpula_
by name, also lives in tube-like structures of its own manufacture and
is remarkable in that a row of nearly two thousand seven-toothed hooks
run along its back and all these thirteen thousand odd teeth are there
merely for the purpose of holding _Serpula_ in its tubular home.
Much interesting work may be done at the seaside in studying the young
stages of various familiar creatures. This work, however, necessitates
the keeping of the adults in an aquarium; for the young ones, in most
cases, are so unlike their parents that, if found swimming about on
their own account, they would never be recognised. Young barnacles,
for instance, have six legs, a tubular mouth and a single eye. At a
later stage they might be mistaken for young shell fish; they possess
a shell, not unlike that of the mussel, containing, however, not a
soft, fleshy mollusc but a six legged creature which swims through the
water in jerks, after the manner of a water flea. Strangely enough, it
is now provided with two stalked eyes like those of a crab, whilst of
mouth parts it has none, or they are so imperfectly developed as to
be useless for feeding. Soon this active youngster settles down for
the rest of its life and becomes a sedate and sedentary barnacle, with
one imperfectly developed eye and a mouth capable of feeding to good
purpose.
Young crabs are equally curious and also totally unlike their parents,
but these curious creatures are hardly accessible to those who only pay
a flying visit to the sea-side. As we have remarked, an aquarium is a
necessity and to keep marine animals inland is a feat beyond the powers
of the ordinary mortal.
The Sea Lemon or Doris is a curious little creature, worthy of
examination. Its habit is to feed upon sponges and strange diet it
is, for we remember that all sponges are fortified with hard flinty
structures, called spicules. This habit of the Sea Lemon is of use to
the microscopist, for the stomach of the creature is always laden with
the indigestible spicules and a very interesting collection of these
beautiful structures may be gathered together in this manner. The egg
masses of Doris may be looked for on rocks during the summer. Enormous
numbers of eggs are laid in a jelly-like mass. Some of this jelly may
be collected and examined under the microscope and, should we have
collected our material at a favourable moment, we may watch the eggs
hatch and observe the young Sea Lemons in their delicate transparent
shells swimming round and round the chamber within which they are
imprisoned during the very early stages of their lives.
Very frequently in the summer, when the seas are warm any agitation
of the water causes a beautiful phosphorescence to appear.
Phosphorescence, by the way, may be described as light without
heat and it is not uncommon in nature. Glow worms and fireflies are
phosphorescent; fish, also, in the dead state, often emit a certain
amount of light as do bones whilst, of course, phosphorous itself is
the best example of a naturally phosphorescent body. Phosphorescent
sea water, however, owes its peculiarity to myriads of minute animals
and they will afford us an interesting half hour with the microscope.
Let us collect a little of this water in a glass jar and take it into
a darkened room that we may the better see the phosphorescence. When
the water in the jar is undisturbed, we can see nothing unusual; if
we stir it or strike it a faint greenish light is given off, but it
does not last for long. Now, on taking the jar into the daylight, as
soon as our eyes are accustomed to the light, we shall just be able
to see that there are some very small living creatures on the surface
of the water. We must examine one, under a fairly high magnification;
it may be transferred, from the jar to a drop of water on our slide,
by means of a paint brush. The little animal which is responsible for
the phosphorescence of sea water is strangely reminiscent of an apple
with its stalk. It is, of course, very minute being only 1/60 inch
in diameter and its tail, which we have compared to the stalk of the
apple, is equal to the diameter in length. The whole creature is quite
transparent. As we watch it swim in our drop of water, we shall notice
that it propels itself by the lashing of its tail.
There are countless animals of the sea we have not so much as
mentioned, but the marine gardens contain plants so interesting and so
totally unlike those which live upon land that we must devote a few
pages to them also.
CHAPTER XVI
THE MICROSCOPE BY THE SEASIDE--PLANT LIFE
The plant life of the sea side may be divided into two natural groups
(i) of plants living on the shore near the sea and (ii) of plants
living in the sea, for part of each day at least. The former group
contains many plants of exactly the same kind as occur far inland,
together with a few typically sea-side plants such as Thrift or Sea
Pink. They are, however, one and all land plants. In this chapter we
shall confine ourselves to the real sea plants, the seaweeds.
Before we study any of these interesting plants under the microscope,
it will be useful to learn a little about seaweeds in general, because
they are so totally unlike land plants in every respect. They belong to
the great plant division known as the _Algæ_, to which also belong the
Diatoms, Desmids, _Volvox_, _Spirogyra_ and many of the other plants
we mentioned in our chapter on Pond Life. So we see that, although all
seaweeds are _Algæ_, not all _Algæ_ are seaweeds. They are higher in
the scale of development than Fungi, to which Bacteria, Yeast, Mould,
etc., belong, though they are not so highly developed as Ferns and not
nearly so advanced as flowering plants. A very short acquaintance with
_Algæ_ will show us that they are either green, brown or red. The green
_Algæ_ are nearly all fresh water forms, though a few are to be found
in the sea; on the other hand brown and red _Algæ_ are common in the
sea and rare in fresh water.
As we study our seaweeds on the shore, if we are really observant,
we shall notice that they live in zones or belts according to their
colour. There are exceptions to this rule but, in general, the green
seaweeds dwell in situations where they are only covered by the sea
at high tide; the red seaweeds are to be found mostly where they are
always below water and, between the two, the brown seaweeds occur.
In some parts, this colour scheme is very striking. Frequently red
seaweeds may be found above high-water mark it is true, but in such
cases they nearly always occur in rock pools and they are invariably
sheltered by brown seaweeds.
In our chapter on plant life we mentioned that many coloured plants
contained the green colouring matter, chlorophyll, just as do the
ordinary green leaves. We showed too that by boiling some green leaves
in methylated spirits we could extract the chlorophyll and that its
solution had the peculiar property of appearing green when light passes
through it and red when light is reflected from it. If now we take
almost any brown or red seaweed, we cannot see a trace of chlorophyll
anywhere. Let us leave our specimen, however, in fresh water for a
few days when we shall find that the brown or red colouring matter as
the case may be, is dissolved by the water and a green plant remains.
By treating the seaweed, deprived of its distinctive colour, with
methylated spirit as described above, we can obtain a solution of
chlorophyll.
The microscopist who is anxious to make a study of seaweeds, will
find little scope for his hobby on a sandy shore. Just as the most
interesting marine animals are to be found where rocks abound, so must
we hunt in similar situations for our _Algæ_. A few thread like _Algæ_
are able to anchor themselves to the sand but most of them require
a substantial support. A bare rock is a much favoured situation and
before we have learned the peculiarities of these plants we may marvel
how they obtain any sustenance from so barren a resting place. As a
matter of fact they derive no nourishment from the rocks on which they
rest. The part of the sea weed which, in our ignorance, we may have
dubbed a root, is nothing of the kind. It bears no relationship to the
roots of higher plants and is a mere anchor, designed to fasten its
owner to a support.
None of these plants have roots, none have true stems or leaves, though
the parts resembling stems and leaves are often so called; none of them
flower and so fruits and seeds are unknown to them. Their food is
absorbed from the sea water over the whole of their surface.
We may well begin our study of the seaweeds with an examination of the
external structure of as many different kinds as we can find. Some of
them are flattened and very thin forms and of them the Sea Lettuce may
be taken as typical. This plant, known to scientists as _Ulva Lactuca_,
occurs at high-water mark. In its fully developed form it is pale
green and so thin as to be almost transparent; its structure may be
studied under the microscope without difficulty. Then there is the very
common, green, Compressed Enteromorpha which grows in great profusion
on the rocks of the shore, rendering them exceedingly slippery. The
closely related Intestinal Enteromorpha as it floats in the water
resembles a green, membranous tube and those of us who have ever done
any zoological dissection will appreciate how well named this plant is.
The structure of both the Enteromorphas can easily be seen. Many of the
brown and red _Algæ_ will provide us with a good deal of occupation
in making out their structure. Some of them, the brown _Ectocarpus
Siliculosus_ for example, may be found, growing in moss like tufts,
which are usually attached to one of the larger _Algæ_ living between
the tide marks. It is one of the simplest of the brown sea weeds,
consisting of branched threads, but one cell in thickness. The Wracks,
of which the Common Bladder Wrack or Pop weed with its little air
filled bladders is familiar to everyone, are more complicated in
structure, in fact they appear to be possessed of stems and leaves, but
we shall return to them in a moment.
Most of the common red _Algæ_ are so delicate in structure that they
require a fairly high magnification for their examination. The thin,
membranous fronds of the beautiful crimson _Delessera Sanguinea_, may
be sought below low tide limit or may be found washed up upon the
shore. Superficially the plant resembles a red hart’s-tongue fern, with
much more delicate fronds than ever fern of that species possessed. We
may well compare its structure with that of the sea lettuce, for it is
equally transparent.
In the rock pools of many parts of the coast we may happen upon a
most curious almost white sea weed, known as Coralline or _Corallina
Officinalis_. Its branched, feathery stems are hard and stony and the
whole plant bears a superficial resemblance to a coral, hence its name.
The plant absorbs a substance known as calcium carbonate from the sea
water and deposits it in the form of a hard, stony covering over its
surface. Calcium carbonate does not occur in sea water everywhere, at
least not in sufficient quantity to be of use to the Coralline, that is
the reason the sea weed is not quite so common as some of the others we
have mentioned. The curious armour-plating of this sea dweller, should
be studied under the microscope.
The chief scientific interest of the sea weeds, however, lies in
their mode of increase, it is so totally different from that of any
of the higher plants. The most simple method of increase is known as
vegetative reproduction, it does not occur in every kind of seaweed
and is nothing more or less than the growth of a broken piece of plant
into a new individual. This form of increase is not unknown higher in
the plant world; begonia leaves may be induced to send forth roots and
grow into new plants, many garden favourites are propagated by means
of cuttings and both these methods are similar to the breaking away
and growth of portions of a seaweed; the garden plants, however, are
assisted by man, the seaweed does its own work.
The simplest forms of increase occur amongst those giants of the
sea, the Laminarias or Tangles as they are often called. These brown
seaweeds often attain enormous sizes, they all grow below the limits of
low tide and appear to thrive best where the water is frequently lashed
by storms. To see these plants at their best we must look down upon
them in their watery home. There are spots on the North-Eastern coast
of Ireland, where one may look from the cliffs upon a veritable forest
of Tangles. There thrives the “Devil’s Apron,” short of stem but with
a flat ribbon of a frond, which may attain a length of a dozen feet
and a width of as many inches. There too we can behold the Fingered
Tangle, with stem, maybe, six feet in length and a crown of large
finger-like fronds, “Sea Laces” or “Dead Men’s Ropes,” with fronds
resembling slender ropes, in length, at times as much as forty feet,
ride gracefully on the ever changing currents. Safely hidden in this
marine forest lurk queer fishes and crabs and shell fish. About the
broad fronds of the “Devil’s Apron” sea mice and sea cucumbers disport
themselves; the Tangle home is a paradise for marine life. Yet with all
their vigorous growth they increase simply by liberating spores which
give rise to new plants.
In our chapter on Plant Life we described spores very briefly; we said
that from a strictly scientific point of view they were not comparable
to seeds but that for our purpose they might be looked upon as seeds
for the reason that, by their germination, new plants were formed.
All the spores of land plants are minute, they are carried from the
mother plant to suitable spots for germination by wind. The spores of
seaweeds also are small, but they are very different to the little
wind-blown, land-dwelling spores. They possess a pair of the curious
little whip-like structures we have observed in so many water plants
and animals. By the lashing of these little whips they are able to swim
about in the water till they find a suitable spot to settle down and
grow into plants similar to those whence they came. On account of their
animal-like movements they are called zoospores.
The formation of zoospores may be easily observed in the brown sea weed
_Ectocarpus Siliculosus_ we have already mentioned. This plant, as we
have already remarked, consists of thin, thread-like rows of single
cells and from time to time it is branched. At certain periods of the
year, moderately large, pear shaped swellings occur on the threads of
the sea-weed. If we are either fortunate or exceptionally patient we
may chance to be examining one of these swellings under the microscope
at the moment when it bursts and sets free its contents. Should we have
this good fortune we must hasten to magnify more highly the zoospores
which have escaped from the pear shaped spore case. Here we may add the
caution that we shall only witness the bursting of the spore case if
we examine our specimen in sea water; we should require more than the
patience of Job to watch for its bursting in the dry state, for it will
never come to pass.
A careful study of a zoospore will show that it swims in a peculiar
manner. One of the little whips is directed forwards, the other
trails behind. After a short period of activity the zoospore comes to
rest, loses all means of propulsion, germinates and grows into a new
_Ectocarpus_ plant.
Sometimes this _Algæ_ reaches a low ebb of vitality, it requires a
new lease of life as it were, when this state is reached another
form of increase takes place. The events in this case may also be
witnessed under the microscope. From very similar spore cases a number
of zoospores are liberated and for a time they swim about freely. If
now we watch carefully we shall notice that one of the active little
bodies comes to rest and that the others lose no time in swarming round
it. One of these swarming zoospores fuses with the individual which
first ceased swimming about, with the result that a much larger,
non-swimming individual is formed which, after a short resting period,
germinates and grows into a new sea weed. The remainder of the
zoospores will come to rest later and germinate to form new plants just
as though no fusion had taken place with two of their number. Here we
see the beginnings of male and female increase amongst sea weeds; the
individual which first comes to rest is looked upon as the female and
the one with which it fuses as the male.
[Illustration:
_By the courtesy of Messrs. F. Davidson & Co._
THE FEELER OF A COCKCHAFER
The end of the feeler consists of a number of plates, which can be
spread fanwise. The pores visible on the plates are the insect’s
organs of smell.
]
Amongst the Wracks, of which there are a number of kinds, the methods
of increase reach a higher stage. First of all let us describe the
plants, so that we may know what to look for. They all belong to the
group of brown _Algæ_. The “Channelled Wrack” is, when fully grown,
about six inches long. It is much branched, often almost yellowish in
colour and grows just below high-water mark. Along one side of the
plant there is a moderately deep groove. Here we may note that the
Wracks grow in zones from just below high water mark to low water mark.
A little nearer the sea than the haunts of the Channelled Wrack, we
shall find the Flat and Bladder Wracks. The former is but six inches
or so in length, with flat, forked fronds, along the centres of which
runs a single rib. The Bladder Wrack varies considerably in size. It
may be smaller than either of the Wracks we have already mentioned or
it may be two or three feet in length. It is the one seaweed familiar
to everybody. Nearer to low tide mark we shall encounter the Knobbed
Wrack, greenish brown in colour and often as much as six feet in
length. It is so named because from the sides of its flat, leathery,
strap-like fronds, there arise little stalked bladders. Right at,
and often beyond, low tide mark there dwells the Notched Wrack; very
similar to, though larger than, the Flat Wrack, from which it may
easily be distinguished by the fact that the edges of its fronds are
toothed, after the manner of a saw.
It is obvious that the structure of any one of these Wracks is much
more complicated than is the case with _Ectocarpus_. The latter Alga
was composed of a number of cells, similar to one another in every
respect except size. If we tease a stem or a frond of one of the Wracks
upon a slide and examine the result of our efforts under the microscope
we shall see that the cells which compose the Wrack are not by any
means similar to one another. Those of us who have mastered the, by
no means difficult, art of section cutting, should cut sections of
stem and frond and compare them with sections of leaf and stem of some
higher plant. The comparison will show us that, although the seaweed
does not exhibit the complicated structure of a flowering plant, it
has at least three kinds of different cells--an outer layer, a central
structure and an intermediate layer.
If we secure a specimen of the Channelled Wrack, at the end of the
summer, we shall notice that the tips of certain of the fronds are
swollen. Examination of these swellings under a low magnification will
reveal a number of wart-like structures, and at the end of each wart
there is a little pore. If we open up one of these little warts, very
carefully with our mounted needles, we shall find that each little pore
opens into a cavity, within which we can find two kinds of structures,
hidden amongst a number of hair like growths. We shall see a number of
dark, oval bodies, at the base of the hairs, these are the egg cells;
more careful search will show us a number of much branched structures
also partly concealed by the hairs, these are the male organs of the
plants. The purpose of the hairs, by the way, is to keep the little
chamber moist, when the plant is left high and dry. If we watch the
pores of the other warts carefully we may be fortunate enough to see
the process of increase taking place, for it occurs outside the plant
and not within the chamber. The egg cell divides into two and its
contents pass out of the chamber by way of the pore; each cell of the
male organs gives rise to sixty-four oval little structures, each
provided with a pair of minute whip-like threads by means of which
it swims from the chamber and goes in search of the egg cells. Many
of these little navigators are lost by the way but one of them will
reach and fuse with each egg cell. After fusion the new-formed cell
germinates at once into a new Channelled Wrack.
That this is an advance is shown by the fact that the little swimming
bodies which fail to fuse with the egg cell, do not develop into new
plants as in _Ectocarpus_ nor does the egg cell, which has failed to
fuse with a swimming body, germinate.
In the Bladder Wrack, a very similar process takes place. There are,
however, certain important differences, differences which show that the
plant is still more highly developed. If we examine the cavities, in
the little warts of the Bladder Wrack we shall find that some of them
contain egg cells, some contain male organs but none contain both. We
noticed that the egg cell of the Channelled Wrack produced two eggs,
that of the Bladder Wrack produces eight. In other respects the two
plants behave similarly.
The methods of increase amongst the red seaweeds are rather more
complicated and as our object is to interest and not to puzzle
our readers we will content ourselves with a few general remarks.
Microscopists who are anxious to probe more deeply into the subject
will soon devise ways and means for themselves. The little swimming
bodies which lend an added attraction to the study of the brown
seaweeds are replaced, amongst the red _Algæ_, by organisms with but
one whip-like structure apiece and that without the power of propelling
its owner through the water. As with _Ectocarpus_, increase may take
place in two ways. On these red plants we may find the now familiar
swellings, which we have learned to know are spore cases, but instead
of the multitudes of free swimming organisms which are set free on
the bursting of the brown sea weed spore case, we now witness the
expulsion of but four inert spores, which settle down in the water and
immediately grow into new plants.
In the second method of increase, where male and female organs are
concerned, we find that both these structures grow on the outside
of the plant and not in cavities. Let us take the common, pink,
much branched seaweed, known by the fearsome name of _Callithamnion
Corymbosum_ as our example. The male organs grow in little fungus
like tufts about the branches of the plant and they give off enormous
numbers of little organisms which have no power of swimming to the
female organs. Either on the same or on another plant we shall find
the female organs; we need not describe them in detail but there is
one point of very great interest. From each of the female organs there
grows a long jelly-like hair. As we have remarked, the organisms set
free by the male organs cannot swim about but float aimlessly in the
water. Obviously the majority of them simply perish, one perchance may
touch a sticky hair to which it adheres, with which it fuses and passes
down to the female cell, resulting in the production of a new seaweed.
It may be remembered that in writing about the pollen grains of
flowering plants, we mentioned that those plants dependent upon
wind for the distribution of their pollen, have stigmas ingeniously
contrived for catching and retaining the grains. It is curious that the
red seaweeds should have very similar contrivances for capturing and
retaining the male cells.
The sea will also provide us with a rich harvest of those beautiful
microscopic objects, the Diatoms. They may be sought on seaweeds, their
yellowish brown colour often betrays their whereabouts, on rocks and
sand and in mud. The salt water forms are as varied and as beautiful as
their fresh water cousins.
To the microscopist who merely uses his microscope for the pleasure he
can derive from it, rather than for serious study, it may appear that
the plant life of the sea falls short of the animal life as far as
interest is concerned. He may disabuse himself at once of this idea.
There is no class of plants more interesting than the seaweeds and in
few branches of plant life is there greater scope for new discoveries.
The garden of the sea is largely an unexplored territory and there
is no coast-line in the world of equal extent which provides so many
different sea dwelling plants as our own.
CHAPTER XVII
THE MICRO-TELESCOPE AND SUPER MICROSCOPE
Those of our readers who have borne with us thus far may quite
excusably have thought that the last word had been attained in the
construction of the microscope. It is true that different makers
have made various improvements to their instruments, from time to
time in recent years, most of them of minor importance but useful
in the aggregate. But a few years ago, however, the advent of
the Micro-Telescope and Super-Microscope marked an epoch in the
manufacture of the microscope. We have shown that great strides
were made in scientific investigation when the first simple lenses
were manufactured, that there was a lull in microscopic research
till the appearance of the compound microscope and now, when the
latest instruments are in the hands of scientific workers, further
possibilities are opened up.
In all microscopes--the best as well as the cheaper instruments--there
is one failing which very early forces itself upon the user. They
have very little “depth of focus.” Let us explain exactly what the
phrase means. Once or twice in our pages, we have recommended that the
fine adjustment should be rotated to and fro while certain objects
are being examined. When potato starch grains, for instance, are
magnified sufficiently highly to show their characteristic markings,
the whole of the grain cannot be seen clearly at one time, because
at that magnification the “depth of focus” is slight. The higher the
magnification the less is the “depth of focus”; when this quality is
absent altogether only one plane of an object can be viewed clearly
without re-adjusting the focus. With low magnifications, we may, to a
limited extent, have more than one plane of an object in focus.
The same question of “depth of focus” occurs in photography and perhaps
an example showing how it affects the camera user may make the matter
clearer. Suppose we wish to photograph a landscape having, let us say,
a tree in the foreground, a cottage in the mid-distance and a hill in
the distance. If our lens is one of large aperture, that is to say
admits a considerable amount of light and is also what is known as a
long focus lens we shall find, when we view the scene on the ground
glass, that when the tree is sharply focussed, the cottage and hill
are not clear. When we rack in the camera to get the cottage sharply
focussed, the tree and hill will be un-sharp. Similarly when we focus
on the hill the tree and cottage remain out of focus. The reason is
that the lens in our case possesses little “depth of focus.” The
experienced landscape photographer, did he wish all three objects to
appear equally sharp could easily attain his object. He would perform
the simple operation known as stopping down his lens, that is to say
he would gradually close its diaphragm, while viewing the scene on
the ground glass. At a certain point everything would be sharp, from
foreground to distance. At the smaller aperture of the lens, caused
by closing the diaphragm, the depth of focus would be considerably
increased, at the same time much less light would be admitted to the
camera. Looking upon our object, under the microscope, as comparable
to a landscape, seen on the ground glass focussing screen of a camera,
it is obvious that, unless our object has no thickness, and this is
impossible, we cannot highly magnify its upper and lower surfaces
at one and the same time. There are no adjustable diaphragms in the
objective so our only course is to examine the two surfaces in turn or
to resort to a lower magnification.
[Illustration:
_By the courtesy of Messrs. F. Davidson & Co._
THE MICRO-TELESCOPE
1. View taken with ordinary camera. The arrow shows the building
illustrated in the plate below; it is 3/4 mile away.
2. The building indicated by the arrow in the plate above, taken from
the same standpoint through the micro-telescope.
]
Apart from any other consideration, the super microscope marks a big
advance from the fact that it possesses great “depth of focus.” It is
possible, for example, with this remarkable instrument to examine a
moss as it grows, with a high magnification and see not a portion of
a leaf or a fragment of the stalk, as with the ordinary microscope,
but the whole upstanding plant, in stereoscopic relief. It shows us
objects exactly as we should see them were we endowed with super eyes,
enormously enlarged, in relief and erect. Objects viewed through this
instrument are not inverted, as with the ordinary microscope.
[Illustration]
A is the microscope proper and is, in all respects, similar to the
instrument with which we are familiar, except that its mirror and
condenser have been removed. In the fitting provided for the condenser
a second microscope is arranged.
[Illustration]
This consists of a tube D, an objective F, an ocular C and an inner
sliding tube E, the whole fits into the metal case B, G is a stage on
which the object is placed and below G, if necessary, a condenser may
be fitted.
The instrument owes its remarkable magnifying powers to the fact that
the additional microscope B, forms a magnified image of the object on
the stage G, at the opening in the stage of the microscope A. This
magnified image is still further enlarged by the original microscope.
In other words, the power of one microscope is employed on that of
another.
The apparatus as we have described it gives enormous magnifications
and it is possible, by using a suitable combination of objectives,
to obtain a magnification of ten thousand diameters. Expressed in
non-technical language, a circle whose actual diameter is equal to
the thickness of the paper upon which these words are printed, could
be so magnified by the super-microscope that it would appear to be as
thick as ten thousand similar pieces of paper but, with this enormous
magnification, it is obvious that, even with such a marvellous
instrument, the whole of the circle from edge to edge could not be
seen at one time. With an ordinary microscope a magnification of one
thousand three hundred diameters is considered highly satisfactory.
So that there is a great probability of being able to see some of
the minute objects which are known to exist, but which have, up to
the present, eluded those who would view them, on account of their
minuteness. There is a possibility also of discovering a new underworld
of which no man has yet dreamed. The man who uses his microscope solely
because of the pleasure he derives from it, rather than he who uses it
because it is essential to his business or profession, will be more
attracted by the micro-telescope. By this we do not infer that the
instrument is not useful, as a fact it is of the greatest importance in
certain cases, especially for Nature-study work, for the observation
of minerals and to the chemist.
Like its sister instrument, the micro-telescope consists of an ordinary
microscope to which is attached a specially designed object glass in
a tube, which take the place of the condenser. The makers, indeed,
provide two of these object glasses--one for viewing objects from one
to two feet away, the other for viewing objects from a distance of
three feet to the planets, if need be. Once when the instrument was
being tested some crumbs were placed on the floor at a distance of
four yards and strongly illuminated, and the microscope with a 1-inch
objective focussed on the crumbs. With this objective in an ordinary
microscope the magnification would be about thirty diameters. Presently
some mice came out, and made themselves at home with the crumbs. The
mice could be examined at this distance, without their being aware of
it, so well that individual hairs were easily visible and about half a
mouse was in the field of view. In point of size each mouse appeared
about the same as a beaver within a foot or two.
Messrs F. Davidson & Co., 29 Great Portland Street, London, are well
known for the high state of perfection to which they have brought the
micro-telescope and other instruments connected with microscopy.
The novelty of the micro-telescope will appeal strongly to the Nature
lover. At a distance of a few feet a spider can be magnified to the
size of a large cat, and it can be watched spinning its web with
spinnerettes the size of teacups. Ants at a distance of six feet are
seen to be fearsome individuals, six inches in length, and their
tiny burdens are so magnified that they appear like yule logs or
goodly-proportioned boulders, according to their nature. At a distance
of ten feet a wasp may be seen scraping tiny shavings of wood from oak
palings by means of its jaws--shavings which it converts into paper
for building its nest. Very small insects may be observed as they come
into the world from their chrysalis stage. The never-tiring jaws of
the caterpillar may be seen at work devouring some favourite leaf--the
whole action of biting and swallowing the vegetable matter can be
plainly seen. Such interesting events as the tending of green flies by
ants, the leaf-cutting habits of the leaf-cutter bee, and a hundred and
one other events are all revealed by the micro-telescope and at such a
distance that the living objects are not disturbed in their activities,
being quite unaware that they are under observation. To the botanist
the instrument is no less useful.
In various manufactures, such as ore smelting, and in the manufacture
of glass, china and pottery, in enamelling and in certain engineering
shops, where it is necessary to examine material at a high temperature,
the micro-telescope is a great boon, and, at least, is the means
of avoiding considerable physical discomfort. It is used also by
architects and surveyors for examining the condition of the factory
chimneys, bridges, derricks and the like. To the engineer the
instrument is invaluable; we have explained elsewhere how necessary it
is that various metals and mixtures of metals should be examined for
fractures. Under the high magnification with the ordinary microscope,
even with an instrument specially designed for the work, only a
very small area can be studied at once. With the micro-telescope a
relatively large area may be examined under a high magnifying power.
There are three features of this ingenious apparatus which cannot fail
to commend themselves equally to the casual worker and the serious
microscopist. All objects are seen erect, just as the eye sees them.
This is brought about because, as in the case of the super-microscope,
we really observe a magnified, inverted image of our object, formed at
the spot where the object would be placed were we using an ordinary
microscope. This image is inverted once more in the microscope and so
it appears to us erect.
All objects appear in relief and in their proper planes; this is seen
in a striking manner by viewing an ordinary photograph through the
apparatus, when the various parts stand out as in nature. There is also
enormous depth of focus. With the attachment for viewing objects at a
short distance, the whole of a tubular shaped flower such as a daffodil
may be in focus at once, from the tip of the petals to the bottom of
the tube. With the attachment designed for long distance work, objects
from twenty yards to sixty miles may be clearly viewed at one and the
same time.
A few of the unsolved problems confronting the microscopist are
reviewed in our concluding chapter. Whether they will ever be solved
we dare not venture to say; but, if they are to be, surely to this
latest arrival in the microscopic world and to its companion, the
super-microscope, the honours will go.
CHAPTER XVIII
CHEMISTRY AND THE MICROSCOPE
To thoroughly comprehend the various uses to which the chemist may put
his microscope, it is necessary to have a knowledge of chemistry. The
science is so wide in its scope that no single chapter could do justice
to it. There are analytical chemists, scientists whose aim is to find
out the composition of various substances; biological chemists who deal
with the many problems of life in which chemistry plays a part, but we
need not attempt to detail all the branches of this highly specialised
science.
Chemical analysis is founded upon the fact that when certain
chemicals are mixed together they will, or they may, unite to form
quite a different chemical. This newly formed chemical is probably
a different colour to the substances which were used in its making,
or again the original chemicals may be soluble in water and the new
chemical insoluble, in which case it will form a cloudiness known as
a precipitate. An example may help to make our description clearer.
Suppose we take some common table salt and dissolve it in a little
water in a glass, then add to this a little solution of nitrate of
silver, which is sold under the name of lunar caustic, we shall find
that a white cloudiness is formed when the liquids mix, although
originally they were both clear. The reason of this cloudiness is that
the two substances, dissolved in water have united with one another to
form a third substance which will not dissolve, therefore it settles
down as a fine powder. Long experience has taught analytical chemists
exactly what chemicals to add to test for all the common substances, by
the formation of these precipitates; so that, if any powder were given
to one of these scientists he could tell its composition by applying
certain tests. In this chemical analysis considerable quantities are
required and it is often necessary to test very small samples, so small
that the ordinary methods are out of the question. This is where the
microscope scores, because with this wonderful instrument, only drops
are required and tests of corresponding delicacy may be made; in fact,
by modern microscopic methods it is possible to detect the presence of
as little as ten thousandth part of a grain of arsenic, or quicksilver
or of the deadly poisons, Strychnine or Prussic Acid.
In testing for poisons the microscope is invaluable. Frequently only
the most minute traces of the poison occur in the system and the modern
microscopist who makes a study of poisons and their detection can
solve mysteries which would have baffled all the scientists in the
world in days gone by.
Our chemical studies with the microscope may well begin with various
common crystals; they are usually easy to prepare; the process of
crystallization is always interesting to watch, and as objects for the
microscope it would be difficult to find anything more beautiful than
these home-made gems. All crystals should be examined by reflected
as well as by transmitted light. When we are working with the former
lighting, a piece of black paper beneath the slide will help to show up
the objects to better advantage.
The easiest method of obtaining crystals of any substance which is
soluble in water is to make a saturated solution in this liquid, to
put a small drop upon a slide, then to tilt the microscope slightly so
that there is a thin film of solution at the upper side of the drop.
The microscope must not be tilted so much that the liquid runs from the
slide on to the stage. Where the film of liquid is thin crystals will
be found first. A word of explanation is necessary concerning the term
saturated solution, especially as we may have occasion to use it many
times. When we add a solid to a liquid in which it is soluble, we shall
find that the liquid will take up a certain amount of the solid and no
more; when, on the addition of more solid it fails to dissolve, we have
reached the saturation point. A saturated solution then is a liquid in
which the maximum amount of solid is dissolved.
We have already described the most simple way in which crystals may
be formed, and we may easily make objects for our microscope in
this manner of all the substances we can find which are soluble in
water. With some we shall find that crystals do not form easily, in
which event we may modify our tactics and warm one drop of saturated
solution on the slide till nearly all the moisture is driven off, then
there should be no difficulty in watching the crystals in process
of formation. Common salt, sugar, alum, borax, washing soda, iron
sulphate, called also green vitriol and copper sulphate, or blue
vitriol, are common and easily obtained substances, all soluble in
water. As we carry our experiments a little further we shall find that
crystals formed from cold solutions as suggested in our first method
differ from those formed from hot solutions. Again, if we use some
other substance than water as our solvent the crystals which separate
out will differ once more. Many very interesting experiments may be
tried on these lines.
Many beautiful crystals may be obtained by dissolving various
substances in gelatine or gum. The method is simple. Gently warm
a little gelatine, to which is added an equal volume of water, in
a chemist’s test-tube. When the gelatine has all dissolved make a
saturated solution of the substance, from which crystals are derived.
Green or blue vitriol are good subjects for the experiment. Add a
little of the saturated solution to the gelatine and stir with a glass
rod, taking care to avoid the formation of air bubbles. A little of
the mixture may now be placed in a thin film on a slide covered with a
cover slip and left to cool. Examination when cool under the microscope
will show lovely fern-like crystals, whose beauty rivals that of ice
crystals familiar to us on our window panes during hard weather in the
winter. Barium chloride also produces very beautiful fern-like crystals
when treated in this manner. Chlorate of potash, familiar to most of
us as a remedy for sore throat, forms crystals totally dissimilar to
those substances we have named. Having made as many crystals as we wish
from water and also from gelatine solutions, we may turn our attention
to gum arabic. The method of obtaining crystals from this substance
is exactly similar to that described for gelatine except, of course,
that we substitute gum for gelatine. This work is of the greatest
interest, for not only does it yield wonderfully beautiful objects for
our microscope, but it is a hobby full of surprises. When we are about
to examine a new crystal for the first time we can never so much as
hazard what shape it will assume. Sugar, it may be mentioned, does not
crystallize at once from a saturated solution in water, and the best
method of obtaining the crystals is to warm the slide, on which we have
placed a drop of solution, and then when dry to set aside for a day; at
the end of that time, especially if the air be moist, the crystals will
have formed.
We shall find it interesting to try experiments in mixing two different
substances, then we shall probably obtain crystals totally unlike those
of either of the ingredients. As an example of this method, let us
make a saturated solution of a mixture of blue vitriol and magnesium
sulphate in water. Place a drop of the solution on a slide, heat over
a flame, not only till dry, but till the substance left on the slide
begins to melt. We must use every care not to crack the slide, and this
may best be accomplished by keeping it moving while over the flame.
Now if we watch our object through our microscope we can witness the
formation of wonderful feathery crystals, as the slide cools.
Some strikingly beautiful results may be obtained in another manner;
the method is used by analysts in their so-called fusion tests. We
take a small grain of some substance, say, lead nitrate, place it in
the centre of our slide, cover with a cover slip, and warm over a
flame till it melts. Then, taking a similar-sized grain of another
substance, such as saltpetre, we place it against the edge of the cover
slip on the side opposite to the lead nitrate. Further warming will
cause the saltpetre to melt, and run below the slide and mix with the
lead nitrate. If we watch the meeting of the two chemicals a wonderful
sight will reward us. The lead compound forms a “beautiful crystalline
skeleton,” whilst the saltpetre forms six-sided stars at the opposite
side of the cover slip. The experiment may be repeated, using lunar
caustic and saltpetre, also the iodides of silver and potassium; in
fact we may try any chemical substances we have at hand, though we
shall find that some do not melt very easily, and potassium chlorate
is somewhat dangerous, for it forms explosive compounds with certain
substances.
The number of interesting and beautiful objects which may be obtained
by so-called solution tests is practically unlimited, and the
enthusiastic microscopist will certainly try all the tests we describe
as well as many others of his own invention. Should the chemist wish to
detect the presence of aluminium in very small quantities, he relies on
his microscope and proceeds in the following manner. He takes a drop of
the solution, suspected of containing alum, and adds to it a drop of
sulphuric acid. This mixture he puts on his slide, which he heats over
a flame till dry. Now he adds a drop of water, and then a very small
amount of calcium chloride is brought to the edge of the water. Beneath
his microscope he can now watch the formation of beautiful colourless,
eight-sided crystals which denote the presence of aluminium.
A word of warning is necessary concerning this and the following
experiments; they may not always succeed, for success largely depends
upon having the solutions at the right strength, and experience alone
can teach us what is correct. Interesting and easily formed crystals
may be obtained from barium sulphate, and the experiment may also be
made to show the phenomenon which we have already mentioned, that the
form of the crystals depends on the nature of their solution. A little
barium sulphate should be dissolved in strong sulphuric acid. Here, by
the way, another warning: every care should be taken in the use of all
acids; they should never be allowed to come into contact with face,
hands or clothes, nor should they touch any part of the microscope.
Some acids give off fumes, and these should not be allowed to reach
the eyes or nose, and the microscope must be protected from them. To
continue our experiment: While hot a drop of the solution of barium
sulphate in sulphuric acid should be transferred by means of a glass
rod to a slide and allowed to cool. Examination with the microscope
will show that the barium sulphate has formed small rectangular
scales. With the remainder of the sulphuric acid we now make a
saturated solution of barium sulphate, and find, on repeating the
method described above, that the chemical has formed curious x-shaped
crystals. Similar experiments may be tried with calcium sulphate with
the certainty of interesting results.
Many crystals of calcium, in the form of calcium oxalate, may be
found in plants, and they are well worth looking for. They may best
be seen in sections of the plants, but, if we have not mastered the
art of cutting sections, we may find them by teasing the plant cells
apart with our mounted needles. In the stems of rhubarb there is the
substance in bundles of long needle-shaped crystals, to which the name
of raphides has been given. In the seed of the garden poppy, just below
the skin, there is a layer known as the crystal layer, where crystals
of calcium oxalate occur as tiny balls called crystal sand. In the leaf
stalks of begonias very beautiful and occasionally very large crystals
of this substance may be found, whilst in shapes they are strikingly
varied. In orris root there are enormous crystals of calcium oxalate;
in fact it is common in many plants.
If we have a photographic friend who will supply us with quite a small
quantity of gold chloride, we shall be in a position to try three most
interesting experiments and to obtain some curious crystals. We require
a very weak solution of gold chloride in water, not more than 3.5 per
cent. for our first experiment. Mix one drop of this solution with the
same quantity of hydrochloric acid on a slide and heat over a flame
till dry. The microscope will now show us probably the most curious
crystals we have ever examined; some are long, some short, and some a
zigzag in form; mixed with these there will be a few flat plate-like
crystals with rectangular projections. All these curious crystals are
yellow.
If instead of hydrochloric acid we use a solution of common salt in
water and repeat the experiment as before, we shall obtain pale yellow
prisms and some crystals of common salt. Gold is costly, so it is
perhaps lucky that one of the tests for this rare metal is one of the
most delicate known to chemists; it is possible, in fact, to detect
very minute quantities of gold. For this experiment we may take an
exceedingly weak solution of gold chloride and place a drop on our
slide; we also require a solution of the chloride of tin, known as
stannous chloride, in an evil smelling liquid called chlorine water. If
now we watch our drop of gold solution under the microscope, and while
watching mix with it a drop of stannous chloride solution, a strikingly
beautiful purple colouration is produced--this purple has been named
the purple of Cassius.
[Illustration:
_By the courtesy of Messrs. F. Davidson & Co._
1. THE EYE OF A COCKCHAFER
This section shows the eyelashes, the convex lenses, and, in the
lower portion of the plate, the nerve fibres leading from the brain
to the eye.
2. HOOKS ON BEE’S WING
The row of hooks on the margin of the bee’s wing are clearly shown.
By their aid the fore and hind wings are fastened together when the
insect flies.
]
If we desire further experiments in the testing of common substances,
and incidentally in the production of beautiful crystals, we might do
worse than try the effect of adding a solution of platinum chloride to
any solution containing a compound of potassium. Charmingly beautiful
crystals will result.
The experiments we have described as well as hundreds of others are
used by analysts every day in the testing of various substances. We
have started in every case by knowing what our solutions contain; the
duty of the analyst is to discover what he has before him. Given an
unlimited quantity of a substance for testing purposes it is not always
easy to determine its composition. With very small quantities, perhaps
less than a tea-spoon full in all, the difficulties of the analyst
are increased tenfold and without the assistance of the microscope
his efforts would be unavailing, he deals in drops and every drop is
precious. Sad to relate this form of testing, known to science as
micro-chemical analysis, has been practised to a far greater extent on
the continent than in this country.
Those of our readers who wish to try the experiments we have described
for themselves can obtain all the necessary substances, except the
poisons, at any chemists and the quantities required will only be the
smallest that can be obtained, in fact any reasonable-minded chemist
would probably let a microscopist whom he knew have a few grains of a
large number of chemical substances, suitable for this work, at the
outlay of a few pence. The chlorides of gold and platinum we fear no
one will give away.
All the experiments we have described thus far have necessitated the
use of what are called inorganic substances, they may be described in
every day language as substances derived from the inanimate world.
There are many equally interesting tests which may be carried out with
animal and vegetable products.
Formic acid is the substance which renders the sting of ants so
painful; it may, however, be prepared artificially and if a little,
dissolved in water, is mixed with a solution of silver nitrate we shall
obtain flat plate like crystals also some resembling fine fibres.
Probably the most curious of the easily obtained crystals from
vegetable products, may be made from citric acid which occurs in
lemons. If a little of the acid be mixed with a solution of caustic
soda and boiled with calcium chloride, a drop of the liquid after
boiling placed on a slide will give crystals readily. When viewed from
above they are an elongated oval shape, described by some authorities
as resembling a whetstone. Viewed from the side they have a striking
similarity to small sheaves of wheat.
With the recognition of poisons under the microscope we need not
trouble ourselves here. It would be useless to describe any of the
experiments, for few of us could obtain such deadly substances as
nicotine, strychnine, aconite, morphia and the like. Nevertheless the
recognition of these and similar substances in very minute quantities
is rendered easy, to those who have the necessary knowledge, by means
of the microscope.
CHAPTER XIX
THE USE OF THE MICROSCOPE IN MANUFACTURES
In how many branches of commerce, we wonder, does the microscope
play its part. It is used in several departments of engineering for
examining steels and many other metals not only for defects but to
see how they are made up. It is used in brewing for studying the
various yeasts and other substances, including the hops which go to
the making of beer. All manufactures which depend upon fermentation,
such as wine and vinegar making, are largely dependent upon the work
of the microscope. In dairy work the microscope is invaluable. In the
examination of various fabrics the assistance of the microscope is
always summoned. Paper manufacture and paper testing give work for
the microscopist but it would, we think, be easier to give a list of
the branches of commerce in which the microscope is not used than to
attempt to enumerate those which make use of the instrument.
We cannot possibly describe all the uses to which the microscope is
put, so we will confine ourselves to one or two of the more important
and, at the same time, to those which can, for the most part be
repeated at home.
The two most important commodities for mankind are food and clothing;
we cannot live without food and those of us who take but little pride
in our appearance, must have clothing of some sort. We have said a
little about food in another chapter and there we have also mentioned
the impurities which find their way, by accident or design, into some
of the commoner foods.
In this chapter we will deal first of all with clothing describing
how many of the raw materials may be recognised under the microscope
and showing very briefly how fraud in connection with the manufacture
of wearing apparel is detected. Practically all clothing is made from
animal or vegetable fibres, some, however, is made of artificial fibres
and these we shall mention.
The vegetable fibres used in the manufacture of wearing apparel are
all either hairs or what are called bast fibres and the latter, in
non-scientific language, may be described as the strands which run
through the roots and stems of most plants. The chief requirements of
vegetable fibres, destined to be woven into fabrics, are strength, it
is obvious that a weak fibre would be useless; length, the longer the
fibre the better and as we shall see later, short fibres are often made
up into inferior material; pliability, a stiff fibre would make an
uncomfortable fabric; firmness and durability. Animal fibres used in
the manufacture are either hairs or silk.
The most important vegetable fibre is cotton, it consists of the hairs
from the seed coats of several species of _Gossypium_, a plant closely
related to our common mallow. There are very many different kinds of
cotton and the qualities of the fibres of these different cottons vary
tremendously. Each hair is one cell and more or less spindle shaped,
that is to say, thicker towards the middle than at the base. If we
can obtain a little raw cotton we should certainly examine it under
the microscope; this may best be done by laying one or two fibres in
a drop of water on a slide. Under the low power, the first thing that
will attract our attention is the fact that the fibres are twisted,
corkscrew fashion, though not regularly nor throughout their whole
length. This curious twisting makes raw cotton easily recognised and it
is, at the same time, a very valuable peculiarity of these plant hairs.
The greater the number of twists and the greater their regularity, the
more valuable the cotton becomes for weaving purposes. Under a higher
magnification, we recognise other characteristics of the cotton fibre.
Each fibre is somewhat flattened, its edges are thick and, running up
the centre, there is a fairly broad lumen, as it is called. Covering
the whole there is a skin which by the way is often wanting in the
fibres of cotton fabric owing to the chemicals with which the raw
cotton has been treated and also to the methods of manufacture.
A very striking experiment may be tried by soaking a few cotton fibres
in cuprammonia, a substance prepared by the action of ammonia solution
on copper filings. Constrictions occur at fairly regular intervals
along the fibre so that, after treatment with cuprammonia, the cotton
fibres resemble strings of little beads.
The manufacture of mercerised cotton has become very important of late
years. The process is named after its inventor, Mercer, and consists
in removing the skin from the fibres, causing them to untwist and, by
doing so, to impart to them a lustre of silk. We may make a little
mercerised cotton for examination under the microscope by soaking some
raw fibres for a short time in a solution of caustic soda or caustic
potash and then washing them in water to which a little acid has been
added. This will cause the fibres to untwist and also destroy the
skin, but we shall probably notice that the fibres have shrunk. In the
process of manufacture precautions are taken to prevent this shrinking
for then the lustre is much better. We shall also observe that in our
mercerised fibres the lumen has become very narrow and it is often
broken, here and there are swellings on the outside of each fibre,
corresponding to the positions of the twists. Mercerised cotton, in
addition to its lustre, is stronger and absorbs dyes more easily than
ordinary cotton.
Flax consists of the bast fibres of the flax plant. Examination
of the raw product under the microscope will reveal both long and
short fibres. The former are the more valuable and are used in the
manufacture of linen, the latter are made into tow. The long fibres,
which are derived from the upper parts of the flax plant have thickened
edges and a very small lumen. The short fibres, used for making tow,
come from the lower part of the stem and the roots of the plant. Each
fibre has a broad lumen and is very similar to hemp fibre. Examination
of all these fibres, by the way, is best made in water as described
under cotton.
Hemp is another bast fibre and as we have remarked it resembles the
short fibres of flax; there is a broad lumen with an indistinct
margin. If we have an opportunity of comparing these fibres under the
microscope we shall see that many of those of hemp have forked ends.
This is very characteristic of the plant and is never found in flax,
therefore it affords a ready means of distinguishing hemp from flax.
Fine linen should never contain hemp, so that if our object be to test
the quality of a sample of linen by microscopic examination, we must
keep a sharp look out for the forked fibres of hemp. In coarse linen
these fibres occur for hemp is used in its manufacture.
Jute, another important fibre is readily distinguished under the
microscope, for its margins have perfectly smooth walls and its lumen
is wide in some places, narrow in others and interrupted altogether in
places.
[Illustration:
_Photos by Flatters & Garnett_
A SPIDER’S FOOT
The toothed claws are well adapted to enable their owner to obtain
a firm grasp of the fine threads of its web.
THE FOOT OF A FLY
The two claws enable the fly to walk up rough surfaces, whilst the
suckers between the claws give it a firm hold on smooth surfaces.
]
There are an extraordinary number of vegetable fibres which are woven
into articles of commerce, of one kind and another. Then again, many
fibres are so short or so brittle that they cannot be woven but are
used for other purposes such as filling cushions, cheap bedding, etc.
There are also a certain number of vegetable fibres which are valuable
because they are stiff and bristle-like as well as durable, and they
are used for brushes, door mats and for similar purposes. To the
microscopist who is interested in this work there is a wide field open.
For the examination of paper, which may be described as a “felt of
finely divided fibres,” the microscope is invaluable. The essentials of
a good paper are that it be durable, that it retain its colour and not
become brittle. The least observant of us cannot fail to have noticed
that there are an extraordinary number of different kinds of paper,
not only the many kinds which the paper manufacturers could show us,
but the obviously varied papers which we meet with every day. Added to
the papers, there are cardboards which are really a kind of paper. It
is clear, therefore, that the man who can tell us exactly how any and
every paper is made and what it is made of has laid up a goodly store
of knowledge. In carrying out tests of paper we rely partly on chemical
and partly on microscopic tests.
A number of substances contribute to the manufacture of paper; linen
and cotton rags, hemp and various fibres are the most commonly used,
not forgetting wood pulp which we shall mention in a moment. The
finest and whitest paper is made from linen rags and that from unused
linen and hemp is the strongest. Without attempting to describe in
detail or even in outline the different processes which the various
vegetable fibres must undergo before they appear in the guise of
paper, we may say the treatment is very drastic. Strong chemicals and
machinery designed to reduce the fibre to the finest possible particles
render the examination of paper, for the purpose of discovering its
composition, far from easy. Such fibres as survive the rough treatment
are mere fragments yet they are often large enough for the lynx eyed
microscope to read their story. Formerly the constituents of most
papers could be separated into three classes according to their
behaviour with iodine solution, but this test has been superseded by
more complicated methods which do not concern us here.
The examination of various papers may prove interesting for example in
linen rag paper, we ought to find some flax fibres, they will be sadly
battered and torn but are usually recognisable under the microscope.
Hemp paper is tough and is used for bank notes, in it some of the
short tow fibres will probably occur and they will give a clue to its
composition. Cotton rag paper is easily recognised for the fibres are
very characteristic, a remark which also applies to jute paper, the
so-called manila used for envelopes, wrappers, etc.
Mechanical wood pulp which enters so largely into the manufacture of
paper is easily recognised by those who have given a fair amount of
attention to the microscopical examination of plant life. Wood pulp is
always used in conjunction with some binding material such as cotton
or flax fibres. Many different kinds of wood are converted into pulp
and of course it requires a considerable amount of experience to say
exactly what kind of wood has been used in a certain paper. Some of
the woods are poplars of various kinds, others spruces and firs. It is
easy to distinguish the conifers as spruces and firs are called, for
the reason that the trees bear cones. The little fragments of wood,
scattered throughout the paper, have minute circular perforations upon
them, resembling miniature quoits, if they belong to the conifers; none
of the other woods possess these “pits” as they are called.
Very many other plant remains may be found in paper, for instance
hop fibres are used sometimes and their presence is usually shown by
remnants of the climbing hooks which, during life, studded the climbing
stem of the hop.
Some of the important animal hairs, used in the manufacture of
clothing, may now claim our attention. Wool and silk are of course the
most important. The best wool is all obtained from the domestic sheep
so let us examine one or two of the easily obtained hairs from this
animal. As with the vegetable fibres we may examine them in a drop of
water but, in this case, we shall find that we cannot make the water
go near the wool for the reason that the latter is covered with a film
of grease, called wool fat. Our first care, therefore, it to get rid of
the wool fat and this may be done by shaking the specimen with a little
ether or chloroform.
Having cleaned the wool fibres we may proceed to examine them under
a high magnification and, to see their structure fully, we must move
the fine adjustment to and fro, for it is not possible to obtain a
true idea of its structure without doing so. We shall see that the
hair consists of two layers, an outer skin and an inner core. The
latter consists of a number of cells, whilst the former is composed of
scales, of which the lower edges are arranged beneath the upper edges
of the previous scales, like tiles on a roof. The free edges of the
scales project outwards a little so that the wool, when not so highly
magnified, appears to have a toothed margin. Further examination will
show us that no single scale completely envelops the strand of wool,
usually two scales make the complete circuit. It is a curious and
easily tested phenomenon that in say an inch of wool from the same
kind of sheep there are always the same number or very nearly the same
number of scales. By counting the scales, experts can tell from what
animal the wool is derived.
If we examine many samples of wool we shall not be long before we
encounter certain specimens showing one or more constrictions. Now we
all know that the reason why some people do not grow very much is
because they are delicate, ill health affects the whole system. The
constrictions in the sheep’s wool occur because the animal has suffered
from some illness, or from great hunger or thirst, which has resulted
in its wool not growing properly for a period corresponding to the
duration of the illness or other calamity.
The examination of various animal hairs will help to while away many
an hour and many of these objects are of the greatest interest. If we
have the opportunity it will be interesting to compare the wool from
different kinds of sheep, that from the Lincoln sheep, for example
differs from that of shortwooled kinds. We may also compare goat’s wool
with sheep’s, then there are differences between the hair of cows and
calves. Comparisons always make microscopic work more interesting.
The microscopic examination of cloth, used for making our coats, is
quite interesting work and withal important. A good deal of the cloth
which is made up into suits is known as shoddy, that is to say that
material that has been worn before; old rags of all sorts and many
other extraordinary things go to the making of this cloth. There are
special factories to which rags are sent to be made up into shoddy. One
might think therefore that this substance would be easily recognised
under the microscope but it is not quite so easy as we shall see.
Absolutely pure wool from the sheep giving the best quality wool is
only used in the very finest and most expensive fabrics. In a great
deal of really good cloth we may recognise other hairs besides those of
the sheep and sometimes vegetable fibres find their way into good cloth
by accident. A piece of suspected cloth should be cut off and separated
as far as possible into its different fibres. In shoddy we shall find
few long fibres and they will all be much torn for the reason that the
rags from which the material is made are cut up in the manufacture. If
we find cotton fibres, we may be certain that our specimen is shoddy,
also if we can find fibres of many different colours, though the final
dyeing may have disguised the fact that the fibres have originally
figured in fabrics of various colours.
Silk is one of the most important of all fibres capable of being woven
into fabric. It is hardly necessary to remark that it is formed by
the fully fed silkworm just before it turns into a chrysalis. A very
large number of caterpillars spin silk but the majority of this silk is
useless for commercial purposes. The silkworm gives off a double thread
of silk from glands in its mouth and, at the same time, it gives off a
sticky substance called silk glue which sticks the two fibres together,
so that, to the eye at least they appear as one fibre.
Most of us have kept silkworms, those who have not may find it worth
while to expend a few pence in some of these insects for the sake of
examining the raw silk. A cocoon, as the work of the caterpillar is
called, consists of three layers, an outer layer of floss, a middle
layer and a so-called layer (the inner layer) of parchment. Only the
middle layer is used in commerce, the floss is too fine and weak and
the parchment is so impregnated with silk glue as to be useless.
If we examine some raw silk, taken from the middle layer of a cocoon,
we can easily see the two parallel fibres of silk and the outer
wrinkled covering of silk glue. Now, magnifying our object more highly,
we shall see that each fibre is a solid rod, with a smooth lustrous
surface and without any sign of lumen or cell structure; the rods
too are continuous and this alone distinguishes silk from all other
fibres animal or vegetable. Two tests are worth trying for they are
characteristic of silk. On the addition of a little strong sulphuric
acid we observe that the silk rapidly dissolves, on the other hand, if
a few fibres are boiled in hydrochloric acid, the silk dissolves but
the envelope of silk glue remains unchanged and appears beneath the
microscope as a cracked and wrinkled tube.
The caterpillar of another moth spins coarser greyish coloured fibres,
which are spun into the well known Tussore silk. As with the common
silkworm these caterpillars spin two fibres and glue them together
with silk glue. In this case, however, under a high magnification we
shall notice that the fibres are marked with a number of very fine
lines running lengthways, whilst every now and then there are fairly
deep indentations. The former markings are natural to the fibres,
the latter are caused by one fibre being pressed against the other.
In countries where the production of silk is of great importance, the
microscope is not only pressed into service for examining the product
of these useful little insects, but also in keeping watch for a very
deadly disease which attacks the caterpillars. It is called “pebrine”
and the great scientist Pasteur, whose name is world famous for his
work on bacteriology, discovered that it was caused by a tiny fungus.
Artificial silk is an important article of commerce. It is made in
several different ways and of various substances. Some artificial silk
is made of collodion, some again is made of cellulose the substance
of which the cell walls of young plants is composed; gelatine is also
used in making this commodity. As a rule it is easy to distinguish
artificial from real silk, for usually the imitation consists of flat
fibres or at least fibres quite different to the smooth rods of real
silk. The iodine test is often sufficient indication, for with this
chemical true silk is coloured brown.
Space or lack of it does not allow us to describe how the microscope
may be and is applied to other manufactures, even the miller uses the
instrument, for it will tell him if his flour is of pure wheat, and in
this manner. He puts a little flour in a drop of water on a slide and
covers with a cover slip; then, for a moment or two, he rubs the cover
glass to and fro over the water and flour and examines his specimen
under the microscope. If his flour be of wheat, he will see fairly
stout spindle shaped strings, if it be of rye no strings appear, whilst
maize flour gives very small strings. These are called gluten strings
and wheat is very rich in gluten.
CHAPTER XX
THE MICROSCOPE AND CAMERA ALLIED
Photography is such a popular hobby in these days, that the enthusiast
who possesses both camera and microscope, is certain, sooner or later,
to wish to take permanent records of some of the beautiful objects
revealed to him by the latter instrument. The production of high-power
photo-micrographs, as the pictures of highly magnified objects are
called, can only be carried out by those who are skilled in the use of
both camera and microscope and are possessed of considerable patience.
There is nothing to prevent any amateur photographer who possesses
a bellows camera--box cameras cannot be used for this work--from
producing excellent low-power photo-micrographs, that is to say
pictures of objects less highly magnified.
It may be disappointing to learn that a microscope is not necessary
for this work. The requirements are, a camera which will open out to
a considerable extent and a short focus lens. Before we show how the
pictures may be taken, we must be quite clear what is meant by a short
focus lens. There are various ways of measuring the focus of a lens; in
the case of a single lens the operation is fairly simple, but single
lenses are made of one glass and most camera lenses are built up of
several pieces of glass, then it is much more difficult to measure the
focus quite accurately. For ordinary purposes and for our purpose, it
is only necessary to know the focus in round figures and to do so we
open up the camera and focus some distant object on the ground glass,
then the distance in inches from the back of the lens to the ground
glass will give us very nearly the correct focus, or as it is often
called focal length, of our lens. Suppose we focus on a church steeple
a mile away and then find that a space of five inches separates the
back of our lens from the ground glass, the lens is of five inches
focal length.
With a lens of such a focal length we shall require a camera with
very long bellows to obtain much magnification of our object but, if
our camera is one with only short bellows, we can still overcome our
difficulties. If the extra expense is no object we can obtain one of
the excellent Aldis lenses of only two inches focal length, especially
designed for this work; if we are ingenious we can construct a device
which will answer our purpose admirably and cost but a few pence. With
the simple apparatus we are about to describe photo-micrographs up to
ten diameters magnification can be obtained in most cameras. The term,
ten diameters, may appear puzzling but really it is quite simple.
Suppose we wish to make a photo-micrograph of a penny stamp and we wish
it to be magnified ten diameters, we should require a large camera
for the operation, but that, for the moment, is beside the point. Our
penny stamp, magnified ten diameters, would be equal in area to ten
horizontal rows of stamps, each row containing ten stamps or, in other
words, it would occupy the same area as one hundred penny stamps.
To find the magnification possible with our camera and lens, extend
the camera bellows to the full and set up a foot rule in front of the
lens. Move the camera from a distance slowly towards the rule till it
is sharply focussed, then carefully measure the distance between the
inch lines on the focussing screen; if the lines are three inches apart
we shall be able to make photo-micrographs three times the size of our
object and we shall probably desire something better.
To obtain a considerably magnified picture we must have a lens of short
focal length and a camera with long bellows, in fact in theory the
amount of magnification is only limited by the length of the bellows,
so that an extraordinarily long camera should give us a much magnified
picture. We cannot lengthen our bellows without considerable expense,
but we can shorten the focal length of our lens. If we obtain, at the
cost of a few pence, a convex lens of short focal length, such as is
used in cheap magnifiers we can easily apply it to our camera lens
and it will have the effect of shortening its focus. This extra lens
can either be fitted in front of our original lens by cutting two
inches of cardboard, of such a size that they will just fit into the
lens hood. In the centre of each piece of cardboard, cut two circles,
slightly smaller in diameter than the diameter of the convex lens.
Place the lens over the opening in one of the pieces of cardboard and
stick the other piece upon it with glue or seccotine. When dry fix the
lens in its cardboard holder in the front of the lens hood and repeat
the focussing experiment with the foot rule. We shall now obtain a much
greater magnification with the same length of bellows, because the
additional lens has shortened the focus of our original lens. Probably
our camera is fitted with a double lens and it is possible to unscrew
the front portion, in this event our extra lens with its cardboard
holder may be fitted inside the front lens up against the diaphragm and
the front lens replaced. The result is practically the same whatever
the position of the new lens, but we must be certain that it is convex,
for a concave lens would increase the focal length of the whole and so
reduce our magnification.
We may find that the amount of enlargement we can now obtain is
sufficient for our purpose; if so we can go ahead and produce
photo-micrographs of all the objects we desire. The methods of doing so
differ in no way from those employed in taking ordinary photographs and
as we are writing about microscopy and not photography in these pages,
there is no need to go into further details. Even the experienced
photographer, however, is liable to overlook one or two important
details. The bugbear in all work of this kind, with low magnifications
as well as high, is vibration. Not only is our object magnified but
every movement is equally increased. No one should walk across the
room while this work is in progress; in large towns, trams and heavy
motors will ruin many a plate, in fact it is only when one takes up
work of this kind that one realises that one’s house is in a perpetual
quiver. Some enthusiasts work at the dead of night, others suspend
their apparatus on springs and invent all kinds of ingenious devices to
overcome these miniature earthquakes.
If time is no object, we have an easy means of still further increasing
the magnification. To do so we cut a thin sheet of copper in such a
manner that it just fits our lens tube, in front of the diaphragm. Then
in the very centre of the copper disc we make a hole with a “number
one” needle, the hole is about one twentieth of an inch in diameter.
Replace the convex lens in its cardboard holder and screw on the front
portion of the camera lens. A trial will show that we have considerably
increased the magnification but decreased the amount of light admitted
by the lens, therefore we shall probably require an exposure as long as
an hour. How about the bugbear vibration during such a long exposure
is a natural question to ask. Unless the vibration is continuous, and
that is unlikely, it is less likely to cause trouble during a very long
exposure than during a short one, because it operates during a small
proportion of the whole time.
All kinds of objects can be depicted with the apparatus we have
described. Minute shells and insects; parts of larger insects, their
legs and wings for example; the feathers of birds; various rocks,
crystals of all kinds; small flowers and their seeds; mosses, lichens
and many kinds of fungus, in fact their number is limited only by the
degree of ingenuity possessed by the photographer. Do not suppose
that these low-power photo-micrographs are interesting only because
of their size, the enthusiast who makes a collection will discover
in his prints hidden beauty of which he had no conception when he
looked at the originals. We have seen a very large number of low-power
photo-micrographs, taken with very inexpensive apparatus, showing
the wonderful sculpturing on the wing cases of beetles, some of them
are marvels of design, yet, observed with the naked eye, many of the
insects appear to be devoid of ornamentation.
The production of high-power photo-micrographs is hardly a subject that
can be described in these pages; the apparatus is costly for, even if
one dispenses with a specially designed micro-photographic camera, it
is necessary to have a good microscope and bellows camera, for really
advanced work. Messrs Swift & Sons supply an excellent fitting, with
which quite satisfactory photo-micrographs may be taken. It consists of
a light metal cone, the more pointed end fits over the upper portion
of the microscope tube; the other end of the cone is provided with
ground and plain glass focussing screens and a dark slide. When using
this apparatus, it is first necessary to find and focus our object in
the ordinary way, before attaching the photographic apparatus. Having
secured our object exactly as we wish it to be depicted and well in
the centre of the illuminated circle, we remove the eyepiece and slip
the metal camera over the top of the microscope tube. If now we place
the ground glass screen in position, we can see an image of our object
upon it. Great attention must be paid to the lighting, it is necessary
that the illumination be perfectly even, otherwise our negative will be
over-exposed in some parts and under-exposed in others. When everything
is in order the plain glass screen is substituted for the one of ground
glass. On this we shall probably not see any image with the naked eye,
but with the help of a magnifying glass we can see a much finer image
than was possible on the ground glass screen. The final focussing is
done at this stage and, having secured as sharp an image as possible,
the focussing screen is removed and the exposure is made. Experience
alone will teach the length of exposure, it depends upon the amount
and nature of the light, upon the transparency of the object and
also upon its colour. There is no more simple apparatus for taking
moderately high-power photo-micrographs; for more advanced work the
micro-telescope and the super-microscope (see Chapter XVII) used with a
camera, will enable the user to obtain pictures of objects magnified as
much as five thousand times.
CHAPTER XXI
HOW THE GLASS USED IN MICROSCOPES IS MADE
Having described all the ordinary uses of the microscope and having
also insisted that the objectives are the most important part of the
instrument, there are probably many of our readers who may wish to know
in what manner this wonderful glass differs from ordinary glass and how
it is made.
Glass has been defined as a substance which, during its manufacture,
passes from the liquid to the solid state so rapidly that no crystals
are formed. Usually when solids are melted and then allowed to cool,
they do so with the formation of crystals, this may be shown in the
case of sulphur by melting a little and then allowing it to cool. After
a while, as cooling takes place a solid crust will be formed on the
surface of the molten sulphur. If two holes are pierced in the crust
and the still liquid sulphur poured out, it will be found that the
sulphur which adhered to the vessel in which the melting took place has
formed beautiful needle shaped crystals.
A great amount of original work has been done on the subject of glass
manufacture and especially on the kind used for optical instruments--as
a result there are many kinds of glass differing from one another in
physical properties and in chemical composition. Although the various
chemicals used and their proportions are fairly well standardised,
as the result of long experience, it is probable that glass is not
a definite chemical compound but a mixture, in which certain of the
components act as solvents for the rest.
The ordinary glass in use in this country, apart from specially
prepared optical glass, may be either English flint glass, plate
glass or Bohemian glass. The first named is composed of sand,
potassium carbonate and red lead; plate glass is made of sand with the
carbonates of sodium and calcium, whilst similar ingredients are used
for Bohemian glass except that carbonate of potassium is substituted
for carbonate of sodium. It is chiefly owing to the requirements of
optical instrument makers that, new kinds of glass containing very many
previously untried chemicals, have been produced. As a result glasses
are now made with specific gravities varying from 2.5 to 5.0, that is
to say the weight of a square inch, or a square foot or a square yard
of glass may weigh anything from two and a half to five times more than
a square inch, foot or yard of water, according to its composition.
Before we describe the details of its manufacture let us consider its
properties as briefly as possible. At high temperatures it is perfectly
fluid and may be poured from vessel to vessel as easily as water; at
lower temperatures it is viscous, _i.e._, semi-fluid and can be rolled
with an iron roller as dough is rolled with a rolling pin; it can be
moulded into any desired shape, blown out into flasks and bottles or
drawn out into threads so fine that they may be woven into a fabric. It
is a bad conductor of heat and for this reason it is safer to pour very
hot liquids into a thin glass vessel than into a thick one. With thick
glass the inner layers expand with the heat before the outer layers
are even warm and the result is a crack or often absolute fracture.
Sometimes during manufacture glass vessels which are suddenly cooled
will appear satisfactory, but the particles of glass remain in so high
a state of tension that at the slightest touch the vessel will break up
into thousands of pieces. On account of this property of glass it must
be cooled very slowly indeed; the process is known as annealing.
Optical glass unlike most other kinds must be manufactured in thick
blocks--some of the large lenses on telescopes are of considerable
thickness. All glass for scientific instruments must also be
homogeneous, which our dictionary tells us means of the same kind. To
be more explicit each particle of optical glass should be precisely the
same in composition and properties as every other particle. In the very
early days of manufacture it was difficult to obtain homogeneous pieces
of glass and Guinard, in the 18th century, conceived the idea of
stirring the molten glass with a rod of fireclay, to ensure a thorough
mixing of the components. This led to considerable improvement and the
method has survived to the present day.
The first real advance in the manufacture of optical glass, was due
to the ingenuity of two Germans, Abbe and Schott, who lived at Jena.
Jena glass became famous for the manufacture of lenses, so much so that
a stupid idea still prevails in many quarters that only the Germans
can make good optical glass. To give them their due it is good but
quite recent events have shown the world that the Britisher can make
better. The two German scientists used new chemicals in making their
glass and they succeeded in producing a substance which possessed
hitherto unheard of properties. In what is now known as the older
crown and flint glass the dispersion and refraction increased with the
density, that is to say, the heavier the glass the more it scatters
and bends light rays passing through it. With the new methods, glass
is made which scatters the light rays very little, though bending them
considerably and vice versa. The ordinary crown glass is composed
of silicates of calcium and sodium or of calcium and potassium or a
mixture of both and it is possible to make it colourless and free from
defects, but its optical properties are never so valuable as those of
Jena glass. The most important components of the newer glass are the
oxides of Barium, Magnesium, Aluminium, Zinc and Boron.
Good optical glass should be transparent and colourless and, as we have
stated, it should be homogeneous--the refraction and dispersion of
light rays should be identical over all parts of the glass. It should
possess no striæ, as they are called. Striæ may be seen at the edge of
a piece of plate glass as little lines just as though the glass had
been formed in layers. Striæ detract from the efficiency of optical
glass, nevertheless, some very cheap lenses are made of plate glass.
Bubbles are almost always present in Jena glass but, unless they are
very numerous they do not appear to render the glass less efficient.
Hardness and chemical stability are other desirable qualities. Most of
these high-grade glasses are soft, as shown by the ease with which they
may be scratched; many of them are not very stable chemically and are
easily affected by chemical fumes with the result that their surfaces
become covered with a coloured film. With all their drawbacks, for
optical work the newer glasses far excel the older.
The manufacture of optical glass is a costly and lengthy process.
The chemicals used in its manufacture are selected with the greatest
care; impurities must be guarded against for they would change the
composition of the glass and in doing so alter its physical properties
on which everything depends. The chemical substances are used either
in the form of oxides, nitrates or carbonates, for the reason that
they are easily decomposed by heat. To assist in the melting of the
substances a few fragments of glass of similar composition are added.
The crucibles, in which the chemical components are melted, are covered
so that no fumes from the furnace may gain access to them, even the
chemical composition of the crucibles is carefully tested that no
impurities may contaminate the glass. No crucible is ever used more
than once and only a single crucible is heated in each furnace, in
order that the temperature may be regulated to a nicety.
The actual manufacture is then begun after all these preliminaries have
been attended to. A clean dry crucible is heated in a furnace--not the
one in which the glass making is to take place--to a dull red heat.
Then, with iron tongs, it is removed to the previously heated glass
making furnace and the temperature is raised very gradually. The next
stage sees the addition of the well mixed chemicals to the heated
crucible, in small quantities at a time. When the full quantity has
been added the crucible contains melted glass full of bubbles, some of
them air bubbles released from the raw materials as they were added
and some bubbles of gas given off from the chemicals as they act upon
one another. The molten glass is then heated strongly so that it will
become perfectly liquid and many of the bubbles will be driven off. To
reach this stage may occupy anything from thirty-six to sixty hours and
constant attention is necessary during the whole time.
The next stage is perhaps the most important in the whole process.
After numerous small samples of the molten glass have been taken, on
the end of iron rods, to see if the air bubbles have been driven off,
the mixture is stirred to render it homogeneous and to eradicate striæ.
The stirring is carried out by means of a cylinder of fireclay which is
first of all heated to the temperature of the molten glass before it is
introduced. To the end of the fireclay cylinder a long, detachable iron
handle is fixed so that the man who undertakes the stirring may stand
at a distance from the hot furnace. The heat is great and the work of
stirring is laborious and for this reason the stirrers are constantly
changed. The iron handles must be watched carefully for, owing to the
heat they rust rapidly and should any of the rust fall into the molten
glass it would impart to it a colour and render it useless for optical
purposes. When stirring begins the glass is liquid as water, but the
stirring is continued during cooling and all the while the glass is
gradually becoming more and more solid. During the final stages the
operation is hard labour indeed and, finally, it is not possible to
stir any more, then the fireclay cylinder is either removed or left in
the glass.
When the glass has solidified and, whilst it is still hot the furnace
is sealed and allowed to cool very gradually till it has reached the
ordinary temperature of its surroundings; this may take several weeks.
When quite cool, the crucible is removed from the furnace and carefully
broken; then the glass, which may be in one mass weighing as much as
1000 lbs., is freed from particles of fireclay and examined for defects.
The next stage consists of moulding and annealing. Large pieces of
glass are heated till they are just soft, then they are passed into
iron or fireclay moulds designed so that the glass is formed into discs
or slabs suitable for grinding by opticians. They are allowed to cool
very slowly in the moulds.
When the glass is taken from the moulds it is not yet ready to be made
into lenses. It is subjected to another and very careful examination,
when all defective parts are cut out. Should there be many defects the
glass is again heated, moulded and annealed. From a crucible containing
1000 lbs. of molten glass it is unusual to obtain much more than 200
lbs. of optically perfect glass, it is obvious therefore that lenses
cannot be cheap.
One would naturally imagine that the minute lenses used in microscope
objectives should be cheaper in comparison than the larger lenses used
for photography or in telescopes, for it is always more difficult
to make minute articles than large ones. As a matter of fact, even
allowing for the greater amount of material used in the larger lenses,
quality being the same in both cases, they are far more expensive
than the smaller microscope lenses; the reason being that it is
exceedingly difficult to obtain a perfect specimen of large size. The
difficulty arises not only in the actual manufacture of the glass but
in the subsequent operations of cutting, grinding and polishing when
fractures are very liable to occur.
From this brief account of the manufacture of optical glass it is clear
that a good lens is always worth a high price. Its components must be
of the purest quality obtainable, the process of manufacture requires
highly skilled labour and it is laborious and exacting work. Constant
attention is needed from start to finish and much of the glass is never
sufficiently good to pass the rigorous tests which it must undergo.
Lastly, in the final preparations of the completed lens, mishaps are
frequent. Added to all these trials of the lens maker is the one
outstanding fact that the process can never be hurried at any stage,
the efficient annealing of optical glass is one of the most important
stages in its manufacture. A very full account of the manufacture of
optical glass is given in the _Encyclopædia Britannica_, whence much of
our information in this chapter is derived.
CHAPTER XXII
THE CHOICE AND USE OF APPARATUS AND ACCESSORIES
In this, our concluding chapter, we propose to give a few hints upon
the choice and use of the microscope and its accessories, to enlighten
our readers concerning stains and staining and to add such other
information as is likely to be useful, information which is better
supplied in a chapter of its own than scattered about our pages and
probably overlooked.
The most important question concerns the choice of a microscope. In
a book of this nature it is obviously impossible to recommend the
wares of any one maker. Some firms are celebrated for the good wearing
qualities of their instruments, some for the delicacy--not fragility by
the way--of their fine adjustments and our readers must select their
maker for themselves. One thing we strongly advise, be patriotic and
select a British maker, it is not only a question of patriotism but of
self interest, for in many respects the British instruments lead the
world. Most of the manufacturers advertise in such papers as _Nature_,
they all print excellent catalogues, a perusal of which may be a useful
preliminary.
The instrument we select will depend on the use to which we intend to
put it and on the length of our purse. Perhaps the question of cost
will be more important to most people. An elaborate instrument is not
a desirable acquisition, till we are somewhat advanced in our work
at anyrate, and one of the models which most makers call Students’
Microscopes will do everything we desire. We should have at least two
objectives, three if we can afford them, a 1 inch and a 1/6 inch will
enable us to examine everything described in these pages, except some
of the bacteria. If we wish to add a third objective we might select
a 2 inch one, for quite low-power work and later we shall probably
become the proud possessors of a 1/12 inch objective, but this would be
of little use to us at first. Two eyepieces will complete our optical
equipment. A condenser for the stage, properly known as a substage
condenser is an addition which we shall appreciate, though quite
interesting work may be done without it.
Our other apparatus is inexpensive and comprises, one or two dissecting
knives, some needles mounted in handles, a pair of fine scissors, a
pair of small forceps, a razor, one or two camel hair brushes, three
or four watch glasses, slides and cover slips, and a pipette or two. A
pipette, by the way, is merely a glass tube pointed at one end and cut
off square at the other; at about its centre it is much wider than at
the top or bottom. It is a most useful piece of apparatus for picking
up small water animals which we wish to examine. By putting the pointed
end of the pipette just over an _Amœba_, for example, and sucking
gently at the other end, we shall draw some water and the animal into
the swollen part of the tube. If we then place one finger over the end
of the pipette to which we have applied suction we can transfer the
contents wherever we wish without it running out, as soon as we remove
our finger the pipette empties itself. The dissecting knives are useful
for cutting up specimens before examination, the mounted needles for
teasing them, that is to say tearing them into fine shreds. The brushes
we may use instead of the pipette, for picking up small objects from
water, or when dry. The watch glasses are useful for examining objects
such as sponges, water fleas, etc., under water. They are just as
satisfactory for most purposes as the specially constructed zoophyte
troughs and, of course, very much cheaper. The razor is required for
section cutting concerning which we will say a few words.
For section cutting, special razors are sold, they are heavy as a rule
and not hollow ground. Many microscopists aver that good sections
cannot be cut with a hollow ground razor, we beg to differ, however.
On this point we would advise our readers to select whichever pattern
suits them best; there is no need to buy an expensive razor, for our
early efforts, at anyrate, will quickly dull the cutting edge and
there is no advantage in going to much expense in this direction.
To cut really good sections is not difficult, it is a question of knack
rather than skill; having once acquired the knack, practice will do the
rest. The great point is to begin properly, some people never take the
necessary trouble in their early days and, as a consequence, they never
learn to cut good sections.
There is nothing mysterious about the operation, a section is merely a
very thin slice. A good section for examination under the microscope
should, of course, be so thin that it is transparent; it should be
equally thin everywhere, not thick in some places and thin in others
and it should be cut in the proper direction. The last statement
requires a little explanation; suppose we wish to make a section
straight across a stem, a transverse section it would be called, then
we must see to it that our section is cut straight across, and not in
such a manner that the portion from which the section is cut tapers
in the slightest degree. Before we begin the actual section cutting
we must always trim up our specimen with one of the dissecting knives
and not attempt to make the surface level with the razor. As a start,
let us take some moderately fleshy plant stem for we shall find it as
easy to cut as anything. Our stem must not be woody, for till we have
had a fair amount of practice we shall damage our razor on its hard
structures; it must not be too soft or it will be crushed out of all
recognition in our fingers; it should not be hollow, for solid stems
are always easier to cut. Having selected our stem and the spot upon it
at which we require a section, we cut straight across it at that spot
with our dissecting knife, in such a manner that the cut edge is as
nearly as possible at right angles to the sides of the stem. A piece
two or three inches long is the most comfortable to hold.
Holding our razor firmly in the right hand we take our specimen in the
thumb and index finger of the left hand. Place the side of the stem
against the first joint of the index finger and slightly bend the tip
of the finger round the object. The end of the stem, from which we
propose to cut our section, should be slightly above the level of the
index finger. With the thumb, the stem is pressed firmly against the
first joint of the index finger and the first joint of the thumb must
not be bent upwards. This last point is very important, as we shall
see in a moment. From our description the method of holding the stem
may seem somewhat cramped; it should not be so, however, and an easy
position will help towards success but let it be the correct position.
We may play the violin, in a manner, without learning the correct
method of bowing but we can never become expert violinists unless we
hold our bow properly, in the same way we can never cut really fine
sections unless we hold our specimen in the correct way. We now lay our
razor along the second joint of the index finger in such a manner that
the heel of the razor, or a point near the heel comes gainst the end of
the stem. For the first time we realize the importance of holding our
specimen correctly, for the index finger forms a convenient platform
on which to steady the razor. To cut our section, we, of course, press
the razor towards us and, at the same time draw it outwards from heel
to toe. We must never attempt to press the razor straight through the
specimen, however soft the latter may be, we must always draw the razor
along, at the same time as we press it through the specimen. Never cut
outwards as in sharpening a pencil and never move the razor backwards
and forwards as though using a saw. The outward cut will never result
in a good section, a sawing movement will give a section alternately
thick and thin. Should the section be somewhat tough we may find the
razor slip suddenly over its surface, an event which will impress
itself upon us painfully if we have forgotten the injunction not to
bend the thumb, for the certain result of the slip will be the loss of
a goodly portion of skin upon one’s joint. A straight thumb will be out
of the danger zone of a slipping razor.
[Illustration:
_By the courtesy of Messrs. F. Davidson & Co._
1. A FLY’S EYE
A fly’s eye highly magnified, showing some of the many lenses of
which it is composed.
2. IMAGES SEEN BY A FLY
Photographs of a statue taken through a fly’s eye. Instead of
seeing one image of an object, the fly sees as many images as there
are lenses in its eyes.
]
Having cut a section it is hardly likely that, at this our first
attempt, it will be good enough to put under the microscope, so we
will continue to cut section after section till we have a dozen or
more from these we can select the most transparent for examination.
The sections of such material as a stem should be kept moist and
to do so we will place them in a watch glass containing water. It is
often easier, also, to cut our sections if the razor be moistened with
water, at anyrate the moisture prevents the sections from adhering to
the razor. The sections should be removed from razor to watch glass
and from watch glass to slide, by means of a brush, never by means of
the fingers. The razor of course, should be well dried with a soft rag
before it is put away; rust, besides being unsightly, ruins the cutting
edge. If we are really anxious to cut our own sections, and every good
microscopist does so, we shall return to the operation again and again,
even cutting objects which we have no desire to examine, for the sake
of the practice. We shall soon reach a stage where our razor will be
dulled and require stropping and the efficient stropping of a razor is,
to many people, a more difficult operation than the cutting of sections.
We may point out here, that all sections are not quite so easy to cut
as the one we have taken as our example. Some objects are so soft that
they need hardening with chemicals before they can be cut, some are
so hard that to attempt to cut them would ruin the edge of the razor,
though it is wonderful what hard substances may be cut when we have
had a little experience; some are so delicate that they must needs be
buried in melted wax, then object and wax are cut together and later
the wax is separated from the section.
Leaves and very small stems may be sectioned by the beginner, as
easily as larger stems. For such objects little sticks of pith are
sold as holders. Having cut of our piece of leaf from which we wish to
derive a section, we make a slit, with our dissecting knife, down the
middle of one end of a piece of pith. The piece of leaf is then placed
in the slit and by holding the pith at the sides our piece of leaf is
held firmly. Sections are cut through pith and leaf, and the two are
floated in water, when the thin slices of pith will float away from the
leaf sections.
Having cut a satisfactory section, let us proceed to describe the
method of making a slide thereof. We will suppose that we do not
wish to make a permanent preparation but one for temporary use. A
clean slide must be selected, all through our pages we emphasise the
cleanliness of slides, at or near its centre we put a drop of water
and, lifting it with a brush, we place our section in the drop of
water. If our examination is to be with a low magnification, we need
not use a cover slip, nevertheless it is worth while to cultivate the
habit of using one. The cover slip is not only a protection for our
object but for our objective. Water you may argue cannot harm the
lenses of the objective. Perhaps not, we will not argue the point but,
when the water dries on the objective, it leaves a certain amount of
deposit on the glass and this deposit must be rubbed off. The less
often the lenses are rubbed the better for them, glass especially very
highly polished optical glass, is far more easily scratched than many
people imagine and a scratched lens is an inefficient lens. When using
high magnifications a cover glass should always cover our object and
the same remark applies to objects examined in Canada Balsam. This
substance is likely to cause serious trouble if it finds its way on to
an objective. It must be removed, that is obvious, but it sets hard,
it must not be scraped away for fear of damaging the optical glass
and, as it is used to cement the lenses together, there is the great
danger that any solvent used to remove the Balsam from the face of the
objectives, may also dissolve their setting. Our digression may seem
somewhat unnecessary, but the very great importance of keeping all
chemicals and even water, from coming into contact with the lenses of
our instrument cannot be insisted upon too strongly.
In whatever substance we examine our object, water, glycerine or
Balsam, there is a right and a wrong way of applying the cover slip. It
must not be dropped or laid down flat upon the object, if we do this
we shall certainly imprison a number of air bubbles and that must be
avoided. One edge of the cover slip must be laid against the edge of
the mountant, as the liquid used for mounting our object is called,
then placing a needle beneath the cover slip it must be gently lowered
into position. We shall now find that all air bubbles are driven out as
the cover slip is lowered.
When we have cut an exceptionally good section or when we have some
specially interesting object we may wish to make a permanent slide.
The exact method of doing so varies somewhat with the nature of our
object and to describe all the methods of mounting microscopic objects
permanently would require a book in itself. We will suppose that we
wish to make a permanent slide of one of the sections floating in
our watch glass of water. The first thing we must do is to get rid
of the water with which our section by now is saturated. This may be
accomplished by means of alcohol of various strengths, which we may
put into three watch glasses. In the first watch glass we have half
pure alcohol and half water; in the second three-quarters alcohol and
one part water; in the third watch glass pure or absolute alcohol. The
section is transferred on a brush to the first watch glass and left
there for five minutes, then to the second watch glass for a similar
time and finally to the third watch glass. The mountant may be either
Canada Balsam or glycerine jelly. Should we decide on Canada Balsam,
we put a small drop of the substance in the centre of a clean slide,
place one section atop of it and then gently lower a cover slip upon
it as already described. Then, without using any force, the cover slip
is pressed down, but we must not fall into the all too common error of
thinking that a thick section can be made thin by pressing upon the
cover slip. If we have taken the correct amount of Canada Balsam it
will just cover the area below the cover slip and no more, if we have
used too much, it will flow out on all sides and perhaps on the top of
the cover slip. A very little experience will teach how big a drop of
Balsam to use.
Should we decide to use glycerine jelly, we take a small globule of
the jelly on one of our mounted needles and place it in the centre of
a clean slide. Then we hold the slide a little above a lighted candle,
or even a match will do, to melt the jelly. Directly it is melted
the section is placed upon it, and we proceed as before. Whichever
mountant we have used, the slides must be put away out of the dust, and
they must be flat and not placed on edge. In a few days the mountant
will set, and we need take no further precautions with the slides in
which Balsam was used, but those mounted in glycerine jelly must be
ringed, for the reason that glycerine absorbs moisture from the air and
gradually liquifies.
The process of ringing is best performed upon a turntable, which any
dealer in microscope accessories will supply. It consists of a circular
brass plate which revolves about its centre and to which the slide to
be ringed is affixed. A bottle of ringing asphalt and a fine paint
brush are essentials. With a little of the asphalt upon our brush, we
revolve the turntable and place the brush against the edge of the cover
slip as it revolves, in such a manner that we paint a narrow rim of
asphalt over the junction of cover slip and slide. The asphalt prevents
moisture from reaching the glycerine jelly. Of course we may ring our
slides without making use of a turntable, but it is not easy to paint
a neat ring without mechanical assistance. For square cover slips it
is obvious that a turntable is useless. Specimens mounted in Canada
Balsam do not need ringing, for the Balsam is unaffected by air or the
moisture therein, when once it has set hard.
Several firms supply prepared microscope slides, and it is often
useful to know where reliable preparations may be obtained. The
slides supplied by Messrs Flatters & Garnett Ltd., 409 Oxford Road,
Manchester, are models of what well-made slides should be.
It always lends greater interest to the hobby if objects are found and
mounted by the microscopist himself. By going out into the fields, by
the pond-side, or along the shore, in search of interesting material
for examination, much will be learned of animal and plant habits which
even the microscope cannot reveal. Some of us, however, have neither
the opportunity to hunt for our specimens, nor the time to mount them
properly, and those of us who are so situated will be glad to know
where objects may be obtained.
Apparatus, useful for students of pond or sea-shore life, may be
obtained from Messrs Flatters & Garnett, who always have a goodly stock
of collecting jars, nets, &c. From the same firm, also, may be obtained
stains and any of the limited number of chemicals required by the
microscopist.
We have given a few simple directions for staining in our chapter on
Bacteria. In many cases it is absolutely necessary to have recourse to
stains in order to see the structure of the objects we are desirous of
examining; in other cases it is necessary to stain when we wish to know
the nature of the various parts of our object. Suppose, for example, we
wish to find out whether a plant section contains starch, we then add
iodine solution, and if any parts stain deep blue we know at once that
starch is present. There are other stains for other plant and animal
substances; stains for woody matter, stains for fats, &c., but the art
of staining is a science in itself, and would require many chapters
to describe fully. Most of the objects described in this book can be
studied in the natural state, but, even so, they may be rendered far
more beautiful by staining.
The method we described for the staining of bacteria does not apply to
such objects as plant sections, &c., and we propose to describe, as
briefly as possible, how to proceed with such objects. Suppose we are
examining one of the common pond plants, _Spirogyra_ for instance, and
we wish to see whether it contains starch. Our specimen is in a slide,
in a drop of water, and covered by a cover slip. In the first place, we
must obtain some fluffless blotting paper--the ordinary filter papers
sold by all chemists are excellent--from it we must cut about half a
dozen pieces, about half an inch by one inch, the exact size is not
important, and they need not be measured. These we must fold in the
centre, so that they can be made to stand up like an inverted V. From
our bottle of iodine solution we take a drop of the liquid on the end
of a glass rod and place it carefully at one edge of the cover slip,
avoiding allowing any of the solution to flow on to the upper surface
of the slip. Now, one of the pieces of blotting paper must be placed
upon the opposite side of the slide so that it will stand up; it must
then be moved till it just touches the edge of the cover slip. The
blotting paper will absorb the water from beneath the cover slip, and
in doing so the iodine solution will be drawn along to take the place
of the water. By proceeding in this manner and replacing the blotting
paper with a new piece as it becomes moist, also replenishing the drop
of iodine as it is used up, we act upon our object with a stronger and
stronger solution, and, in _Spirogyra_, the object which we took as our
example, we can see beautiful rosettes of starch grains, arranged at
regular intervals along the green bands of chlorophyll. This method of
staining may be used in most cases where we merely require a temporary
stain; by reversing the process and drawing water over the object by
means of blotting paper, it may be used in washing sections and parts
of plants. For very small objects, such as starch grains separated from
the plants in which they are formed, the method is hardly suitable, for
they are liable to be drawn along in the stream of liquid and lost.
For more permanent staining processes, we must use our watch glasses,
into which we pour the various liquids necessary for the operation. The
precise methods of staining, the periods during which objects should
remain in the staining solution, and the chemicals used for removing
excessive stain vary, as may be guessed, according to circumstances.
Some chemicals act very quickly, and staining takes place in a few
minutes; others act slowly, and with them it is necessary to subject
our specimens to their action for hours or even days. Then again, it is
obvious that large specimens take longer to stain than small ones, hard
objects are not so readily acted upon as soft ones. Experience alone
will show what is required in various cases.
Suppose, for example, we desire to stain a section in Carmalum, a
mixture of Carminic Acid 1 grain, Alum 10 grains, hot distilled water
200 c.c. We take three watch glasses, in one we place a few drops of
our stain, in another water, and in the third alcohol. Our section is
placed in the watch glass containing the carmalum and is left there
for about two minutes, then with the help of a small brush it is
transferred for a similar period to the watch glass containing water,
and finally it is placed in the alcohol. From the last watch glass it
may be transferred to glycerine jelly on a slide and mounted as already
described; Carmalum stains our section a beautiful pink.
There is a temptation to buy a large stock of stains of all hues and
of varied composition. The temptation should not be allowed to get the
better of us. In our early days at anyrate, we shall do far better if
we use but few stains and learn to understand their peculiarities.
The most useful selection for the beginner will comprise Haematoxylin,
Safranin, Eosin, Carbol-Fuchsin, Methylene Blue, and Carmalum. These
should all be obtained ready for use: this will save errors in
compounding, and, in certain cases, will save time; some of the stains,
Haematoxylin for example, are not fit for use till months after they
have been mixed with the other ingredients which form the complete
stain.
Frequently when examining small but lively water animals we may
feel the necessity of some method of sobering them. All the little
organisms, both plants and animals, which are provided with the little
whip-like structures mentioned so often in our pages, are difficult to
examine whilst they are in motion. There are many substances, we might
add, which would kill them, but in doing so we shall nearly always find
that they contract to such an extent as to lose all semblance of their
natural shape, and become useless as objects for our microscope. Should
our inclinations lead us in the direction of much study of these little
beings, we shall do well to keep handy a small bottle of Rousselet’s
solution; it is composed of 2 per cent. solution of hydrochlorate
of cocaine, 3 parts; Methylated Spirit, 1 part; water, 6 parts. Any
chemist will mix the solution for us. When we have occasion to examine
a too lively specimen, we simply run a drop of the solution under the
cover slip, as described in our remarks on staining with iodine, and
the creature we are examining will abandon its frolics and conveniently
remain in a fully expanded state.
Many of our readers will be anxious to carry their microscopic
investigations to a more advanced stage than we have reached in our
pages. Some will have the advantage of a teacher, and it is a great
advantage to have someone who can show rather than merely explain what
should be done; others not so well placed may like to know a few really
useful books which will help them in their work. “The Microscope and
its Revelations,” by B. H. Dallinger, is a large and therefore costly
work, but it contains a rare fund of information for the microscopist.
Smaller and eminently suitable for the general worker is “Modern
Microscopy,” by Messrs Cross & Cole. Those who wish to specialize in
one of the sciences, such as Botany, Zoology, or Geology, will find no
lack of books dealing with the subject that most appeals to them. For
photographers who are also microscopists, we know of no better books
than “Practical Principles of Plain Photomicrography,” by G. West, or
the more advanced “Handbook of Photomicrography,” by Messrs Hind &
Randles.
Neither books nor teacher will be able to reveal to any of us all the
secrets of the microscope. By its means a new world is unfolded before
the eyes of mankind, a world of unlimited possibilities. No man will
ever see all that the microscope can show him, each day some fresh
wonder is looked upon for the first time.
As a hobby, microscopy can hardly be excelled. It is a sensible hobby
and, after the initial outlay, need cost us but a few pence each year.
We hear someone say that he prefers an outdoor hobby, but surely the
microscope, if used intelligently, will take us out of doors. The
search for specimens is one of the attractions of microscopy. As an
instrument for the serious worker the microscope compares favourably
with any other invention of mankind.
Think for a moment what we owe to the microscope in a hundred and one
walks of life, and then you will realize the romance attached to the
instrument, and its revelations from the days of Euclid to the present
time.
INDEX
Abbe, 285.
Aberration, chromatic, 47.
Aberration, spherical, 49.
Aconite, 259.
_Actinophrys Sol_, 75.
Adjustment, coarse, 58;
fine, 59.
Adulteration of food, 137.
_Algæ_, 225.
Alkaloids, 144.
American water-weed, 98.
_Amœba Proteus_, 67.
_Anatome Plantarum_, 32.
Animal cells, 168.
Animalcule, bell, 70;
protean, 67;
slipper, 68;
sun, 75;
wheel, 74.
Animal fibres, 262.
Annealing, 284, 289.
Aphids, 36.
Apparatus, choice of, 137.
Aristotle, 32.
Bacilli, 159.
_Bacillus Subtilis_, 160.
Bacteria, 36, 37, 38, 152;
cultures of, 165;
examination of, 160;
origin of, 155;
size of, 156;
splitting of, 157;
staining of, 162.
Barnacle, 221.
Bees, 34, 195, 199.
Bird’s head, 76, 216.
Bladderwort, 104.
Blood, circulation of, 23, 171.
Blood corpuscles, red, 34, 170;
white, 170.
Blood crystals, 173, 174.
Boreel, 20.
Borel, 26.
_Botrydium Granulatum_, 92.
Bread, 142.
Brewing, 260.
Broomrape, 180.
Brown, 39.
Browne, 24.
Bubbles in glass, 286.
Buffalo gnat, 201.
Buffon, 39.
Burning glass, 17.
Butter, 148;
renovated, 149.
Caddis fly, 202.
Calcium carbonate, 131, 229.
_Callithamnion Corymbosum_, 237.
Canada Balsam, 300.
_Carchesium Spectabile_, 72.
Cells, 99.
Chelifer, 31, 200.
Chemical analysis, 248.
Chemistry, 25, 200.
Chicory, 145.
_Chlamydomonas Angulosa_, 85.
Chlorophyll, 98, 226.
Cicada, 204.
Citric acid, 258.
_Cladophora Glomerata_, 90.
Classification, 39.
Clay, 129.
Cleavers, 111.
Club moss, 136.
Cluster cups, 186.
Cocaine, 144.
Cocci, 159.
Cocoa, 144.
Coffee, 144.
Colonial animals, 212.
Colour of plants, 97.
Comb bearer, 210.
Compound microscope, 20;
theory of, 51;
to make, 52.
Compressed enteromorpha, 228.
Condenser, 60, 292.
_Corallina Officinalis_, 229.
Coralline, 229.
Cork, 31.
Corn flour, 141.
Cotton, 262.
Cover slip, 64.
Crab, 222.
Cricket, 205.
_Cristatella Mucedo_, 77.
Crystal layer, 256.
Crystallization, 250.
Crystals in plants, 255.
Crystals of alum, 251;
barium chloride, 252;
borax, 251;
calcium oxalate, 255;
copper sulphate, 251;
gold chloride, 256;
iron sulphate, 251;
potassium chlorate, 252;
salt, 251;
sugar, 251;
washing soda, 251.
Cuprammonia, 263.
Cuttlefish, 122.
Cuvier, 39.
_Cyclops_, 81.
_Cypris_, 80.
Darwin, 39.
“Dead Men’s Ropes,” 230;
“Toes,” 211.
_Delessera Sanguinea_, 229.
_Demodex Folliculorum_, 176.
Depth of focus, 240.
Descartes, 18.
Desmids, 85, 225.
Devil’s apron, 230.
Diaphragm, 61;
iris, 62.
Diatoms, 82, 225, 238.
_Didymograptus_, 135.
Digges, L., 18, 20.
Digges, T., 18.
Diplococci, 159.
_Diplograptus_, 135.
Divini, 19.
Dodder, 180.
Doris, 222.
Double staining, 164.
_Draparnalda Glomerata_, 91.
Drebbel, 21.
Dyes in food, 150.
Earliest microscopes, 18.
Early naturalists, 37.
Ears of insects, 205.
_Ectocarpus Siliculosus_, 228, 231.
Egg raft, 202.
Eggs of frogs, 124.
Engyoscopes, 19.
Entomology, 192.
Euclid, 17.
_Euglena Viridis_, 88.
Eyepiece, 56;
choice of, 292;
Huygens, 63.
Fallen stars, 94.
Feathered hair of bees, 199.
Feathers, 118.
Feelers of beetles, 198;
gnats, 198;
moths, 198.
Ferments, 154.
Finger prints, 33.
Flax, 262.
Fleas, 23, 31, 37;
water, 79.
Flint, 129.
Flour, 272.
Flowers, grass, 106.
Focus of lens, 275.
Food adulteration, 137.
Foot of butterfly, 197;
fly, 31, 198;
louse, 198.
_Foraminifera_, 30, 135.
Formic acid, 258.
Fossils, 39, 130, 134.
Fringe wings, 207.
Fruits of burdock, 111;
dandelion, 111;
goosegrass, 111;
groundsel, 111;
thistle, 111.
Fusion tests, 253.
Galileo, 21, 22.
Geology, 125.
Germination, 32.
Glass, 282;
Bohemian, 283;
crown, 285;
flint, 283, 285;
Jena, 285;
optical, 284;
plate, 283.
Glucose, 151.
Gluten strings, 273.
Glycerine jelly, 301.
Gordon, 17.
_Gossypium_, 262.
Graaf, 19.
Granite, 126.
Graptolites, 135.
Grasshopper, 205.
Grew, 33.
Guinard, 284.
Hair, 101, 261, 267;
root, 103.
Hardness of glass, 286.
Harvestmen, 115.
Hay Bacillus, 160.
Hemp, 264.
Hodierna, 24.
Honey, 149.
Hooke, 19, 29, 35, 215, 216.
Horsetails, 110, 136.
House fly, 25, 195, 198.
Human blood, 169.
Humboldt, 39.
_Hydra_, 37, 69, 135, 209, 215.
_Hydrodictyon Reticulatum_, 93.
Image, inverted, 56;
real, 52, 55;
virtual, 52, 53, 55.
Infection, 26.
Infusorians, 36.
Insect transformation, 34.
Insectivorous plants, 104.
_Insectorum Theatrum_, 34.
Intestinal enteromorpha, 228.
Jute, 264.
Killing specimens, 306.
Kircher, 23, 26, 36.
Laminaria, 230.
Leaf, bloom, 101;
cabbage, 100;
folded, 101;
hair, 101;
heather, 101;
iris, 100;
marram grass, 101;
rolled, 101;
structure, 100;
veins, 103;
water lily, 100.
Leeuwenhoek, 19, 35, 36, 37, 38.
Lenses, perfect, 49;
shapes of, 44.
Lice, 23, 31, 198;
water, 79;
wood, 79, 135.
Light, 40;
rays, 41.
Limestone, 131.
Linen, 264.
Linnæus, 39.
Liver fluke, 33, 76, 188.
_Lophopus Crystallinus_, 76.
Lung books, 116.
Magnetite, 129.
Magnification, to increase, 64.
Malaria, 173.
Malpighi, 23, 31, 32.
Marble bleb, 210.
Mealy bug, 206.
Mechanical stage, 65.
Medical work, 26.
Mercer, 263.
Mercerised cotton, 263.
Mermaid’s fingers, 211.
Miall, 37.
Mica, 126, 129.
Micrococci, 159.
Micrographia, 29.
Micro-chemical analysis, 257.
Microscope, choice of, 291;
making, 29;
stand, 56;
tube, 57;
use of, 59.
Microscopia, ludicra, 19;
seria, 19.
Mildew, 183;
gooseberry, 188;
rose, 187.
Milk, examination of, 138;
bacteria in, 154.
Money spinner, 115.
_Monograptus_, 135.
Morphia, 259.
Mother of pearl, 122.
Mouffet, 18, 23.
Mould, 289.
Mouth of butterfly, 195;
fly, 195;
gnat, 196;
green fly, 196;
pangonia, 196;
stable fly, 196.
Mud, 129.
Mushroom, 182.
Musical insects, 204.
Mussel, swan, 77.
Mustard, 145.
Nectar, 32.
Nicotine, 93.
_Nostoc_, 144, 259.
Ocular, 56.
Objective, 56, 63;
choice of, 292.
_Oscillatoria_, 89.
Paper, 145, 265;
cotton rag, 266;
hemp, 266;
linen rag, 266;
manufacture, 126, 260.
Parasites of house fly, 200.
Pasteur, 39.
Pebrine, 272.
Pepper, 145.
Phosphorescence, 223.
Photo-micrographs, 274.
Piperine, 146.
Plague, 175.
Plant animals, 89.
Plant structure, 31.
_Plumatella Repens_, 77.
Pocket magnifier, 50.
Poisons, 259;
testing for, 249.
Polariscope, 141.
Pollen grains, 106;
of _Eschscholtzia_, 107;
mallow, 107;
Scotch fir, 107.
Pond life, apparatus for collecting, 66.
Pop weed, 228.
Porta, 18, 20.
Potato disease, 183.
Preservatives in food, 149.
Principal focus, 45, 54.
Prisms, 43.
Purple of Cassius, 257.
Pure cultures, 165.
Quartz, 126, 129.
Raphides, 255.
Rasping organs of snails, 123.
Réaumur, 38.
Refraction, 42.
Rocks, 126;
primary, 128;
secondary, 128.
Rotifers, 36, 74.
Rousselet’s solution, 306.
Rust of wheat, 185.
Sand, 126, 128.
Sandstone, 130.
Scales of fishes, 120;
lizards, 120;
snakes, 120.
Scheiner, 23.
Schott, 285.
Sea anemone, 208;
cucumber, 218;
lace, 230;
lemon, 222;
lettuce, 228;
mat, 31, 76, 215;
mouse, 220;
pink, 225;
slug, 218;
urchin, 218;
weed, 225.
Section cutting, 294;
in pith, 298.
Sections, plant, 97;
rock, 133.
Seeds, 111;
of campion, 180;
clover, 179;
corn buttercup, 180;
cornflower, 180;
evening primrose, 180;
grass, 179;
larkspur, 180;
rib wort plantain, 180;
spurrey, 180;
wild carrot, 180.
Seed testing, 178.
Seneca, 17.
_Serpula_, 221.
Shale, 129.
Shell binder, 221.
Shells, 122, 129.
Shoddy, 269.
Shrimp, fresh water, 79.
Silk, 270;
artificial, 272;
glue, 270;
tussore, 271;
weed, 90;
worm, 32.
Silver fish, 31.
_Simulium_, 201.
Slate, 129.
Sleeping sickness, 173.
Slides, 64.
Slide making, 295;
mounting, 300;
ringing, 301.
Slugs, 123.
Smuts, 184.
Snail, 34, 35, 37, 123;
rasping organ of, 123;
water, 77.
Solution tests, 254.
Spicules of Doris, 222;
sponge, 132, 138.
Spider, 31, 37, 112;
crab, 115;
drassid, 114;
garden, 112;
house, 113;
wolf, 114;
zebra, 114.
Spinnerets, 116.
_Spirilla_, 159.
_Spirogyra_, 94, 225.
Sponge, bath, 72;
pond, 72;
purse, 73;
river, 72;
spicules, 132, 138.
_Spongia Coalita_, 213.
Spores, 109, 162, 182.
Stains, 291, 308.
Stalactites, 132.
_Staphylococci_, 159.
Starch, 139;
arrowroot, 141;
banana, 142;
barley, 141;
bean, 142;
lentil, 142;
maize, 141;
pea, 142;
potato, 140;
rice, 141;
rye, 141;
sago, 142;
spurge, 142;
tapioca, 142;
to stain, 142.
Star fish, 213.
Stinging darts, 209;
hairs, 32, 102.
Stomata, 32.
“Stops,” 61.
_Streptococci_, 159.
Striæ in glass, 286.
Strychnine, 259.
Sundew, 104.
Swammerdam, 33.
_Synapta Inhærens_, 219.
Tadpole, 34, 124.
Tangle, 230;
fingered, 230.
Tea, 143.
Telescope, 20.
Theine, 144.
Thrips, 207.
_Trichinella Spiralis_, 176.
Trilobites, 135.
_Ulva Lactuosa_, 228.
_Vaucheria Sessilis_, 91.
Vegetable fibres, 261.
Vinci, 18.
Vinegar eels, 31.
Vitrea muscaria, 19;
publicaria, 19.
_Volvox_, 37, 87, 225.
_Vorticella_, 71.
Water net, 93.
Water testing, 146.
Wings of bees, 195;
butterflies, 193;
dragonflies, 194;
flies, 194;
gnats, 194.
Wool, 268;
fat, 268.
Wrack, bladder, 228, 233, 236;
channelled, 233, 234;
flat, 233;
knobbed, 234;
notched, 234.
Yeast, 36.
Zacharias, 20, 21, 22.
Zoospores, 231.
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7. =ARCTIC REGIONS.= H. W. G. HYRST.
8. =RED INDIANS.= H. W. G. HYRST.
9. =TRAPPERS & HUNTERS.= E. YOUNG, B.SC.
10. =SOUTHERN SEAS.= R. STEAD, B.A.
* * * * *
SEELEY’S MISSIONARY LIVES FOR CHILDREN.
With Covers in 4 Colours. Illustrated. Price 1s. nett.
[Illustration]
“With attractive coloured covers, well written, and at the
wonderfully low price of one shilling. Will bring delight to the boys
and girls who get them.”--_Missionary Herald._
BARBROOKE GRUBB OF PARAGUAY. C. T. BEDFORD.
MOFFAT OF AFRICA. N. J. DAVIDSON.
ARNOT OF AFRICA. NIGEL B. M. GRAHAME.
BISHOP BOMPAS OF THE FROZEN NORTH. NIGEL B. M. GRAHAME, B.A.
LIVINGSTONE OF AFRICA. C. T. BEDFORD.
JOHN WILLIAMS OF THE SOUTH SEA ISLANDS. N. J. DAVIDSON.
HANNINGTON OF AFRICA. NIGEL B. M. GRAHAME, B.A.
PENNELL OF THE INDIAN FRONTIER. NORMAN J. DAVIDSON, B.A.(Oxon).
JUDSON OF BURMA. NIGEL B. M. GRAHAME, B.A.
* * * * *
THE PRINCE’S LIBRARY
Handsome Gift Books for Young People. Many Illustrations. 4s. 6d.
nett each.
LATEST VOLUME
17. THE COUNT OF THE SAXON SHORE. Prof. A. J. CHURCH.
RECENTLY ISSUED
1. =LAST OF THE WHITECOATS.= G. I. WHITHAM.
2. =DIANA POLWARTH, ROYALIST.= J. CARTER.
3. =THE FALL OF ATHENS.= Prof. CHURCH, M.A.
4. =THE KING’S REEVE.= EDWARD GILLIAT, M.A.
5. =CABIN ON THE BEACH.= M. E. WINCHESTER.
6. =THE CAPTAIN OF THE WIGHT.= F. COWPER.
7. =GRIMM’S FAIRY TALES.= New Translation.
8. =THE WOLF’S HEAD.= EDWARD GILLIAT, M.A.
9. =ANDERSEN’S FAIRY TALES.=
10. =CAEDWALLA.= FRANK COWPER.
11. =THE ARABIAN NIGHTS’ ENTERTAINMENTS.=
12. =ROBINSON CRUSOE.= DANIEL DEFOE.
13. =THE VICAR OF WAKEFIELD.= GOLDSMITH.
14. =CRANFORD.= Mrs. GASKELL.
15. =PATRIOT AND HERO.= Prof. CHURCH.
16. =MINISTERING CHILDREN.= CHARLESWORTH.
* * * * *
HEROES OF THE WORLD LIBRARY
Each Volume fully Illustrated. Ex. Crown 8vo. 6s. nett.
[Illustration:
THE STORMING OF JHANSI.
_From “Heroes of the Indian Mutiny.”_
]
LATEST VOLUME
9. HEROES OF THE INDIAN MUTINY.
EDWARD GILLIAT, M.A., sometime Master at Harrow School.
With Full-page Illustrations.
“A capital book.... Full of stories of heroic deeds, intrepidity,
and determination performed in the face of fearful odds.”--_The
Field._
RECENTLY ISSUED IN THIS SERIES.
HEROES OF
1. =MISSIONARY ENTERPRISE.= CLAUD FIELD, M.A.
2. =PIONEERING.= EDGAR SANDERSON, M.A.
3. =MISSIONARY ADVENTURE.= CANON DAWSON, M.A.
4. =MODERN CRUSADES.= E. GILLIAT, M.A.
5. =MODERN INDIA.= E. GILLIAT, M.A.
6. =THE ELIZABETHAN AGE.= E. GILLIAT, M.A.
7. =MODERN AFRICA.= E. GILLIAT, M.A.
8. =THE SCIENTIFIC WORLD.= C. R. GIBSON, F.R.S.E.
* * * * *
THE DICK VALLIANT SERIES
By Lieut.-Commander JOHN IRVING, R.N.
With Coloured Illustrations. Extra Crown 8vo. 5s. nett.
DICK VALLIANT, NAVAL CADET.
A stirring story of the life & adventures of a naval cadet in peace &
war.
_Dick Valliant_ is the first volume of a new series of books for boys
which will deal with the most exciting & interesting naval incidents of
the Great War. It will be followed in 1928 by the second volume entitled
DICK VALLIANT IN THE DARDARNELLES.
A new volume will be added each year thereafter.
* * * * *
THE PINK LIBRARY
Books for Boys & Girls. Illustrated. 2s. 6d. nett each.
NEW VOLUME
29. THE PIRATE. CAPT. MARRYAT.
SOME OF THE 29 VOLS. IN THIS SERIES
1. =LIONHEARTED.= Canon E. C. DAWSON.
3. =TO THE LIONS.= Prof. A. J. CHURCH.
9. =A GREEK GULLIVER.= A. J. CHURCH.
15. =MISSIONARY HEROES IN ASIA.= J. G. LAMBERT, M.A., D.D.
16. =LITTLE WOMEN.= L. M. ALCOTT.
17. =GOOD WIVES.= L. M. ALCOTT.
18. =MISSIONARY HEROES IN AFRICA.= J. G. LAMBERT, D.D.
21. =GRIMM’S FAIRY TALES.= A New Translation.
22. =WHAT KATY DID AT HOME.= SUSAN COOLIDGE.
28. =MISSIONARY HEROINES IN INDIA.= Canon DAWSON.
[Illustration:
BOXES FROM HOME.
_From “What Katy Did.”_
]
* * * * *
MISSIONARY BIOGRAPHIES--a new series
With many Illustrations and a Frontispiece in Colours, 3s. 6d. nett
each.
LATEST VOLUME
12. BISHOP PATTESON OF THE CANNIBAL ISLANDS. E. GRIERSON.
“The life story of Bishop Patteson is of such thrilling interest
that boys and girls cannot help being fascinated by it.”--_Teachers’
Times._
[Illustration: Vol. 3.]
[Illustration: Vol. 4.]
[Illustration: Vol. 1.]
[Illustration: Vol. 2.]
EARLIER VOLUMES
1. =A HERO OF THE AFGHAN FRONTIER.= The Life of Dr. T. L. PENNELL, of
Bannu. A. M. PENNELL, M.B., B.S.(Lond.), B.SC.
2. =MISSIONARY CRUSADERS.= Rev. CLAUD FIELD, M.A.
3. =JUDSON, THE HERO OF BURMA.= JESSE PAGE, F.R.G.S.
4. =ON TRAIL AND RAPID BY DOG-SLED AND CANOE.= Bishop Bompas’s Life
amongst Red Indians & Esquimo. Rev. H. A. CODY, M.A.
5. =MISSIONARY KNIGHTS OF THE CROSS.= Rev. J. C. LAMBERT, M.A., D.D.
6. =MISSIONARY HEROINES OF THE CROSS.= By CANON DAWSON.
7. =LIVINGSTONE, THE HERO OF AFRICA.= By R. B. DAWSON, M.A.(Oxon).
8. =BY ESKIMO DOG-SLED & KAYAK.= By Dr. S. K. HUTTON, M.B., C.M.
9. =MISSIONARY EXPLORERS.= By R. M. A. IBBOTSON.
10. =ARNOT, A KNIGHT OF AFRICA.= Rev. E. BAKER.
11. =BARBROOKE GRUBB, PATHFINDER.= N. J. DAVIDSON, B.A.(Oxon)
* * * * *
[Illustration: _Binding of the Russell Series._]
THE RUSSELL SERIES FOR BOYS & GIRLS
A Series of handsome Gift Books for Boys & Girls. With Coloured and
other Illustrations. 3s. 6d. nett each.
LATEST VOLUME
12. STORIES OF EARLY EXPLORATION. By A. WILLIAMS, B.A.(Oxon.)
SOME OF THE 11 EARLIER VOLUMES
=STORIES OF RED INDIAN ADVENTURE.= Stirring narratives of bravery and
peril. H. W. G. HYRST.
=STORIES OF ELIZABETHAN HEROES.= E. GILLIAT, M.A., sometime Master at
Harrow.
=STORIES OF POLAR ADVENTURE.= H. W. G. HYRST.
=STORIES OF GREAT PIONEERS.= EDGAR SANDERSON, M.A.
=STORIES OF GREAT SCIENTISTS.= C. R. GIBSON, F.R.S.E.
* * * * *
BOOKS ON POPULAR SCIENCE
By CHARLES R. GIBSON, LL.D., F.R.S.E.
“Mr. Gibson has fairly made his mark as a populariser of scientific
knowledge.”--_Guardian._
“Mr. Gibson has a fine gift of exposition.”--_Birmingham Post._
MODERN ELECTRICAL CONCEPTIONS. Illustrated. 12s. 6d. nett.
IN THE SCIENCE OF TO-DAY SERIES. Illustrated. 6s. nett each.
SCIENTIFIC IDEAS OF TO-DAY. A Popular Account of the Nature of
Matter, Electricity, Light, Heat, &c. &c.
ELECTRICITY OF TO-DAY.
WIRELESS OF TO-DAY. By CHARLES R. GIBSON, F.R.S.E., & W. B. COLE.
IN THE ROMANCE LIBRARY. Illustrated. 6s. nett each.
THE ROMANCE OF MODERN ELECTRICITY.
THE ROMANCE OF MODERN PHOTOGRAPHY.
THE ROMANCE OF MODERN MANUFACTURE.
THE ROMANCE OF SCIENTIFIC DISCOVERY.
THE ROMANCE OF COAL.
HEROES OF THE SCIENTIFIC WORLD. 6s. nett.
WHAT IS ELECTRICITY? Long 8vo. With 8 Illustrations. 6s. nett.
* * * * *
THE AMUSEMENTS SERIES
By CHARLES R. GIBSON, LL.D., F.R.S.E.
Extra Crown 8vo. Fully illustrated. 5s. nett each.
[Illustration]
VOL. III.
CHEMICAL AMUSEMENTS & EXPERIMENTS.
NEW VOLUME.
VOL. II.
SCIENTIFIC AMUSEMENTS & EXPERIMENTS.
VOL. I.
ELECTRICAL AMUSEMENTS & EXPERIMENTS.
“Dr. C. R. GIBSON’S BOOKS ON SCIENCE HAVE A WORLD-WIDE REPUTATION.
Every experiment is so clearly explained and skilfully handled that
the lesson is learnt at once without difficulty. He has such an
engaging way of imparting his knowledge that he is entertainingly
educative. An admirable gift book, packed with illustrations showing
how the experiments can be carried out without much trouble or
expense.”--_Yorkshire Observer._
* * * * *
CLASSICAL STORIES. By Prof. A. J. CHURCH
“The Headmaster of Eton (Dr. the Hon. E. Lyttelton) advised his
hearers, in a recent speech at the Royal Albert Institute, to read
Professor A. J. Church’s ‘Stories from Homer,’ some of which, he
said, he had read to Eton boys after a hard school day, and at an
age when they were not in the least desirous of learning, but were
anxious to go to tea. The stories were so brilliantly told, however,
that those young Etonians were entranced by them, and they actually
begged of him to go on, being quite prepared to sacrifice their
tea-time.”
Profusely illustrated. Extra Crown 8vo. 5s. nett each.
The Children’s Æneid
The Children’s Iliad
The Children’s Odyssey
The Faery Queen & her Knights
The Crusaders
Greek Story and Song
Stories from Homer
Stories from Virgil
The Crown of Pine
Stories of the East from Herodotus
Story of the Persian War
Stories from Livy
Roman Life in the Days of Cicero
Count of Saxon Shore
The Hammer
Story of the Iliad
Story of the Odyssey
Heroes of Chivalry & Romance
Helmet and Spear
Stories of Charlemagne
* * * * *
THE KING’S LIBRARY
With many Coloured Illustrations. 5s. nett each.
LATEST VOLUME
7. CRANFORD. MRS. GASKELL.
=THE CHILDREN’S ODYSSEY.= Prof. A. J. CHURCH
=THE CHILDREN’S ILIAD.= Prof. A. J. CHURCH
=A KNIGHT ERRANT.= N. J. DAVIDSON, B.A.
=THE CHILDREN’S AENEID.= Prof. A. J. CHURCH
=VICAR OF WAKEFIELD.= GOLDSMITH
=THE FAERY QUEEN.= Prof. A. J. CHURCH
* * * * *
THE MARVEL LIBRARY
Fully Illustrated. Each Volume, 4s. nett.
LATEST VOLUME
10. MARVELS OF ANIMAL INGENUITY. By C. A. EALAND, M.A.
=MARVELS OF=
=1. SCIENTIFIC INVENTION.= By T. W. CORBIN.
=2. AVIATION.= Maj. C. C. TURNER, R.A.F.
=3. GEOLOGY.= By E. S. GREW, M.A.
=4. WAR INVENTIONS.= By T. W. CORBIN.
=5. PHOTOGRAPHY.= C. R. GIBSON.
=6. THE SHIP.= By Lieut-Com. E. KEBLE CHATTERTON, R.N.V.R.
=7. MECHANICAL INVENTION.= By T. W. CORBIN.
=8. WORLD’S FISHERIES.= By SIDNEY WRIGHT.
=9. RAILWAYS.= By A. WILLIAMS.
[Illustration: _Specimen Binding._]
* * * * *
TWO EXCELLENT RECITERS
Each volume consists of over 700 pages, cloth, 6s. nett each. Also
Thin Paper Edition (measuring 6-3/4 × 3/4 ins., and only 3/4 of an
inch thick), price 7s. 6d. nett each. They also contain practical
Introductions by Prof. Cairns James, Professor of Elocution at the
Royal College of Music.
THE GOLDEN RECITER
Selected from the works of Rudyard Kipling, R. L. Stevenson, Conan
Doyle, Thomas Hardy, Austin Dobson, Christina Rossetti, Maurice
Hewlett, A. W. Pinero, Sydney Grundy, &c., &c.
“An admirable collection of pieces both in prose and
verse.”--_Spectator._
“Offers an unusual wealth from authors of to-day.”--_The Athenæum._
“Far superior to anything we have yet seen.”--_Western Press._
THE GOLDEN HUMOROUS RECITER
From Anstey, Barrie, Major Drury, J. K. Jerome, Barry Pain, A. W.
Pinero, Owen Seaman, G. Bernard Shaw, &c., &c.
“A most comprehensive & well-chosen collection of some hundreds of
pieces. A small encyclopædia of English humour.”--_The Spectator._
“Unquestionably the best collection of modern humorous pieces for
recitation which has yet been issued.”--_The Dundee Advertiser._
THE PILGRIM’S WAY
By Prof. SIR A. T. QUILLER-COUCH
A Little Book for Wayfaring Men. In Prose and Verse. Fcap. 8vo,
cloth, 3s. 6d. nett; thin paper, 5s. nett; buff leather yapp, in a
box, 6s. nett.
“The very flower of a cultivated man’s reading.”--_Country Life._
* * * * *
MICROSCOPES & ACCESSORIES
MICROSCOPICAL PREPARATIONS.
Large Stock of Slides in Botany, Zoology, Petrology, Diatoms, and
Commercial Fibres.
PHOTOMICROGRAPHIC AND OTHER SCIENTIFIC LANTERN SLIDES.
See Illustrations in this book. Thousands in Stock.
[Illustration: MERSOL]
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POND LIFE and other Collecting Apparatus.
APLANATIC MAGNIFIERS.
FLATTERS & GARNETT LTD.,
309 OXFORD ROAD (Opposite the University),
MANCHESTER.
* * * * *
THE ROMANCE OF AERONAUTICS
AN INTERESTING ACCOUNT OF THE GROWTH AND ACHIEVEMENTS OF ALL KINDS OF
AERIAL CRAFT
BY CHARLES C. TURNER
Holder of the Royal Aero Society’s Aviation Certificate, Author of
“Aerial Navigation of To-day,” &c. &c.
_With Fifty-two Illustrations and Diagrams. Extra Crown 8vo. 5s._
“A capital survey by an expert writer.”--_Guardian._
“Brightly written, exceptionally well illustrated, and shows a due
regard for historical accuracy.”--_Aeronautics._
“A most interesting book.”--_Spectator._
“A most complete history ... told with lucidity and enthusiasm, and
brimful of interest.”--_Truth._
“Set forth with a persuasive and accomplished pen.”--_Pall Mall
Gazette._
THE ROMANCE OF ANIMAL ARTS & CRAFTS
DESCRIBING THE WONDERFUL INTELLIGENCE OF ANIMALS REVEALED IN THEIR
WORK AS MASONS, PAPER MAKERS, RAFT & DIVING-BELL BUILDERS, MINERS,
TAILORS, ENGINEERS OF ROADS & BRIDGES, &c. &c.
BY H. COUPIN, D.SC. & JOHN LEA, B.A. (CANTAB.)
_With Thirty Illustrations. Extra Crown 8vo. 5s._
“Will carry most readers, young and old, from one surprise to
another.”--_Glasgow Herald._
“A charming subject, well set forth, and dramatically
illustrated.”--_Athenæum._
“It seems like pure romance to read of the curious ways of Nature’s
craftsmen, but it is quite a true tale that is set forth in this
plentifully illustrated book.”--_Evening Citizen._
“This popular volume of Natural History is written by competent
authorities, and besides being entertaining is instructive and
educative.”--_Liverpool Courier._
THE ROMANCE OF MISSIONARY HEROISM
TRUE STORIES OF THE INTREPID BRAVERY AND STIRRING ADVENTURES OF
MISSIONARIES WITH UNCIVILIZED MEN, WILD BEASTS, AND THE FORCES OF
NATURE IN ALL PARTS OF THE WORLD
BY JOHN C. LAMBERT, M.A., D.D.
_With Thirty-six Illustrations. Extra Crown 8vo. 5s._
“A book of quite remarkable and sustained interest.”--_Sheffield
Telegraph._
“The romantic aspect of missionary careers is treated without undue
emphasis on the high prevailing motive. But its existence is the
fact which unifies the eventful history.”--_Athenæum._
“We congratulate Dr. Lambert and his publishers. Dr. Lambert
has proved that the missionary is the hero of our day, and
has written the most entrancing volume of the whole romantic
series.”--_Expository Times._
THE ROMANCE OF EARLY BRITISH LIFE
FROM THE EARLIEST TIMES TO THE COMING OF THE DANES
BY PROFESSOR G. F. SCOTT ELLIOT
M.A. (Cantab.), B.Sc. (Edin.), F.R.G.S., F.L.S.
Author of “The Romance of Savage Life,” “The Romance of Plant Life,”
&c. &c.
_With over Thirty Illustrations. Extra Crown 8vo. 5s._
“Calculated to fascinate the reader.”--_Field._
“Every chapter is full of information given in fascinating form.
The language is simple, the style is excellent, and the information
abundant.”--_Dundee Courier._
* * * * *
THE IAN HARDY SERIES
BY
COMMANDER E. HAMILTON CURREY, R.N.
_Each Volume with Illustrations in Colour. 5s. each_
IAN HARDY’S career in H.M. Navy is told in four volumes, which are
described below. Each volume is complete in itself, and no knowledge
of the previous volumes is necessary, but few boys will read one of
the series without wishing to peruse the others.
IAN HARDY, NAVAL CADET
“A sound and wholesome story giving a lively picture of a naval
cadet’s life.”--_Birmingham Gazette._
“A very wholesome book for boys, and the lurking danger of Ian’s ill
deeds being imitated may be regarded as negligible in comparison
with the good likely to be done by the example of his manly, honest
nature. Ian was a boy whom his father might occasionally have reason
to whip, but never feel ashamed of.”--_United Service Magazine._
IAN HARDY, MIDSHIPMAN
“A jolly sequel to his last year’s book.”--_Christian World._
“The ‘real thing.’... Certain to enthral boys of almost any age who
love stories of British pluck.”--_Observer._
“_Commander E. Hamilton Currey, R.N., is becoming a serious rival to
Kingston as a writer of sea stories._ Just as a former generation
revelled in Kingston’s doings of his three heroes from their middy
days until they became admirals all, so will the present-day boys
read with interest the story of Ian Hardy. Last year we knew him
as a cadet; this year we get _Ian Hardy, Midshipman_. The present
instalment of his stirring history is breezily written.”--_Yorkshire
Observer._
IAN HARDY, SENIOR MIDSHIPMAN
“Of those who are now writing stories of the sea, Commander Currey
holds perhaps the leading position. He has a gift of narrative, a
keen sense of humour, and above all he writes from a full stock of
knowledge.”--_Saturday Review._
“_It is no exaggeration to say that Commander Currey bears worthily
the mantle of Kingston and Captain Marryat._”--_Manchester Courier._
“The Ian Hardy Series is just splendid for boys to read, and the best
of it is that each book is complete in itself. But not many boys will
read one of the series without being keenly desirous of reading all
the others.”--_Sheffield Telegraph._
IAN HARDY FIGHTING THE MOORS
“By writing this series the author is doing national service, for he
writes of the Navy and the sea with knowledge and sound sense....
What a welcome addition the whole series would make to a boy’s
library.”--_Daily Graphic._
“The right romantic stuff, full of fighting and hairbreadth
escapes.... Commander Currey has the secret of making the men and
ships seem actual.”--_Times._
“By this time Ian Hardy has become a real friend and we consider him
all a hero should be.”--_Outlook._
* * * * *
A HERO OF THE AFGHAN FRONTIER
THE SPLENDID LIFE STORY OF T. L. PENNELL, M.D., B.Sc., F.R.C.S.
RETOLD FOR BOYS & GIRLS
BY ALICE M. PENNELL, M.B., B.S. (Lond.), B.Sc.
_With many Illustrations & a Frontispiece in Colour. Extra Crown 8vo.
2s. 6d._
“This is the glorious life story of Dr T. L. Pennell retold for
boys and girls.”--_Church Family Newspaper._
“The life story of a fearless Englishman of the best kind.”--_Daily
Telegraph._
“One of the very finest men who ever devoted his life to the
Missionary cause.”--_Guardian._
“A great story of a grand Christian hero.”--_Christian World._
MISSIONARY KNIGHTS OF THE CROSS
TRUE STORIES OF THE SPLENDID COURAGE & PATIENT ENDURANCE OF
MISSIONARIES IN THEIR ENCOUNTERS WITH UNCIVILIZED MAN, WILD BEASTS &
THE FORCES OF NATURE IN ALL PARTS OF THE WORLD
BY CANON E. C. DAWSON, M.A. (Oxon.)
_With Thirty-six Illustrations & a Frontispiece in Colour. Extra
Crown 8vo. 2s. 6d._
“After all there are few men who see so much of adventure as the
missionaries who go fother among the uncivilised. No better book
could be put into the hands of a lad than this present record of
derring-do.... A volume of thrilling adventure.”--_Eastern Morning
News._
“A most entrancing book.”--_Aberdeen Daily Journal._
“Most readable. Written with rare skill and attractively
illustrated.”--_Expository Times._
ON TRAIL & RAPID BY DOG-SLED & CANOE
THE STORY OF BISHOP BOMPAS’S LIFE AMONGST THE RED INDIANS & ESKIMO.
RETOLD FOR BOYS & GIRLS
BY THE REV. H. A. CODY, M.A.
_With Twenty-six Illustrations & a Frontispiece in Colours. Extra
Crown 8vo. 2s. 6d._
“A book of golden deeds, full of inspiration.”--_Queen._
“An admirable picture of a great career.”--_Spectator._
“An admirable book for the young, full of interest and of healthy
romance.”--_Irish Times._
“Should prove an inspiration and a help to all young
people.”--_Record._
A CHARMING ANTHOLOGY BY “Q”
THE PILGRIMS’ WAY
A LITTLE SCRIP OF GOOD COUNSEL FOR TRAVELLERS
BY SIR A. T. QUILLER-COUCH
Professor of English Literature at Cambridge University
_Cloth, price, nett, 5s. Thin paper edition in leather, 6s. nett;
buffed leather, yapp, in a box, price, nett, 6s._
“Prof. Quiller-Couch is the prince of anthologists.”--_The Glasgow
Evening News._
“A little book of grave and beautiful thoughts. It would be
difficult to better the selections.”--_The Guardian._
“The poems and prose passages are chosen--as might be safely
foretold--with taste and discrimination, and the volume will be
found a heartening companion.”--_The Tribune._
“The very flower of a cultivated man’s reading.”--_Country Life._
“Prof. Quiller-Couch’s anthologies are the best of their kind in
modern English literature.”--_The Morning Post._
THE GOLDEN RECITER
RECITATIONS AND READINGS IN PROSE AND VERSE SELECTED FROM THE
WRITINGS OF
RUDYARD KIPLING, R. L. STEVENSON, CONAN DOYLE, THOMAS HARDY, AUSTIN
DOBSON, CHRISTINA ROSSETTI, MAURICE HEWLETT, A. W. PINERO, SYDNEY
GRUNDY, &c.
WITH A PRACTICAL INTRODUCTION
BY PROF. CAIRNS JAMES
Professor of Elocution at the Royal College of Music and the
Guildhall School of Music
_Extra crown 8vo, over 700 pages, cloth, nett, 6s.; also a thin paper
pocket edition, with coloured edges, nett, 6s. 6d._
“An admirable collection of pieces, both in prose and
verse.”--_Spectator._
“Far superior to anything we have yet seen.”--_Western Press._
“A more admirable book of its kind could not well be
desired.”--_Liverpool Courier._
THE GOLDEN HUMOROUS RECITER
RECITATIONS AND READINGS IN PROSE AND VERSE SELECTED FROM THE
WRITINGS OF
F. ANSTEY, J. M. BARRIE, MAJOR DRURY, JEROME K. JEROME, BARRY PAIN,
A. W. PINERO, OWEN SEAMAN, G. B. SHAW, &c. &c.
WITH A PRACTICAL INTRODUCTION
BY PROF. CAIRNS JAMES
_Extra crown 8vo, over 700 pages, cloth, nett, 6s.; also a thin paper
pocket edition, with coloured edges, nett, 6s. 6d._
“Unquestionably the best collection of modern humorous pieces for
recitations which has yet been issued.”--_The Dundee Advertiser._
“Packed with things that are fresh and unhackneyed.”--_Bookman._
“An excellent selection, three-fifths of them being taken from the
work of the best modern writers.”--_The World._
“A most comprehensive and well-chosen collection of some hundreds
of pieces--a most catholic array of all that is good in English
literature, and a small encyclopædia of English humour.”--_The
Spectator._
* * * * *
THE NEW ART LIBRARY
“The admirable New Art Library.”--_Connoisseur._
_New Volume. Just Ready._
WATER COLOUR PAINTING
ALFRED W. RICH. With over Sixty Illustrations. Price 10s. 6d. nett.
“No artist living is better qualified to undertake a text-book on
water colour painting than Mr Rich. Not only is he one of the most
distinguished exponents of the art in this country, but he has had
considerable experience and success as a teacher. This admirable
volume ...”--_Studio._
“A book on the art of water colour painting by one of its
best living practitioners.... Mr Rich’s technique, clean,
direct, and scrupulous, is the best possible foundation for the
student.”--_Times._
THE PRACTICE OF OIL PAINTING
SOLOMON J. SOLOMON, R.A. With Eighty Illustrations. Price 10s. 6d.
nett.
“Eminently practical.... Can be warmly recommended to all
students.”--_Daily Mail._
“The work of an accomplished painter and experienced
teacher.”--_Scotsman._
“If students were to follow his instructions and, still more, to
heed his warnings, their painting would soon show a great increase
in efficiency.”--_Manchester Guardian._
HUMAN ANATOMY FOR ART STUDENTS
Sir ALFRED D. FRIPP, K.C.V.O., C.B., Lecturer upon Anatomy at Guy’s
and RALPH THOMPSON. Drawings by INNES FRIPP, A.R.C.A., Master of Life
Class, City Guilds Art School. 151 Illustrations. 15s. nett.
“The character of this book all through is clearness, both
in the letterpress and the illustrations. The latter are
admirable.”--_Spectator._
“Just such a work as the art student needs, and is probably all
that he will need. It is very fully illustrated. There are 9 plates
showing different views of the skeleton and the muscular system, 23
reproductions of photographs from life, and over 130 figures and
drawings.”--_Glasgow Herald._
MODELLING & SCULPTURE
ALBERT TOFT, Hon. Associate of the Royal College of Art, Member of
the Society of British Sculptors. With 118 Illustrations. 15s. nett.
“Mr Toft’s reputation as a sculptor of marked power and
versatility guarantees that the instruction he gives is thoroughly
reliable.”--_Connoisseur._
“Will be exceeding useful and indispensable to all who wish to
learn the art of sculpture in its many branches. The book will also
appeal to those who have no intention of learning the art, but wish
to know something about it. Mr Toft writes very clearly.”--_Field._
THE PRACTICE & SCIENCE OF DRAWING
HAROLD SPEED, Member of the Royal Society of Portrait Painters. With
93 Illustrations, 10s. 6d. nett.
“This book is of such importance that everyone interested In the
subject must read it.”--WALTER SICKERT in _The Daily News_.
“Altogether this is one of the best volumes in the admirable series
to which it belongs.”--_Literary World._
“There are many new and original ideas in the book.”--_The Outlook._
THE ARTISTIC ANATOMY OF TREES
REX VICAT COLE. With 500 Illustrations & Diagrams. 15s. nett.
“No work on art published during recent years is better calculated
to be of practical assistance to the student.”--_Connoisseur._
“Excellently and copiously illustrated.”--_Times._
“Like all volumes of the New Art Library, thorough in its
teaching, eminently practical in its manner of presenting it,
and so splendidly illustrated that not a rule is laid down or a
piece of advice given but what a drawing accompanies it. Mr Vicat
Cole’s ability as a landscape painter is well known, and he unites
to his executive talents the qualifications of an accomplished
teacher.”--_Connoisseur._
SEELEY, SERVICE & CO., LIMITED
* * * * *
Transcriber’s Notes:
The one footnote has been moved to the end of its chapter.
Illustrations have been moved to paragraph breaks near where they are
mentioned.
Punctuation has been made consistent.
Variations in spelling and hyphenation were retained as they appear in
the original publication, except that obvious typos have been corrected.
p. 50: Page reference is missing (described on p. .)
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