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Title: On the Construction of a Sivered Glass Telescope - Fifteen and a half inches in aperture and its use in - celestial photography
Author: Draper, Henry
Language: English
As this book started as an ASCII text book there are no pictures available.
Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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Transcriber’s notes:

The text of this book has been preserved in its original form
apart from correction of two typographic errors: embarrasment →
embarrassment, Cassegranian → Cassegrainian. Inconsistent hyphenation
has not been altered. A lengthy preliminary section concerning the
Smithsonian Institution precedes the actual subject matter.


                      CONTRIBUTIONS TO KNOWLEDGE.

                               VOL. XIV.



                          CITY OF WASHINGTON:




This volume forms the fourteenth of a series, composed of original
memoirs on different branches of knowledge, published at the expense,
and under the direction, of the Smithsonian Institution. The
publication of this series forms part of a general plan adopted for
carrying into effect the benevolent intentions of JAMES SMITHSON,
Esq., of England. This gentleman left his property in trust to the
United States of America, to found, at Washington, an institution which
should bear his own name, and have for its objects the “_increase_ and
_diffusion_ of knowledge among men.” This trust was accepted by the
Government of the United States, and an Act of Congress was passed
August 10, 1846, constituting the President and the other principal
executive officers of the general government, the Chief Justice of the
Supreme Court, the Mayor of Washington, and such other persons as they
might elect honorary members, an establishment under the name of the
AMONG MEN.” The members and honorary members of this establishment are
to hold stated and special meetings for the supervision of the affairs
of the Institution, and for the advice and instruction of a Board of
Regents, to whom the financial and other affairs are intrusted.

The Board of Regents consists of three members _ex officio_ of the
establishment, namely, the Vice-President of the United States, the
Chief Justice of the Supreme Court, and the Mayor of Washington,
together with twelve other members, three of whom are appointed by the
Senate from its own body, three by the House of Representatives from
its members, and six persons appointed by a joint resolution of both
houses. To this Board is given the power of electing a Secretary and
other officers, for conducting the active operations of the Institution.

To carry into effect the purposes of the testator, the plan of
organization should evidently embrace two objects: one, the increase
of knowledge by the addition of new truths to the existing stock;
the other, the diffusion of knowledge, thus increased, among men. No
restriction is made in favor of any kind of knowledge; and, hence, each
branch is entitled to, and should receive, a share of attention.

The Act of Congress, establishing the Institution, directs, as a part
of the plan of organization, the formation of a Library, a Museum, and
a Gallery of Art, together with provisions for physical research and
popular lectures, while it leaves to the Regents the power of adopting
such other parts of an organization as they may deem best suited to
promote the objects of the bequest.

After much deliberation, the Regents resolved to divide the annual
income into two parts--one part to be devoted to the increase
and diffusion of knowledge by means of original research and
publications--the other part of the income to be applied in accordance
with the requirements of the Act of Congress, to the gradual formation
of a Library, a Museum, and a Gallery of Art.

The following are the details of the parts of the general plan of
organization provisionally adopted at the meeting of the Regents, Dec.
8, 1847.


  I. TO INCREASE KNOWLEDGE.--_It is proposed to stimulate research,
  by offering rewards for original memoirs on all subjects of

1. The memoirs thus obtained, to be published in a series of volumes,
in a quarto form, and entitled “Smithsonian Contributions to Knowledge.”

2. No memoir, on subjects of physical science, to be accepted
for publication, which does not furnish a positive addition to
human knowledge, resting on original research; and all unverified
speculations to be rejected.

3. Each memoir presented to the Institution, to be submitted for
examination to a commission of persons of reputation for learning
in the branch to which the memoir pertains; and to be accepted for
publication only in case the report of this commission is favorable.

4. The commission to be chosen by the officers of the Institution, and
the name of the author, as far as practicable, concealed, unless a
favorable decision be made.

5. The volumes of the memoirs to be exchanged for the Transactions
of literary and scientific societies, and copies to be given to all
the colleges, and principal libraries, in this country. One part of
the remaining copies may be offered for sale; and the other carefully
preserved, to form complete sets of the work, to supply the demand from
new institutions.

6. An abstract, or popular account, of the contents of these memoirs
to be given to the public, through the annual report of the Regents to

  II. TO INCREASE KNOWLEDGE.--_It is also proposed to appropriate a
  portion of the income, annually, to special objects of research,
  under the direction of suitable persons._

1. The objects, and the amount appropriated, to be recommended by
counsellors of the Institution.

2. Appropriations in different years to different objects; so that, in
course of time, each branch of knowledge may receive a share.

3. The results obtained from these appropriations to be published,
with the memoirs before mentioned, in the volumes of the Smithsonian
Contributions to Knowledge.

4. Examples of objects for which appropriations may be made:--

(1.) System of extended meteorological observations for solving the
problem of American storms.

(2.) Explorations in descriptive natural history, and geological,
mathematical, and topographical surveys, to collect material for the
formation of a Physical Atlas of the United States.

(3.) Solution of experimental problems, such as a new determination of
the weight of the earth, of the velocity of electricity, and of light;
chemical analyses of soils and plants; collection and publication of
articles of science, accumulated in the offices of Government.

(4.) Institution of statistical inquiries with reference to physical,
moral, and political subjects.

(5.) Historical researches, and accurate surveys of places celebrated
in American history.

(6.) Ethnological researches, particularly with reference to the
different races of men in North America; also explorations, and
accurate surveys, of the mounds and other remains of the ancient people
of our country.

  I. TO DIFFUSE KNOWLEDGE.--_It is proposed to publish a series of
  reports, giving an account of the new discoveries in science, and of
  the changes made from year to year in all branches of knowledge not
  strictly professional._

1. Some of these reports may be published annually, others at longer
intervals, as the income of the Institution or the changes in the
branches of knowledge may indicate.

2. The reports are to be prepared by collaborators, eminent in the
different branches of knowledge.

3. Each collaborator to be furnished with the journals and
publications, domestic and foreign, necessary to the compilation of his
report; to be paid a certain sum for his labors, and to be named on the
title-page of the report.

4. The reports to be published in separate parts, so that persons
interested in a particular branch, can procure the parts relating to
it, without purchasing the whole.

5. These reports may be presented to Congress, for partial
distribution, the remaining copies to be given to literary and
scientific institutions, and sold to individuals for a moderate price.

_The following are some of the subjects which may be embraced in the


1. Physics, including astronomy, natural philosophy, chemistry, and

2. Natural history, including botany, zoology, geology, &c.

3. Agriculture.

4. Application of science to arts.


5. Ethnology, including particular history, comparative philology,
antiquities, &c.

6. Statistics and political economy.

7. Mental and moral philosophy.

8. A survey of the political events of the world; penal reform, &c.


9. Modern literature.

10. The fine arts, and their application to the useful arts.

11. Bibliography.

12. Obituary notices of distinguished individuals.

  II. TO DIFFUSE KNOWLEDGE.--_It is proposed to publish occasionally
  separate treatises on subjects of general interest._

1. These treatises may occasionally consist of valuable memoirs
translated from foreign languages, or of articles prepared under the
direction of the Institution, or procured by offering premiums for the
best exposition of a given subject.

2. The treatises to be submitted to a commission of competent judges,
previous to their publication.


This part contemplates the formation of a Library, a Museum, and a
Gallery of Art.

1. To carry out the plan before described, a library will be required,
consisting, 1st, of a complete collection of the transactions and
proceedings of all the learned societies of the world; 2d, of the more
important current periodical publications, and other works necessary in
preparing the periodical reports.

2. The Institution should make special collections, particularly
of objects to verify its own publications. Also a collection of
instruments of research in all branches of experimental science.

3. With reference to the collection of books, other than those
mentioned above, catalogues of all the different libraries in the
United States should be procured, in order that the valuable books
first purchased may be such as are not to be found elsewhere in the
United States.

4. Also catalogues of memoirs, and of books in foreign libraries, and
other materials, should be collected, for rendering the Institution a
centre of bibliographical knowledge, whence the student may be directed
to any work which he may require.

5. It is believed that the collections in natural history will increase
by donation, as rapidly as the income of the Institution can make
provision for their reception; and, therefore, it will seldom be
necessary to purchase any article of this kind.

6. Attempts should be made to procure for the gallery of art, casts of
the most celebrated articles of ancient and modern sculpture.

7. The arts may be encouraged by providing a room, free of expense,
for the exhibition of the objects of the Art-Union, and other similar

8. A small appropriation should annually be made for models of
antiquity, such as those of the remains of ancient temples, &c.

9. The Secretary and his assistants, during the session of Congress,
will be required to illustrate new discoveries in science, and to
exhibit new objects of art; distinguished individuals should also be
invited to give lectures on subjects of general interest.

       *       *       *       *       *

In accordance with the rules adopted in the programme of organization,
each memoir in this volume has been favorably reported on by a
Commission appointed for its examination. It is however impossible,
in most cases, to verify the statements of an author; and, therefore,
neither the Commission nor the Institution can be responsible for more
than the general character of a memoir.

       *       *       *       *       *

The following rules have been adopted for the distribution of the
quarto volumes of the Smithsonian Contributions:--

1. They are to be presented to all learned societies which publish
Transactions, and give copies of these, in exchange, to the Institution.

2. Also, to all foreign libraries of the first class, provided they
give in exchange their catalogues or other publications, or an
equivalent from their duplicate volumes.

3. To all the colleges in actual operation in this country, provided
they furnish, in return, meteorological observations, catalogues of
their libraries and of their students, and all other publications
issued by them relative to their organization and history.

4. To all States and Territories, provided there be given, in return,
copies of all documents published under their authority.

5. To all incorporated public libraries in this county, not included in
any of the foregoing classes, now containing more than 10,000 volumes;
and to smaller libraries, where a whole State or large district would
be otherwise unsupplied.


                                OF THE

                       SMITHSONIAN INSTITUTION.


                _Ex officio_ SECOND PRESIDING OFFICER.

                           SALMON P. CHASE,

                             JOSEPH HENRY,
                     SECRETARY OF THE INSTITUTION.

                           SPENCER F. BAIRD,
                         ASSISTANT SECRETARY.

                       W. W. SEATON, TREASURER.

              ALEXANDER D. BACHE, }
              RICHARD DELAFIELD,  }


  ---- ----               _Vice-President of the United States_.
  SALMON P. CHASE,        _Chief Justice of the United States_.
  RICHARD WALLACH,        _Mayor of the City of Washington_.
  LYMAN TRUMBULL,         _Member of the Senate of the United States_.
  WILLIAM P. FESSENDEN,       "     "       "     "       "     "
  GARRETT DAVIS,              "     "       "     "       "     "
  SAMUEL S. COX,          _Member of the House of Representatives U. S._
  JAMES W. PATTERSON,         "     "       "     "       "         "
  HENRY W. DAVIS,             "     "       "     "       "         "
  WILLIAM B. ASTOR,       _Citizen of New York_.
  THEODORE D. WOOLSEY,        "   _of Connecticut_.
  LOUIS AGASSIZ,              "   _of Massachusetts_.
        (Vacancy.)           ---- ----
  ALEXANDER D. BACHE,         "   _of Washington_.
  RICHARD DELAFIELD,          "   _of Washington_.


  ANDREW JOHNSON,         _President of the United States_.
  ---- ----               _Vice-President of the United States_.
  WILLIAM H. SEWARD,      _Secretary of State_.
  HUGH MCCULLOCH,         _Secretary of the Treasury_.
  EDWIN M. STANTON,       _Secretary of War_.
  GIDEON WELLES,          _Secretary of the Navy_.
  WILLIAM DENNISON,       _Postmaster-General_.
  JAMES SPEED,            _Attorney-General_.
  SALMON P. CHASE,        _Chief Justice of the United States_.
  DAVID P. HOLLOWAY,      _Commissioner of Patents_.
  RICHARD WALLACH,        _Mayor of the City of Washington_.


  JAMES HARLAN.           _The Secretary of the Interior_.


  [1] Each memoir is separately paged and indexed.

      Advertisement                                                 iii
      List of Officers of the Smithsonian Institution                ix

      PHILADELPHIA, IN 1840, 1841, 1842, 1843, 1844, AND 1845.
      Third Section, comprising Parts VII, VIII, AND IX.
      LL. D., F. R. S., Mem. Corr. Acad. Sc. Paris; Prest. Nat.
      Acad. Sciences; Superintendent U. S. Coast Survey. Pp. 72.
      (Published May, 1864.)

      PHILADELPHIA, IN 1840, 1841, 1842, 1843, 1844, AND 1845.
      Fourth Section, comprising Parts X, XI, AND XII. DIP AND
      A. D. BACHE, LL. D., F. R. S., Mem. Corr. Acad. Sc. Paris;
      Prest. Nat. Acad. Sciences; Superintendent U. S. Coast
      Survey. Pp. 44. (Published January, 1865.)

      of Natural Science in the University of New York. Pp. 60.
      (Published July, 1864.)

      §1. Grinding and Polishing the Mirrors                          2
      §2. The Telescope Mounting                                     27
      §3. The Clock Movement                                         38
      §4. The Observatory                                            41
      §5. The Photographic Laboratory                                46
      §6. The Photographic Enlarger                                  51

      COMMAND OF LIEUT. G. K. WARREN, U. S. Top. Engrs., IN 1855
      M. D. Part I. Pp. 158, and five Plates. (Published April,

           Introductory Remarks                                     vii
        I. Silurian Age. Potsdam Period                               1
       II. Carboniferous Age. Carboniferous Period                   11
      III. Carboniferous Age. Permian Period                         48
       IV. Reptilian Age. Jurassic Period                            66
           Index                                                    121
           Explanations of Plates.

      By JOSEPH LEIDY, M. D., Professor of Anatomy in the
      University of Pennsylvania, Curator of the Academy of
      Natural Sciences of Philadelphia. Pp. 140 and twenty
      plates. (Published May, 1865.)

      Introduction                                                    1
      Sauria                                                          5
      Chelonia                                                      104
      A Synopsis, in which an attempt is made to define more
        closely the Genera and Species of Reptiles whose remains
        are described in the preceding pages                        115
      Index                                                         121
      References to the Plates                                      123


                          ON THE CONSTRUCTION

                                 OF A

                       SILVERED GLASS TELESCOPE,





                         HENRY DRAPER, M. D.,




                 Prof. WOLCOTT GIBBS.
                 Com. J. M. GILLISS, U. S. N.

                              JOSEPH HENRY,
                                     _Secretary S. I._




§1. GRINDING AND POLISHING THE MIRRORS                                2

    1. _Experiments on a metal speculum._ Corrosion by aqua regia;
        voltaic grinding                                              2

    2. _Silvering glass._ Foucault’s and Cimeg’s processes; details
        of silvering a mirror; thickness and durability of silver
        films; their use in daguerreotyping                           2

    3. _Grinding and polishing glass._ Division of subject            6

       _a._ Peculiarities of glass; effects of pressure; effects of
            heat; oblique mirrors                                     6

       _b._ Emery and rouge; elutriation of emery                    10

       _c._ Tools of iron, lead, pitch; the gauges; the leaden
            tool; the iron tool; the pitch polisher                  10

       _d._ Methods of examination; two tests, eyepiece and opaque
            screen; appearance of spherical surface; oblate
            spheroidal surface; hyperbolic surface; irregular
            surface; details of tests; atmospheric movements;
            correction for parallel rays by measure; appearances
            in relief on mirrors                                     13

       _e._ Machines; Lord Rosse; Mr. Lassell; spiral stroke
            machine; its construction and use; the foot-power;
            method of local corrections; its advantages and
            disadvantages; machine for local corrections;
            description and use                                      19

    4. _Eyepieces, plane mirrors, and test objects_                  26

§2. THE TELESCOPE MOUNTING                                           27

        Stationary eyepiece; method of counterpoising                27

       _a._ The tube; the mirror support; air sac; currents
            in the tube                                              28

       _b._ The supporting frame                                     31

§3. THE CLOCK MOVEMENT                                               33

       _a._ The sliding plateholder; the frictionless slide          33

       _b._ The clepsydra; the sand-clock                            36

       _c._ The sun camera                                           40

§4. THE OBSERVATORY                                                  41

       _a._ The building                                             41

       _b._ The dome; its peculiarities                              44

       _c._ The observer’s chair                                     45

§5. THE PHOTOGRAPHIC LABORATORY                                      46

       _a._ Description of the apartment                             46

       _b._ Photographic processes; washed plates; difficulties
            of celestial photography                                 47

§6. THE PHOTOGRAPHIC ENLARGER                                        51

       _a._ Low powers; use of a concave mirror, its novelty and
            advantages; of the making of reverses                    51

       _b._ High powers; microscopic photography                     54

                              AN ACCOUNT



The construction of a reflecting telescope capable of showing every
celestial object now known, is not a very difficult task. It demands
principally perseverance and careful observation of minutiæ. The cost
of materials is but trifling compared with the result obtained, and I
can see no reason why silvered glass instruments should not come into
general use among amateurs. The future hopes of Astronomy lie in the
multitude of observers, and in the concentration of the action of many
minds. If what is written here should aid in the advance of that noble
study, I shall feel amply repaid for my labor.

A short historical sketch of this telescope may not be uninteresting.
In the summer of 1857, I visited Lord Rosse’s great reflector, at
Parsonstown, and, in addition to an inspection of the machinery
for grinding and polishing, had an opportunity of seeing several
celestial objects through it. On returning home, in 1858, I determined
to construct a similar, though smaller instrument; which, however,
should be larger than any in America, and be especially adapted for
photography. Accordingly, in September of that year, a 15 inch speculum
was cast, and a machine to work it made. In 1860, the observatory was
built, by the village carpenter, from my own designs, at my father’s
country seat, and the telescope with its metal speculum mounted.
This latter was, however, soon after abandoned, and silvered glass
adopted. During 1861, the difficulties of grinding and polishing that
are detailed in this account were met with, and the remedies for many
of them ascertained. The experiments were conducted by the aid of
three 15-1/2 inch disks of glass, together with a variety of smaller
pieces. Three mirrors of the same focal length and aperture are almost
essential, for it not infrequently happens that two in succession will
be so similar, that a third is required for attempting an advance
beyond them. One of these was made to acquire a parabolic figure, and
bore a power of 1,000. The winter was devoted to perfecting the art
of silvering, and to the study of special photographic processes.
A large portion of 1862 was spent with a regiment in a campaign in
Virginia, and but few photographs were produced till autumn, when
sand clocks and clepsydras of several kinds having been made, the
driving mechanism attained great excellence. During the winter, the
art of local corrections was acquired, and two 15-1/2 inch mirrors,
as well as two of 9 inches for the photographic enlarging apparatus,
were completed. The greater part of 1863 has been occupied by lunar
and planetary photography, and the enlargement of the small negatives
obtained at the focus of the great reflector. Lunar negatives have
been produced which have been magnified to 3 feet in diameter. I have
also finished two mirrors 15-1/2 inches in aperture, suitable for a
Herschelian telescope, that is, which can only converge oblique pencils
to a focus free from aberration. This work has all been accomplished in
the intervals of professional labor.

The details of the preceding operations are arranged as follows: §1.



My first 15 inch speculum was an alloy of copper and tin, in the
proportions given by Lord Rosse. His general directions were closely
followed, and the casting was very fine, free from pores, and of
silvery whiteness. It was 2 inches thick, weighed 110 pounds, and was
intended to be of 12 feet focal length. The grinding and polishing
were conducted with the Rosse machine. Although a great amount of time
was spent in various trials, extending over more than a year, a fine
figure was never obtained--the principal obstacle to success being
a tendency to polish in rings of different focal length. It must,
however, be borne in mind that Lord Rosse had so thoroughly mastered
the peculiarities of his machine as to produce with it the largest
specula ever made and of very fine figure.

During these experiments there was occasion to grind out some
imperfections, 8/100 of an inch deep, from the face of the metal.
This operation was greatly assisted by stopping up the defects with a
thick alcoholic solution of Canada balsam, and having made a rim of
wax around the edge of the mirror, pouring on nitro-hydrochloric acid,
which quickly corroded away the uncovered spaces. Subsequently an
increase in focal length of 15 inches was accomplished, by attacking
the edge zones of the surface with the acid in graduated depths.

An attempt also was made to assist the tedious grinding operation by
including the grinder and mirror in a Voltaic circuit, making the
speculum the positive pole. By decomposing acidulated water between
it and the grinder, and thereby oxidizing the tin and copper of the
speculum, the operation was much facilitated, but the battery surface
required was too great for common use. If a sufficient intensity was
given to the current, speculum metal was transferred without oxidation
to the grinder, and deposited in thin layers upon it. It was proposed
at one time to make use of this fact, and coat a mirror of brass with a
layer of speculum metal by electrotyping. The gain in lightness would
be considerable.

During the winter of 1860 the speculum was split into two pieces, by
the expansion in freezing of a few drops of water that had found their
way into the supporting case.

(2.) Silvering Glass.

At Sir John Herschel’s suggestion (given on the occasion of a visit
that my father paid him in 1860), experiments were next commenced with
silvered glass specula. These were described as possessing great
capabilities for astronomical purposes. They reflect more than 90 per
cent. of the light that fulls upon them, and only weigh one-eighth as
much as specula of metal of equal aperture.

As no details of Steinheil’s or Foucault’s processes for silvering in
the cold way were accessible at the time, trials extending at intervals
over four months were made. A variety of reducing agents were used, and
eventually good results obtained with milk sugar.

Soon after a description of the process resorted to by M. Foucault in
his excellent experiments was procured. It consists in decomposing an
alcoholic solution of ammonia and nitrate of silver by oil of cloves.
The preparation of the solutions and putting them in a proper state of
instability are very difficult, and the results by no means certain.
The silver is apt to be soft and easily rubbed off, or of a leaden
appearance. It is liable to become spotted from adherent particles of
the solutions used in its preparation, and often when dissolved off a
piece of glass with nitric acid leaves a reddish powder. Occasionally,
however, the process gives excellent results.

In the winter of 1861, M. Cimeg published his method of silvering
looking-glasses by tartrate of potash and soda (Rochelle salt). Since I
have made modifications in it fitting the silver for being polished on
the reverse side, I have never on any occasion failed to secure bright,
hard, and in every respect, perfect films.

The operation, which in many details resembles that of M. Foucault, is
divided into: 1st, cleaning the glass; 2d, preparing the solutions;
3d, warming the glass; 4th, immersion in the silver solution and stay
there; 5th, polishing. It should be carried on in a room warmed to
70° F. at least. The description is for a 15-1/2 inch mirror.

1st. Clean the glass like a plate for collodion photography. Rub it
thoroughly with nitric acid, and then wash it well in plenty of water,
and set it on edge on filtering paper to dry. Then cover it with a
mixture of alcohol and prepared chalk, and allow evaporation to take
place. Rub it in succession with many pieces of cotton flannel. This
leaves the surface almost chemically clean. Lately, instead of chalk I
have used plain uniodized collodion, and polished with a freshly-washed
piece of cotton flannel, as soon as the film had become semi-solid.

2d. Dissolve 560 grains of Rochelle salt in two or three ounces of
water and filter. Dissolve 800 grains of nitrate of silver in four
ounces of water. Take an ounce of strong ammonia of commerce, and add
nitrate solution to it until a brown precipitate remains undissolved.
Then add more ammonia and again nitrate of silver solution. This
alternate addition is to be carefully continued until the silver
solution is exhausted, when some of the brown precipitate should remain
in suspension. The mixture then contains an undissolved excess of
oxide of silver. Filter. Just before using, mix with the Rochelle salt
solution, and add water enough to make 22 ounces.

[Illustration: Fig. 1. The Silvering Vessel.]

The vessel in which the silvering is to be performed may be a circular
dish (Fig. 1) of ordinary tinplate, 16-1/2 inches in diameter, with a
flat bottom and perpendicular sides one inch high, and coated inside
with a mixture of beeswax and rosin (equal parts), At opposite ends of
one diameter two narrow pieces of wood, _a a′_, 1/8 of an inch thick,
are cemented. They are to keep the face of the mirror from the bottom
of the vessel, and permit of a rocking motion being given to the glass.
Before using such a vessel, it is necessary to touch any cracks that
may have formed in the wax with a hot poker. A spirit lamp causes
bubbles and holes through to the tin. The vessel too must always,
especially if partly silvered, be cleaned with nitric acid and water,
and left filled with cold water till needed. Instead of the above,
India-rubber baths have been occasionally used.

3d. In order to secure fine and hard deposits in the shortest time
and with weak solutions, it is desirable, though not necessary, to
warm the glass slightly. This is best done by putting it in a tub or
other suitably sized vessel, and pouring in water enough to cover the
glass. Then hot water is gradually stirred in, till the mixture reaches
100° F. It is also advantageous to place the vessels containing the
ingredients for the silvering solution in the same bath for a short

4th. On taking the glass out of the warm water, carry it to the
silvering vessel--into which an assistant has just previously poured
the mixed silvering solution--and immediately immerse it face
downwards, dipping in first one edge and then quickly letting down the
other till the face is horizontal. The back of course is not covered
with the fluid. The same precautions are necessary to avoid streaks
in silvering as in the case of putting a collodion plate in the bath.
Place the whole apparatus before a window. Keep up a slow rocking
motion of the glass, and watch for the appearance of the bright silver
film. The solution quickly turns brown, and the silver soon after
appears, usually in from three to five minutes. Leave the mirror in the
liquid about six times as long. At the expiration of the twenty minutes
or half hour lift it out, and look through it at some very bright
object. If the object is scarcely visible, the silver surface must
then be washed with plenty of water, and set on edge on bibulous paper
to dry. If, on the contrary, it is too thin, put it quickly back, and
leave it until thick enough. When polished the silver ought, if held
between the eye and the sun, to show his disk of a light blue tint. On
coming out of the bath the metallic surface should have a rosy golden
color by reflected light.

5th. When the mirror is thoroughly dry, and no drops of water remain
about the edges, lay it upon its back on a thoroughly dusted table.
Take a piece of the softest thin buckskin, and stuff it loosely with
cotton to make a rubber. Avoid using the edge pieces of a skin, as they
are always hard and contain nodules of lime.

Go gently over the whole silver surface with this rubber in circular
strokes, in order to commence the removal of the rosy golden film, and
to condense the silver. Then having put some very fine rouge on a piece
of buckskin laid flat on the table, impregnate the rubber with it.
The best stroke for polishing is a motion in small circles, at times
going gradually round on the mirror, at times across on the various
chords (Fig. 2). At the end of an hour of continuous gentle rubbing,
with occasional touches on the flat rouged skin, the surface will be
polished so as to be perfectly black in oblique positions, and, with
even moderate care, scratchless. The process is like a burnishing. Put
the rubber carefully away for another occasion.

[Illustration: Fig. 2. Polishing Strokes.]

The thickness of the silver thus deposited is about 1/200,000 of an
inch. Gold leaf, when equally transparent, is estimated at the same
fraction. The actual value of the amount on a 15-1/2 inch mirror is not
quite a cent--the weight being less than 4 grains (239 milligrammes on
one occasion when the silver was unusually thick), if the directions
above given are followed.

Variations in thickness of this film of silver on various parts of the
face of the mirror are consequently only small fractions of 1/200,000
of an inch, and are therefore of no optical moment whatever. If a glass
has been properly silvered, and shows the sun of the same color and
intensity through all parts of its surface, the most delicate optical
tests will certainly fail to indicate any difference in figure between
the silver and the glass underneath. The faintest peculiarities of
local surface seen on the glass by the method of M. Foucault, will be
reproduced on the silver.

The durability of these silver films varies, depending on the
circumstances under which they are placed, and the method of
preparation. Sulphuretted hydrogen tarnishes them quickly. Drops of
water may split the silver off. Under certain circumstances, too,
minute fissures will spread all over the surface of the silver, and it
will apparently lose its adhesion to the glass. This phenomenon seems
to be connected with a continued exposure to dampness, and is avoided
by grinding the edge of the concave mirror flat, and keeping it covered
when not in use with a sheet of flat plate glass. Heat seems to have
no prejudicial effect, though it might have been supposed that the
difference in expansibility would have overcome the mutual adhesion.

Generally silvered mirrors are very enduring, and will bear polishing
repeatedly, if previously dried by heat. I have some which have been
used as diagonal reflectors in the Newtonian, and have been exposed
during a large part of the day to the heat of the sun concentrated by
the 15-1/2 inch mirror. These small mirrors are never covered, and yet
the one now in the telescope has been there a year, and has had the
dusty film--like that which accumulates on glass--polished off it a
dozen times.

In order to guard against tarnishing, experiments were at first made in
gilding silver films, but were abandoned when found to be unnecessary.
A partial conversion of the silver film into a golden one, when it will
resist sulphuretted hydrogen, can be accomplished as follows: Take
three grains of hyposulphite of soda, and dissolve it in an ounce of
water. Add to it slowly a solution in water of one grain of chloride of
gold. A lemon yellow liquid results, which eventually becomes clear.
Immerse the silvered glass in it for twenty-four hours. An exchange
will take place, and the film become yellowish. I have a piece of glass
prepared in this way which remains unhurt in a box, where other pieces
of plain silvered glass have changed some to yellow, some to blue, from
exposure to coal gas.

I have also used silvered glass plates for daguerreotyping. They iodize
beautifully if freshly polished, and owing probably to the absence
of the usual copper alloy of silver plating, take impressions with
very short exposures. The resulting picture has a rosy warmth, rarely
seen in ordinary daguerreotypes. The only precaution necessary is in
fixing to use an alcoholic solution of cyanide of potassium, instead of
hyposulphite of soda dissolved in water. The latter has a tendency to
split up the silver. The subsequent washing must be with diluted common

Pictures obtained by this method will bear high magnifying powers
without showing granulation. Unfortunately the exposure required for
them in the telescope is six times as great as for a sensitive wet
collodion, though the iodizing be carried to a lemon yellow, the
bromizing to a rose red, and the plate be returned to the iodine.


Some of the facts stated in the following paragraphs, the result of
numerous experiments, may not be new to practical opticians. I have
had, however, to polish with my own hands more than a hundred mirrors
of various sizes, from 19 inches to 1/4 of an inch in diameter, and to
experience very frequent failures for three years, before succeeding
in producing large surfaces with certainty and quickly. It is well
nigh impossible to obtain from opticians the practical minutiæ which
are essential, and which they conceal even from each other. The long
continued researches of Lord Rosse, Mr. Lassell, and M. Foucault are
full of the most valuable facts, and have been of continual use.

The subject is divided into: a. The Peculiarities of Glass; b. Emery
and Rouge; c. Tools of Iron, Lead and Pitch; d. Methods of Examining
Surfaces; e. Machines.

a. _Peculiarities of Glass._

_Effects of Pressure._--It is generally supposed that glass is
possessed of the power of resistance to compression and rigidity in a
very marked manner. In the course of these experiments it has appeared
that a sheet of it, even when very thick, can with difficulty be set on
edge without bending so much as to be optically worthless. Fortunately
in every disk of glass that I have tried, there is one diameter on
either end of which it may stand without harm.

In examining lately various works on astronomy and optics, it appears
that the same difficulty has been found not only in glass but also in
speculum metal. Short used always to mark on the edge of the large
mirrors of his Gregorian telescopes the point which should be placed
uppermost, in case they were removed from their cells. In achromatics
the image is very sensibly changed in sharpness if the flint and
crown are not in the best positions; and Mr. Airy, in mounting the
Northumberland telescope, had to arrange the means for turning the
lenses on their common axis, until the finest image was attained. In
no account, however, have I found a critical statement of the exact
nature of the deformation, the observers merely remarking that in some
positions of the object glass there was a sharper image than in others.

Before I appreciated the facts now to be mentioned, many fine mirrors
were condemned to be re-polished, which, had they been properly set in
their mountings, would have operated excellently.

In attempting to ascertain the nature of deformations by pressure, many
changes were made in the position of the disk of glass, and in the
kind of support. Some square mirrors, too, were ground and polished.
As an example of the final results, the following case is presented:
A 15-1/2 inch unsilvered mirror 1-1/4 inch thick was set with its
best diameter perpendicular, the axis of the mirror being horizontal
(Fig. 8). The image of a pin-hole illuminated by a lamp was then
observed to be single, sharply defined, and with interference rings
surrounding it as at _a_, Fig. 3. On turning the glass 90 degrees,
that is one quarter way round, its axis still pointing in the same
direction, it could hardly be realized that the same concave surface
was converging the rays. The image was separated into two of about
equal intensity, as at _b_, with a wing of light going out above and
below from the junction. Inside and outside of the focal plane the cone
of rays had an elliptical section, the major axis being horizontal
inside, and perpendicular outside. Turning the mirror still more
round the image gradually improved, until the original diameter was
perpendicular again--the end that had been the uppermost now being the
lowest. A similar series of changes occurred in supporting the glass
on various parts of the other semicircle. It might be supposed that
irregularities on the edge of the glass disk, or in the supporting arc
would account for the phenomena. But two facts dispose of the former
of these hypotheses: in the first place if the glass be turned exactly
half way round, the character of the image is unchanged, and it is
not to be believed that in many different mirrors this could occur
by chance coincidence. In the second place, one of these mirrors has
been carefully examined after being ground and polished three times
in succession, and on each occasion required the same diameter to be
perpendicular. As to the second hypothesis no material difference is
observed whether the supporting arc below be large or small, nor when
it is replaced by a thin semicircle of tinplate lined with cotton wool.

[Illustration: Fig. 3. Effect of Pressure on a Reflecting Surface.]

I am led to believe that this peculiarity results from the structural
arrangement of the glass. The specimens that have served for these
experiments have probably been subjected to a rolling operation when
in a plastic state, in order to be reduced to a uniform thickness.
Optical glass, which may be made by softening down irregular fragments
into moulds at a temperature below that of fusion, may have the same
difficulty, but whether it has a diameter of minimum compression can
only be determined by experiment. Why speculum metal should have the
same property might be ascertained by a critical examination of the
process of casting, and the effect of the position of the openings in
the mould for the entrance of the molten metal.

_Effects of Heat._--The preceding changes in glass when isolated
appear very simple, and their remedy, to keep the proper diameter
perpendicular, is so obvious that it may seem surprising that they
should have given origin to any embarrassment. In fact it is now
desirable to have a disk in which they are well marked. But in practice
they are complicated in the most trying manner with variations produced
by heat pervading the various parts of the glass unequally. The
following case illustrates the effects of heat:--

[Illustration: Fig. 4. Effects of Heat on a Reflecting Surface.]

A 15-1/2 inch mirror, which was giving at its centre of curvature a
very fine image (_a_, Fig. 4) of an illuminated pin-hole, was heated
at the edge by placing the right hand on the back of the mirror, at
one end of the horizontal diameter. In a few seconds an arc of light
came out from the image as at _b′_, and on putting the left hand on
the other extremity of the same diameter the appearance _c′_ was that
of two arcs of light crossing each other, and having an image at each
intersection. The mirror did not recover its original condition in ten
minutes. Another person on a subsequent occasion touching the ends of
the perpendicular diameter at the same time that the horizontal were
warmed, caused the image _d′_ to become somewhat like two of _c′_, put
at right angles to each other. A little distance outside the focus the
complementary appearances, _b_, _c_, _d_, were found.

By unsymmetrical warming still more remarkable forms emerged in
succession, some of which were more like certain nebulæ with their
milky light, than any regular geometrical figure.

If the glass had, after one of these experiments, been immediately
put on the polishing machine and re-polished, the changes in surface
would to a certain extent have become permanent, as in Chinese
specula, and the mirror would have required either re-grinding or
prolonged polishing to get rid of them. This occurred unfortunately
very frequently in the earlier stages of this series of experiments,
and gave origin on one occasion to a surface which could only show the
image of a pin-hole as a lozenge (_b_, Fig. 5), with an image at each
angle inside the focus, and as an image a with four wings outside.

[Illustration: Fig. 5. Effects of Heat rendered permanent.]

But it must not be supposed that such apparent causes as these are
required to disturb a surface injuriously. Frequently mirrors in the
process for correction of spherical aberration will change the quality
of their images without any perceptible reason for the alteration. A
current of cold or warm air, a gleam of sunlight, the close approach
of some person, an unguarded touch, the application of cold water
injudiciously will ruin the labor of days. The avoidance of these and
similar causes requires personal experience, and the amateur can only
be advised to use too much caution rather than too little.

Such accidents, too, teach a useful lesson in the management of a large
telescope, never, for instance, to leave one-half the mirror or lens
exposed to radiate into cold space, while the other half is covered
by a comparatively warm dome. Under the head of the Sun-Camera, some
further facts of this kind may be found.

_Oblique Mirrors._--Still another propensity of glass and speculum
metal must be noted. A truly spherical concave can only give an image
free from distortion when it is so set that its optical axis points
to the object and returns the image directly back towards it. But I
have polished a large number of mirrors in which an image free from
distortion was produced _only_ when oblique pencils fell on the mirror,
and the image was returned along a line forming an angle of from 2
to 3 degrees with the direction of the object. Such mirrors, though
exactly suited for the Herschelian construction, will not officiate in
a Newtonian unless the diagonal mirror be put enough out of centre in
the tube, to compensate for the figure of the mirror. Some of the best
photographs of the moon that have been produced in the observatory,
were made when the diagonal mirror was 6 inches out of centre in the
16 inch tube. Of course the large mirror below was not perpendicular
to the axis of the tube, but was inclined 2° 32′. The figure of such
a concave might be explained by the supposition that it was as if cut
out of a parabolic surface of twice the diameter, so that the vertex
should be on the edge. But if the mirror was turned 180° it apparently
did just as well as in the first position, the image of a round object
being neither oval nor elliptical, and without wings. The image,
however, is never quite as fine as in the usual kind of mirrors. The
true explanation seems rather to be that the radius of curvature is
greater along one of the diameters than along that at right angles. How
it is possible for such a figure to arise during grinding and polishing
is not easy to understand, unless it be granted that glass yields more
to heat and compression in one direction than another.

After these facts had been laboriously ascertained, and the method
of using such otherwise valueless mirrors put in practice as above
stated, chance brought a letter of Maskelyne to my notice. He says,
“I hit upon an extraordinary experiment which greatly improved the
performance of the six-feet reflector”.... It was one made by Short.
“As a like management may improve many other telescopes, I shall here
relate it: I removed the great speculum from the position it ought to
hold perpendicular to the axis of the tube when the telescope is said
to be rightly adjusted, to one a little inclined to the same and found
a certain inclination of about 2-1/2° (as I found by the alteration of
objects in the finer one of Dollond’s best night glasses with a field
of 6°), which caused the telescope to show the object (a printed paper)
incomparably better than before; insomuch that I could read many of
the words which before I could make nothing at all of. It is plain,
therefore, that this telescope shows best with a certain oblique
pencil of rays. Probably it will be found that this circumstance is by
no means peculiar to this telescope.” This very valuable observation
has lain buried for eighty-two years, and ignorance of it has led to
the destruction of many a valuable surface.

As regards the method of combating this tendency, it is as a general
rule best to re-grind or rather re-fine the surface, for though pitch
polishing has occasionally corrected it in a few minutes, it will
not always do so. I have polished a surface for thirteen and a half
hours, examining it frequently, without changing the obliquity in the
slightest degree.

Glass, then, is a substance prone to change by heat and compression,
and requiring to be handled with the utmost caution.

b. _Emery and Rouge._

In order to excavate the concave depression in a piece of glass, emery
as coarse as the head of a pin has been commonly used. This cuts
rapidly, and is succeeded by finer grained varieties, till flour emery
is reached. After that only washed emeries should be permitted. They
are made by an elutriating process invented by Dr. Green.

Five pounds of the finest sifted flour emery are mixed with an ounce
of pulverized gum arabic. Enough water to make the mass like treacle
is then added, and the ingredients are thoroughly incorporated by the
hand. They are put into a deep jar containing a gallon of water. After
being stirred the fluid is allowed to come to rest, and the surface be
skimmed. At the end of an hour the liquid containing extremely fine
emery in suspension is decanted or drawn off with a siphon, nearly down
to the level of the precipitated emery at the bottom, and set aside to
subside in a tall vessel. When this has occurred, which will be in the
lapse of a few hours, the fluid is to be carefully poured back into the
first vessel, and the fine deposit in the second put into a stoppered
bottle. In the same way by stirring up the precipitate again, emery
that has been suspended 30, 10, 3, 1 minutes, and 20, 3, seconds is to
be secured and preserved in wide-mouthed vessels.

The quantity of the finer emeries consumed in smoothing a 15-1/2 inch
surface is very trifling--a mass of each as large as two peas sufficing.

Rouge, or peroxide of iron, is better bought than prepared by the
amateur. It is made by calcining sulphate of iron and washing the
product in water. Three kinds are usually found in commerce: a very
coarse variety containing the largest percentage of the cutting black
oxide of iron, which will scratch glass like quartz; a very fine
variety which can hardly polish glass, but is suitable for silver
films; and one intermediate. Trial of several boxes is the best method
of procuring that which is desired.

c. _Tools of Iron, Lead, and Pitch._

In making a mirror, one of the first steps is to describe upon two
stout sheets of brass or iron, arcs of a circle with a radius equal to
twice the desired focal length, and to secure, by filing and grinding
them together, a concave and convex gauge. When the radius bar is very
long, it may be hung against the side of a house. By the assistance
of these templets, the convex tools of lead and iron and the concave
surface of the mirror are made parts of a sphere of proper diameter.

The excavation of a large flat disc of glass to a concave is best
accomplished by means of a thick plate of lead, cast considerably more
convex than the gauge. The central parts wear away very quickly, and
when they become too flat must be made convex again by striking the
lead on the back with a hammer. The glass is thus caused gradually
to approach the right concavity. Ten or twelve hours usually suffice
to complete this stage. The progress of the excavating is tested
sufficiently well by setting the convex gauge on a diameter of the
mirror, and observing how many slips of paper of a definite thickness
will pass under the centre or edge, as the case may be. This avoids the
necessity of a spherometer. The thickness of paper is found correctly
enough by measuring a half ream, and dividing by the number of sheets.
In this manner differences in the versed sine of a thousandth of an
inch may be appreciated, and a close enough approximation to the
desired focal length reached--the precision required in achromatics not
being needed. The preparation of the iron tools on which the grinding
is to be finished is very laborious where personal exertion is used.
They require to be cast thin in order that they may be easily handled,
and hence cannot be turned with very great exactness.

The pair for my large mirrors are 15-1/2 inches in diameter, and were
cast 3/8 of an inch thick, being strengthened however on the back by
eight ribs 3/4 of an inch high, radiating from a solid centre two
inches in diameter (_a_, Fig. 6). They weighed 25 pounds apiece. Four
ears, with a tapped hole in each, project at equal distances round the
edge, and serve either as a means of attachment for a counterpoise
lever, or as handles.

[Illustration: Fig. 6. The Iron Grinder.]

After these were turned and taken off the lathe chuck, they were found
to be somewhat sprung, and had to be scraped and ground in the machine
for a week before fitting properly. The slowness in grinding results
from the emery becoming imbedded in the iron, and forming a surface as
hard as adamant.

Once acquired, such grinders are very valuable, as they keep their
focal length and figure apparently without change if carefully used,
and only worked on glass of nearly similar curvature. At first no
grooves were cut upon the face, for in the lead previously employed
for fining they were found to be a fruitful source of scratches, on
account of grains of emery imbedding in them, and gradually breaking
loose as the lead wore away. Subsequently it appeared, that unless
there was some means of spreading water and the grinding powders
evenly, rings were likely to be produced on the mirror, and the iron
was consequently treated as follows:--

A number of pieces of wax, such as is used in making artificial
flowers, were procured. The convex iron was laid out in squares of 3/4
of an inch on the side, and each alternate one being touched with a
thick alcoholic solution of Canada balsam, a piece of wax of that size
was put over it. This was found after many trials to be the best method
of protecting some squares, and yet leaving others in the most suitable
condition to be attacked. A rim of wax, melted with Canada balsam, was
raised around the edge of the iron, and a pint of aqua regia poured in.
In a short time this corroded out the uncovered parts to a sufficient
depth, leaving an appearance like a chess-board, except that the
projecting squares did not touch at the adjoining angles (_b_, Fig. 6).
I should have chipped the cavities out, instead of dissolving them
away, but for fear of changing the radius of curvature and breaking the
thin plate. However as soon as the iron was cleaned, it proved to have
become flatter, the radius of curvature having increased 7-3/4 inches.
This shows what a state of tension and compression there must be in
such a mass, when the removal of a film of metal 1/50 of an inch thick,
here and there, from one surface, causes so great a change.

When the glass has been brought to the finest possible grain on such
a grinder, a polishing tool has to be prepared by covering the convex
iron with either pitch or rosin. These substances have very similar
properties, but the rosin by being clear affords an opportunity of
seeing whether there are impurities, and therefore has been frequently
used, straining being unnecessary. It is, however, too hard as it
occurs in commerce, and requires to be softened with turpentine.

A mass sufficiently large to cover the iron 1/8 of an inch thick
is melted in a porcelain or metal capsule by a spirit lamp. When
thoroughly liquid the lamp is blown out, and spirits of turpentine
added, a drachm or two at a time. After each addition a chisel or
some similar piece of metal is dipped into the fluid rosin, and then
immersed in water at the temperature of the room. After a minute or two
it is taken out, and tried with the thumb-nail. When the proper degree
of softness is obtained, an indentation can be made by a moderate

The iron having been heated in hot water is then painted in stripes
1/8 of an inch deep with this resinous composition. The glass concave
to be polished being smeared with rouge, is pressed upon it to secure
a fit, and the iron is then put in cold water. With a narrow chisel
straight grooves are made, dividing the surface into squares of one
inch, separated by intervals of one-quarter of an inch (Fig. 7). Under
certain circumstances it is also desirable to take off every other
square, or perhaps reduce the polishing surface irregularly here and
there, to get an excess of action on some particular portion of the

[Illustration: Fig. 7. The Polishing Tool.]

It is well, on commencing to polish with a tool made in this way, to
warm the glass as well as the tool in water (page 4) before bringing
the two in contact. If this is not done the polishing will not go on
kindly, a good adaptation not being secured for a length of time, and
the glass surface being injured at the outset. The rosin on a polisher
put away for a day or two suffers an internal change, a species of
irregular swelling, and does not retain its original form. Heating,
too, has a good effect in preventing disturbance by local variations of
temperature in the glass.

The description of “Local Polishers” will be given under _Machines_.

d. _Methods of Examining Surfaces._

I have been in the habit of testing mirrors exclusively at the centre
of curvature, not putting them in the telescope tube until nearly
parabolic or finished. The means of trial are so excellent, the
indications obtained so precise, and the freedom from atmospheric
disturbances so complete, that the greatest facilities are offered
for ascertaining the nature of a surface. In addition the observer
is entirely independent of day or night, and of the weather. I do
not think that anything more is learned of the telescope, even under
favorable circumstances, than in the workshop. For the improvement
of these methods of observation, Science is largely indebted to M.
Foucault, whose third test--the second in the next paragraph--is
sufficient to afford by itself a large part of the information required
in correcting a concave surface.

There are two distinct modes of examination: 1st, observing with an
eye-piece the image of an illuminated pin-hole at the focus, and the
cone of rays inside and outside that plane; 2d, receiving the entire
pencil of light coming from the mirror through the pupil on the retina,
and noticing the distribution of light and shade, and the appearances
in relief on the face of the mirror.

[Illustration: Fig. 8. Testing a Concave at the Centre of Curvature.]

The arrangements for these tests are as follows: Around the flame of a
lamp (_a_, Fig. 8) a sheet of tin is bent so as to form a cylindrical
screen. Through it at the height of the brightest part of the flame,
as at _b_, two holes are bored, a quarter of an inch apart, one 1/32
of an inch in diameter, the other as small as the point of the finest
needle will make--perhaps 1/200 of an inch. This apparatus is to be set
at the centre of curvature of the mirror _c_--the optical axis of the
latter being horizontal--and so adjusted that the light which diverges
from the illuminated hole in use, may, after impinging on the concave
surface of the glass, return to form an image close by the side of
the tin screen. In the case of the first test, the returning rays are
received into an eye-piece or microscope, _d_, magnifying 20 times, and
moving upon a divided scale to and from the mirror. In the second test
the eye-piece is removed away from before the eye, and a straight-edged
opaque screen, _e_, is put in its place. The mirror is supported in
these trials by an arc of wood _f_, lined with thick woollen stuff, and
above two wooden latches, _g_, _g_, prevent it from falling forward,
but do not compress it. It is, of course, unsilvered. In the figure the
table is represented very much closer to the mirror than it should be.
In trials on the 15-1/2 inch it has to be 25 feet distant.

The appearance that a truly spherical concave surface presents with
the first test is: the image of the hole is sharply defined without
any areola of aberration around it, and is surrounded by interference
rings. Inside and outside the focus the cone of rays is exactly
similar, and circular in section. It presents no trace of irregular
illumination, nor any bright or dark circles. With the second test,
when the eye is brought into such a position that it receives the whole
pencil of reflected rays, and the opaque screen is gradually drawn
across in front of the pupil, the brightness of the surface slowly
diminishes, until just as the screen is cutting off the last relic of
the cone of rays (Fig. 9), the mirror presents an uniform grayish tint,
followed by total darkness, and gives to the eye the sensation of a

[Illustration: Fig. 9. Action of the Opaque Screen.]

[Illustration: Fig. 10. Caustic of Oblate Spheroidal Mirror.]

If, however, the mirror is not spherical, but instead gradually
_decreases_ in focal length toward the edge, the following changes
result: The image at the best focus is surrounded by a nebulosity,
stronger as the deviation from the sphere is greater, and neither can
a sharp focus be obtained nor interference fringes seen. In order to
include this nebulosity in the image, it will be necessary to push the
eye-piece toward the mirror. Before the cone of rays has completed its
convergence, the mass of light will be seen to have accumulated at the
periphery, and after the focus is past and divergence has commenced,
the accumulation will be around the axis. That is, a caustic (Fig. 10)
is formed with its summit from the mirror. By the second test, in
gradually eclipsing the light coming from the mirror, just before all
the rays are obstructed, a part of those which have constituted the
nebulosity will escape past the screen (Fig. 11) into the eye, and
cause there an extremely exaggerated appearance in relief of the solid
superposed upon the true surface beneath. The glass will no longer
seem to be a plane, but to have a section as in Fig. 12. Let us examine
by the aid of M. Foucault’s diagrams why it is that the surface seems
thus curved. If the dotted line, Fig. 13, represents the section of
the mirror, and the solid line a section of a spherical mirror of the
same mean focal length, it will be seen that the curves touch at two
points, but are separated by an interval elsewhere. If this interval be
projected by means of the differences of the ordinates, the resulting
curve will be found to be the same as that which the mirror apparently

[Illustration: Fig. 12. Apparent Section of Oblate Spheroidal Mirror.]

If the opaque screen be drawn a short distance from the mirror, the
appearance of the section curve will seem to change, the bottom of the
groove (Fig. 12) between the centre and edge advancing inwards, and the
mound in the middle growing smaller. If the screen be pushed toward the
mirror the reverse takes place, the central mound becoming larger, but
the edge decreasing. The reason for these variations becomes apparent
by considering the three diagrams, Fig. 14. The dotted curve in each
instance represents the real curve of the mirror described in the
last paragraph, while the solid lines are circles drawn with radii
progressionally shorter in _a_, _b_ and _c_, and represent sections of
three spherical mirrors whose focal lengths also progressively shorten.

[Illustration: Fig. 11. Action of the Opaque Screen.]

[Illustration: Fig. 13. Section of Spherical and Spheroidal Mirrors.]

[Illustration: Fig. 14. Relation of Spheres to Oblate Spheroid.]

When the opaque screen is at a given distance from the mirror under
examination, the only parts of the mirror which can officiate well are
those which have a curvature corresponding to a radius equal to the
same distance. All the other parts seem as if they were covered by
projecting circular masses. In looking at Fig. 14, it is plain, then,
if the opaque screen is at a maximum distance from the mirror, that the
central parts alone will seem to operate, because the two curves (_a_)
only touch there. If the screen is moved toward the mirror the curves
(_b_) will coincide at some point between the centre and edge, while if
carried still farther in only the edges touch and the appearance will
be as if a large mound were fixed upon the centre. I have been careful
in explaining how a surface may thus seem to present entirely different
characteristics if examined from points of view which vary slightly
in distance, because a knowledge of these facts is of the utmost
importance in correcting such an erroneous figure. It is now obvious
that the correction will be equally effectual if the mirror be polished
with a small rubber on the edge, or on the centre, or partly on each.
The only difference in the result will be, that the mean focal length
will be increased in the first instance, and decreased in the second,
while it will remain unchanged in the third.

[Illustration: Fig. 15. Caustic of Hyperbolic Mirror.]

[Illustration: Fig. 16. Apparent Section of Hyperbolic Mirror.]

If the mirror, instead of having a section like that of an oblate
spheroid, should have either an ellipse, parabola, or hyperbola, as
its section curve, the appearances seen above are reversed. Whilst by
the first test there is still an aberration round the image at the
best focus, the eye-piece must now be drawn from the mirror to include
it. The cone of rays is most dense round the axis inside, and at the
periphery outside the focus, and the summit of the caustic (Fig. 15)
is turned towards the mirror. The second test shows a section as in
Fig. 16, a depression at the centre, and the edges turned backwards.
The nature of the movement necessary to reduce the surface to a sphere
is very plainly indicated, action on a zone _a_ between the centre and
edge. If, however, a parabolic section is required, the zone _a_ must
not be entirely removed, and the surface rendered apparently flat, but
as much of it must be left as experience shows to be desirable.

[Illustration: Fig. 17. Action of the Opaque Screen.]

[Illustration: Fig. 18. Apparent Section of Mirror with Rings.]

If, in still a fourth instance, the mirror is not formed by the
revolution of any regular curve upon its axis, but has upon its surface
zones of longer and shorter radius intermixed irregularly, a very
common case, the two tests still indicate with precision the parts in
fault, and the correction demanded. Thus the mirror seen in section in
Fig. 17, when the principal mass of light was obstructed by the opaque
screen, would still permit that coming from certain parts to find its
way into the eye.

Figure 18 represents an irregular mirror, that was produced in the
process of correction of a hyperbolic surface, which had an apparent
section like Fig. 16 previously. The zone _a_ had been acted upon with
a small local polisher, and the mirror was finished by subsequently
softening down _b_ and _c_ with a larger tool.

After having gained from the preceding paragraphs a general idea of
the value and nature of these tests at the centre of curvature, a more
particular description of their use is desirable. M. Foucault in his
methods first brings the mirror to a spherical surface, and then by
moving the luminous pin-hole toward the mirror, and correspondingly
retracting the eye-piece or opaque screen, carries it, avoiding
aberration continually by polishing, through a series of ellipsoidal
curvatures, advancing step by step toward the paraboloid of revolution.
The length of the apartment, however, soon puts a termination to this
gradual system of correction, and he is forced to perform the last
steps of the conversion by an empirical process, and eventually to
resort to trial in the telescope.

With my mirrors of 150 inches focal length, demanding from the outset
a room more than 25 feet long, this successive system had to be
abandoned. It was not found feasible to place the lamp in the distant
focus of the ellipse--the workshop being less than 30 feet long--and
putting the luminous source on stands outside, introduced several
injurious complications, not the least of which was currents in the
layers of variously refracting air in the apartment. In a still room
the density and hygrometric variations in its various parts only give
rise to slight embarrassment. The moment, however, that currents are
produced, satisfactory examination of a mirror becomes difficult.
The air is seen only too easily to move in great spiral convolutions
between the mirror and the eye, areolæ of aberration appear around
a previously excellent image, and were it not for the second test,
any determination of surface would be impossible. By that test the
real deviations from truth of figure can be distinguished from the
atmospheric, and to a practised eye sufficient indications of necessary
changes given. Such a movement as that caused by placing the hand in
or under the line of the converging rays, will completely destroy the
beauty of an image, and by the second test give origin in the first
case to the appearance Fig. 19. In order to be completely exempt at
all times from aërial difficulties, it is desirable to have control
of a long underground apartment, the openings of which can be tightly
closed. As no artificial warmth is needed, there is the minimum of
movement in the inclosed air, and conclusions respecting a surface may
be arrived at in a very short time. The mirror may also be supported
from the ground, so that tremulous vibrations which weary the eye, and
interfere with the accuracy of criticism, may be avoided.

[Illustration: Fig. 19. Atmospheric Motions.]

Driven then from observing an image kept continually free from
aberration, through advancing ellipsoidal changes, it became necessary
to study the gradual increase of deformation, produced by the greater
and greater departures from a spherical surface, as the parabola was
approached. It was found that a sufficient guide is still provided in
these tests, by modifying them properly. The longitudinal aberration
of a mirror of small angular opening is easily calculated--being
equal to the square of half the aperture, divided by eight times
the principal focal length. That is, if a 15-1/2 inch mirror of
150 inches focal length were spherical, and were used to converge
parallel rays, those from its edge would reach a focus 5/100 of an
inch nearer the mirror than those from its central parts. If now the
converse experiment be tried, and a mirror of the same size and focal
length which can converge parallel rays, falling on all its parts, to
one focus, be examined at the centre of curvature, it gives there an
amount of longitudinal aberration 10/100 of an inch, equal to twice
the preceding. This latter, then, is the condition at the centre of
curvature, to which such mirror must be brought in order to converge
parallel rays with exactness. In addition, strict watch must be kept
upon the zones intermediate between the centre and edge, both by
measurement with diaphragms of their aberration, and better yet, by
observation of the regularity of the curve of that apparent solid,
Fig. 16, seen by the second test.

This modification of the first test is literally a method of
parabolizing by measure, and is capable of great precision when the
eye learns to estimate where the exact focus of a zone is. The little
irregularities found round the edges of the holes through the tin
screen, Fig. 8, are in this respect of material assistance. They show,
too, the increased optical or penetrating power that is gained by
increase of aperture. Minute peculiarities, not visible under very high
powers with a 10 inch diaphragm, become immediately perceptible even
with less magnifying when the whole aperture is used, provided the
mirror is spherical.

[Illustration: Fig. 20. Adjusting the Opaque Screen.]

In the use of the second test precautions have to be taken, as may be
inferred from page 15, to set the opaque screen exactly in the proper
position. The best method for ascertaining its location is, having
received the image into the eye, placed purposely too near the mirror,
to cause the screen to move across the cone of rays from the right
towards the left side. A jet black shadow begins to advance at the same
time, and in the same direction across the mirror. If the eye is then
moved from the mirror sufficiently, this black shadow can be made to
originate by the same motion of the screen as before, from the left or
opposite side of the mirror. Midway between these extremes there is a
point where the advance is from neither side. This is the true position
for the screen when it is desired to see the imperfections of the
surface in highly exaggerated relief, as in Fig. 20, which represents
the appearance of Fig. 12.[2]

  [2] In order to examine Fig. 20, the book should be held with the
  left side of the page toward a window or lamp. The eye should also be
  at least two feet distant. The centre will then be seen to protrude,
  and the surface present the apparent section engraved below it.

The interpretation of the lights and shadows upon the face of a mirror
in this test is always easy, and the observer is not likely to mistake
an elevation for a depression, if he bears in mind the fact that the
surface under examination must always be regarded as illuminated by an
oblique light coming from a source on the side opposite to that from
which the screen advances, coming for instance from the left hand side,
in the above description.

In practice, the diaphragms commonly used for a 15-1/2 inch mirror have
been as small as the light from the unsilvered surface would allow. A
six inch aperture at the centre, a ring an inch wide round the edge,
and a two inch zone midway between the two.

e. _Machines._

In the beginning of this section the difficulties into which I fell
with Lord Rosse’s machine were stated. These caused it at the time
to be abandoned. A machine based on the same idea as Mr. Lassell’s
beautiful apparatus was next constructed. It varied, however, in this,
that the hypocycloidal curve was described partly by the rotation of
the mirror, and partly by the motions of the polisher--the axes of
the spindles carrying the two being capable either of coincidence or
lateral separation to a moderate extent. A great deal of time and labor
was expended in grinding and polishing numerous mirrors with it, but
still the difficulty that had been so annoying in the former machine
persisted. Frequently, in fact generally, from six to eight zones of
unequal focal length were visible, although on some occasions when the
mirror was hyperbolic, the number was reduced to two. At first it was
supposed that the fault lay with the polishing, the pitch accumulating
irregularly from being of improper softness, for it was found to be
particularly prone to heap up at the centre. But after I had introduced
a method of fine grinding with elutriated hone powder, which enabled
the glass to reject light before the pitch polishing, it became evident
that the zones were connected with the mode of motion of the mechanism.
Many changes were made in the speed of its various elements, and a
contrivance to control the irregular motion of the polisher introduced,
but a really fine and uniform parabolic surface was never obtained,
the very best showing when finished zones of different focal lengths.
Although it cannot be said that I have tried this machine thoroughly,
for Mr. Lassell has produced specula of exquisite defining power with
it, and must have avoided these imperfections to a great extent, yet
the evident necessity of complicating the movement[3] considerably, to
avoid the polishing in rings, led me to adopt an entirely different
construction, which was used until quite recently. Although it has now
been replaced by another machine, which is still better in principle,
and gives fine results much more quickly, yet as it produced one
parabolic surface that bore a power of more than 1000, and as it serves
to introduce the process of grinding, it is worthy of description.
The action of machines for grinding and polishing has been thoroughly
examined in my workshop, no less than seven different ones having been
made at various times.

  [3] Messrs. De La Rue and Nasmyth, who used one of Mr. Lassell’s
  machines, as I have since learned, met with the same trouble, and
  were led to make two additions to the mechanism: 1, to control
  the rotation of the polisher rigorously; and 2, to give the whole
  speculum a lateral motion, by which the intersecting points of the
  curves described by the polisher were regularly changed in distance
  from the centre of the mirror. Mr. Lassell had previously, however,
  introduced a contrivance for this latter purpose himself.

The machine, which is a simplification of Lord Rosse’s, was intended
to give spiral strokes. It differed from the original, however, in
demanding a changeable stroke, and in the absence of the lateral
motion. In another most essential feature it varied from both that
and Mr. Lassell’s, _the mirror was always uppermost while polishing_,
and being uncounterpoised escaped to as great an extent as possible
from the effects of irregular pressure. To any one who has studied the
deformations of a reflecting surface, and knows how troublesome it is
to support a mirror properly, the advantage is apparent.

[Illustration: Fig. 21. Polishing Machine.]

[Illustration: Fig. 22. The Foot Power.]

The construction is as follows: A stout vertical shaft, _a_, Fig. 21,
carries at its top a circular table _b_, upon which the polisher _c_
is screwed. Below a band-wheel _d_ is fixed. Above the table, at a
distance of four inches, a horizontal bar _e_ is arranged, so as to
move back and forward in the direction of its length, and to carry
with it by means of a screw _l_, the mirror _m_, and its iron back or
chuck _n_. The bar is moved by a connecting rod _f_, attached to it
at one end, and at the other to a pin _g_ moving a slot. This slot is
in a crank _h_, carried by a vertical shaft _i_, near the former one
_a_. The band-wheel _k_ is connected with the foot power, Fig. 22. The
machine, except those parts liable to wear by friction, is made of
wood. The ends _o o′_ of the horizontal bar _e_, are defended by brass
tubes working in mahogany, and have even now but little shake, though
many hundred thousands of reciprocations have been made.

The foot power consists of an endless band with wooden treads _a a′_,
passing at one end of the apparatus over iron wheels _b b′_, which
carry the band-wheel c upon their axle. At the other end it goes over
the rollers _d d′_. Two pairs of intermediate wheels _e e′_, serve
to sustain the weight of the man or animal working in it. The treads
are so arranged that they interlock, and form a platform, which will
not yield downwards. Owing to its inclination when a weight is put on
the platform _a′_, it immediately moves from _b_ toward _d_ and the
band-wheel turns. By a moderate exertion, equivalent to walking up a
slight incline at a slow rate, a power more than sufficient to polish
a 15-1/2 inch mirror is obtained. This machine, in which very little
force is lost in overcoming friction, is frequently employed for dairy
use, and is moved commonly in the State of New York by a sheep. I have
generally myself walked in the one used by me, and have travelled some
days, during five hours, more than ten miles.

In order to give an idea of the method of using a grinding and
polishing machine, the following extract from the workshop note-book is

“A disk of plate glass 15-1/2 inches in diameter, and 1-1/4 inch
thick was procured. It had been polished flat on both sides, so that
its internal constitution might be seen.[4] It was fastened upon the
table _b_ of the machine, by four blocks of wood as at _c_, Fig. 21.
Underneath the glass were three thick folds of blanket, 15 inches in
diameter, to prevent scratching of the lower face, and avoid risk of
fracture. A convex disk of lead weighing 40 pounds having been cast,
was laid upon the upper surface of the glass, and then the screw _l_
was depressed so as to catch in a perforated iron plate _n_, at the
back of the lead _m_, and press downward strongly.

  [4] The glass that I have used has generally been such as was
  intended for dead-lights and sky-lights in ships.

“Emery as coarse as the head of a pin having been introduced, through
a hole in the lead, motion was commenced and continued for half an
hour, an occasional supply of emery being given. The machine made 150
eight-inch cross strokes, and the mirror 50 revolutions per minute. The
grinder _m_ was occasionally restrained from turning by the hand. At
the end of the time the detritus was washed away, and an examination
with the gauge made. A spot 11 inches in diameter, and 1/60 of an
inch deep, was found to have been ground out. The same process was
continued at intervals for ten hours, measurements with the gauge being
frequently made. The concave was then sufficiently deep. The leaden
grinder was kept of the right convexity by beating it on the back when
necessary. A finer variety of coarse emery, and after that flour emery
were next put on, each for an hour. These left the surface moderately
smooth, and nearly of the right focal length. The leaden grinder was
then dismissed, and the iron one, Fig. 6, put in its stead. The mirror
was removed from its place, and ground upon a large piece of flat glass
for ten minutes, to produce a circular outline to the concavity. It
was cemented with soft pitch to the concave iron disk, the counterpart
of Fig. 6, and again recentred on the blanketed table _b_. Emeries
of 3 and 20 seconds, and 1, 3, 10, 30, 60 minutes’ elutriation were
worked on it, an hour each. The rate of cross motion was reduced to
25 per minute to avoid heating, the mirror still revolving once for
every three cross strokes. The screw pressure of _l_ was stopped. This
produced a surface exquisitely fine, semi-transparent, and appearing as
if covered with a thin film of dried milk. It could reflect the light
from objects outside the window until an incidence of 45 degrees was
reached, and at night was found to be bright enough for a preliminary
examination at the centre of curvature.

“The polisher was constructed in the usual way (page 12), and being
smeared with rouge was fastened to the table _b_, where the mirror
had been. The latter warmed in water to 120° F., was then put face
downwards upon the former, and the screw _l_ so lowered as to cause no
pressure. The machine was allowed to make 20 four-inch cross strokes
per minute, and the polisher to revolve once for every three strokes.
The mirror being unconstrainedly supported on the polisher, was
irregularly rotated by hand, or rather prevented from rotating with the
polisher. The tendency of this method is to produce an almost spherical
surface. To change it to a paraboloid, it was only necessary when the
glass was polished all over to increase the length of the stroke to 8
inches, and continue working fifteen minutes at a time, examining in
the intervals by the tests at the centre of curvature. The production
of a polish all over occupied about two hours, but the correction of
figure took more time, on account of the frequent examinations, and
the absolute necessity of allowing the mirror to come back to a state
of equilibrium from which it had been disturbed when worked on the
machine.” I have seen a mirror which was parabolic when just off the
machine, by cooling over night become spherical. And these heat changes
are often succeeded by other slower molecular movements, which continue
to modify a surface for many days after.

This correction, where time and not length of stroke is the governing
agent, has once or twice been accomplished in fifteen minutes, but
sometimes has cost several hours. If the figure should have become a
hyperboloid of revolution, that is, have its edge zones too long in
comparison with the centre, it is only necessary to shorten the stroke
to bring it back to the sphere, or even to overpass that and produce
a surface in which at the centre of curvature the edge zones have too
short a focal length (Fig. 12).

Very much less trouble from zones of unequal focal length was
experienced after this machine and system of working were adopted.
This was owing probably partly to the element of irregularity in the
rotation of the mirror, and partly to the fact that the surface is
kept spherical until polished, and is then rapidly changed to the
paraboloid. Where the adjustments of an apparatus are made so as to
attempt to keep a surface parabolic for some hours, there is a strong
tendency for zones to appear, and of a width bearing a fixed relation
to the stroke.

The method of producing reflecting surfaces next to be spoken of, is
however that which has finally been adopted as the best of all, being
capable of forming mirrors which are as perfect as can be, and yet
only requiring a short time. It is the correction of a surface by local
retouches. In the account published by M. Foucault, it appears that he
is in France the inventor of this improvement.

The mode of practising the retouches is as follows: Several disks
of wood, as _a_, Fig. 23, varying from 8 inches to 1/2 an inch in
diameter, are to be provided, and covered with pitch or rosin of the
usual hardness, in squares as at _c_, on one side.[5] On the other a
low cylindrical handle _b_, is to be fixed. The mirror _a_, Fig. 24,
having been fined with the succession of emeries before described, is
laid face upward on several folds of blanket, arranged upon a circular
table, screwed to an isolated post in the centre of the apartment,
which permits the operator to move completely round it. An ordinary
barrel has generally supplied the place of the post, the head _c_,
Fig. 24, serving for the circular table, and the rim _b_ preventing
the mirror sliding off. The other end is fastened to the floor by four
cleets _d d´_.

  [5] M. Foucault used plano-convex lenses of glass, of a radius of
  curvature slightly less than that of the mirror, and covered with
  paper on the convex face.

[Illustration: Fig. 23. Local Polisher.]

[Illustration: Fig. 24. Section of Optician’s Post.]

The large polisher is first moved over the surface in straight strokes
upon every chord, and a moderate pressure is exerted. As soon as the
mirror is at all brightened, perhaps in five minutes, the operation
is to be suspended, and an examination at the centre of curvature
made. By carefully turning round, the best diameter for support is to
be found, and marked with a rat-tail file on the edge, and then the
curve of the mirror ascertained. If it is nearly spherical, as will be
the case if the grinding has been conducted with care and irregular
heating avoided, it is to be replaced on the blanketed support, and
the previous action kept up until a fine polish, free from dots
like stippling, is attained. This stage should occupy three or four
hours. Another examination should reveal the same appearances as the
preceding. It is next necessary to lengthen the radius of curvature of
the edge zones, or what is much better shorten that of the centre, so
as to convert the section curve into a parabola. This is accomplished
by straight strokes across every diameter of the face, at first with
a 4 inch, then with a 6 inch, and finally with the 8 inch polisher.
Examinations must, however, be made every five or ten minutes, to
determine how much lateral departure from a direct diametrical stroke
is necessary, to render the curve uniform out to the edge. Care must be
taken always to warm the polisher, either in front of a fire or over a
spirit lamp, before using it.

Perhaps the most striking feature in this operation is that the mirror
presents continually a curve of revolution, and is not diversified with
undulations like a ruffle. By walking steadily round the support, on
the top of which the mirror is placed, there seems to be no tendency
for such irregularities to arise.

If the correction for spherical aberration should have proceeded too
far, and the mirror become hyperbolic, the sphere can be recovered
by working a succession of polishers of increasing size on the zone
_a_, Fig. 16, intermediate between the centre and edge, causing their
centres to pass along every chord that can be described tangent to the

A most perfect and rapid control can thus be exercised over a surface,
and an uniform result very quickly attained. It becomes a pleasant
and interesting occupation to produce a mirror. But two effects have
presented themselves in this operation, which unfortunately bar the
way to the very best results. In the first place the edge parts of
such mirrors, for more than half an inch all around, bend backwards
and become of too great focal length, and the rays from these parts
cannot be united with the rest forming the image. In the second place,
the surface, when critically examined by the second test, is found
to have a delicate wavy or fleecy appearance, not seen in machine
polishing.[6] Although the variations from the true curve implied by
these latter greatly exaggerated imperfections are exceedingly small,
and do not prevent a thermometer bulb in the sunshine appearing like
a disk surrounded by rings of interference, yet they must divert some
undulations from their proper direction, or else they would not be
visible. All kinds of strokes have been tried, straight, sweeping
circular, hypocycloidal, &c. without effecting their removal. M.
Foucault, who used a paper polisher, also encountered them. Eventually
they were imputed to the unequal pressure of the hand, and in
consequence a machine to overcome the two above mentioned faults of
manual correction was constructed.

  [6] By this it is not meant that there is a rippled polish, like that
  produced by buckskin.

[Illustration: Fig. 25. Machine for Local Corrections.]

The mirror _a_, is carried by an iron chuck or table _b_, covered with
a triple fold of blanket, and is prevented from slipping off by four
cleets _c c′_. The vertical shaft _d_ passes through a worm-wheel
_e_, the endless screw of which _f_, is driven by a band _g_, from
the primary shaft _h_. At _i_ is the band-wheel for connection to
the foot-power. At one end of the primary shaft is firmly fixed the
cogwheel _k_, which drives the crank-shaft _l_. Attached to the
horizontal part of _l_, is the crank-pin _m_. The two bolts _n n′_
move in a slot, so that the crank-pin may be set at any distance from
0 to 2 inches, out of line with _l_. Above, the crank-pin carries
one end of the bar _o_, the other end passing through an elliptical
hole in the oak-block _p_. Down the middle of the bar runs a long
slot, through which the screw-pin _q_ passes, and which permits _q_
to be brought over any zone from the centre to the edge of the mirror
_a_. It is retained by the bolts _r r′_, which are tapped into _s_.
The local polisher is seen at _t_. The curve which the centre of the
local polisher describes upon the face of the mirror, varies with the
adjustments. Fig. 26 is a reduction from one traced by the machine,
the overlapping being seen on the left side. The mirror is not tightly
confined by the cleets _c c′_, for that would certainly injure the
figure, but performs a slow motion of rotation, so that in no two
successive strokes are the same parts of the edge pressed against them.

[Illustration: Fig. 26. Hypocycloidal Curve.]

The local polishers are made of lead, alloyed with a small proportion
of antimony, and are 8, 6, and 4 inches in diameter, respectively. The
largest and smallest are most used, the former on account of its size
polishing most quickly, but the latter giving the truest surface. The
rosin that covers them is just indentable by the thumb nail, and is
arranged in a novel manner. The leaden basis, as seen at _t_, Fig. 25,
is perforated in many places with holes, which permit evaporation,
serve for the introduction of water where needed, and allow the rosin
to spread freely. Grooves are made from one aperture to another, and
the rosin thus divided into irregular portions. The effects of the
production of heat are in this way avoided.

The mirror may be ground and fined on this machine, in the same manner
as on that described at page 21, or it may be ground with a small
tool 8 inches in diameter, as recently suggested by M. Foucault, the
results in the latter case being just as good a surface of revolution
as in the former. It is best polished with the 8 inch, and a moderate
pressure may be given by the screw _q_, if the pitch is not too soft.
This, however, tends to leave an excavated place at the centre of the
mirror, the size depending on the stroke of the crank _m_, which should
be about 2 inches. The pin _q_ ought to be half way from the centre to
the edge of the mirror, but must be occasionally moved right or left an
inch along the slot. When the surface is approaching a perfect polish,
the warmed 4 inch polisher must be put in the place of the 8 inch.
The pin _q_ must be set exactly half-way between the centre and edge
of the mirror, and the crank must have a stroke of two inches radius.
The polisher then just goes up to the centre of the glass surface
with one edge, and to the periphery with the other, while the outer
excursion of the inner edge and inner excursion of the outer edge meet,
and neutralize one another at a midway point. Wherever the edge of a
polisher changes direction many times in succession, on a surface, a
zone is sure to form, unless avoided in this manner. All the foregoing
description is for a 15-1/2 inch mirror.

By this system of local polishing the difficulties of heat,
distribution of polishing powders, irregular contact of the rosin,
&c. that render the attainment of a fine figure so uncertain usually,
entirely disappear. A spherical surface is produced as above described,
and afterwards by moving _q_ towards the edge, and at the same time
increasing the stroke, it is converted into a paraboloid. The fleecy
appearance spoken of on a former page is not perceived, and the surface
is good almost up to the extreme edge.


The telescope is furnished with several eye-pieces of various
construction, giving magnifying powers from 75 to 1200, or if it were
desired even higher. For the medium powers 300 and 600 Ramsden, or
rather positive eye-pieces have been adopted. They differ, however,
from the usual form in being achromatic, that is, each plano-convex
is composed of a flint and crown, arranged according to formulas
calculated by Littrow. In this way a large flat field and absence of
color are secured, and the fine images yielded by the mirror are not
injured. For the higher powers, single achromatic lenses are used, and
for the highest of all a Ross microscope.

With these means it has been found that the parabolic surfaces
yielded by the processes before described, will define test objects
excellently. Of close double stars they will separate such as γ^{2}
Andromedæ, and show the colors of the components. In the case of
unequal stars which seem to be more severe tests, they can show the
close companion of Sirius--discovered by Mr. Alvan Clark’s magnificent
refractor--the sixth component of θ^{1} Orionis, and a multitude of
other difficult objects.

As an example of light collecting power, Debillisima between ε and 5
Lyræ is found to be quintuple, as first noticed by Mr. Lassell. In the
18-1/2 inch specula of Herschel, it was only recorded as double, and,
according to Admiral Smyth, Lord Rosse did not notice the fourth and
fifth components. Jupiter’s moons show with beautiful disks, and their
difference in diameter is very marked. As for the body of that planet,
it is literally covered with belts up to the poles. The bright and dark
spots on Venus, and the fading illumination of her inner edge, and its
irregularities are perceived even when the air is far from tranquil.
Stars are often seen as disks, and without any wings or tails, unless
indeed the mirror should be wrongly placed, so that the best diameter
for support is not in the perpendicular plane, passing through the axis
of the tube.

It has been found that no advantage other than the decrease of
atmospheric influence on the image, results from cutting down the
aperture of these mirrors by diaphragms, while the disadvantage
of reducing the separating power, is perceived at the same time.
Faint objects can be better seen with the whole surface than with a
reduced aperture, and this though apparently a property common to all
reflectors and object glasses is not so in reality. A defective edge
will often cause the whole field to be filled with a pale milky light,
which will extinguish the fainter stars. Good definition is just as
important for faint as for close objects.

The properties of these mirrors have been best shown by the excellence
of the photographs taken with them. Although these are not as sharp as
the image seen in the telescope, yet it must not be supposed that an
imperfect mirror will give just as good pictures. A photograph which
is magnified to 3 feet, represents a power of 380. As the original
negative taken at the focus of the mirror is not quite 1-1/2 inch in
diameter when the moon is at its mean distance, it has to be enlarged
about 25 times, and has therefore to be very sharp to bear it.

The light collecting power of an unsilvered mirror is quite surprising.
With a 15-1/2 inch, the companion of α Lyræ can be perceived, though it
is only of the eleventh magnitude. The moon and other bright objects
are seen with a purity highly pleasing to the eye, some parts being
even more visible than after silvering.

In order to finish this description, one part more of the optical
apparatus requires to be noticed--the plane mirrors. In the Newtonian
reflector the image is rejected out at the side of the tube by a flat
surface placed at 45° with the optical axis of the large concave.[7]
If this secondary mirror is either convex or concave, it modifies the
image injuriously, causing a star to look like a cross, and this though
the curvature be so slight as hardly to be perceptible by ordinary
means. For a long time I used a piece 3 × 5 inches, which was cut from
the centre of a large looking-glass accidentally broken, but eventually
found that by grinding three pieces of 6 inches in diameter against
one another, and polishing them on very hard pitch, a nearer approach
to a true plane could be made. They were tested by being put in the
telescope, and observing whether the focus was lengthened or shortened,
and also by trial on a star. When sufficiently good to bear these
tests, a piece of the right size was cut out with a diamond, from the
central parts.

  [7] A right-angled prism cannot be used with advantage to replace the
  plane silvered mirrors, because it transmits less light than they
  reflect, is more liable to injure the image, and the glass is apt to
  be more or less colored. Its great size and cost, one three inches
  square on two faces being required for my purposes, has also to be


The telescope is mounted as an altitude and azimuth instrument, but
in a manner that causes it to differ from the usual instrument of
that kind. The essential feature is, that _the eye-piece or place of
the sensitive plate is stationary at all altitudes_, the observer
always looking straight forward, and never having to stoop or assume
inconvenient and constrained positions.

[Illustration: Fig. 27. Miss Herschel’s Telescope.]

The stationary eye-piece mounting was first used by Miss Caroline
Herschel, who had a 27 inch Newtonian arranged on that plan. Fig. 27.
(Smyth’s Celestial Cycle.)

Subsequently it was applied to a large telescope by Mr. Nasmyth, the
eminent engineer, but no details of his construction have reached
me. He used it for making drawings of the moon, which are said to be
excellently executed.

When it became necessary to determine how my telescope should be
mounted, I was strongly urged to make it an equatorial. But after
reflecting on the fact that it was intended for photography, and that
absolute freedom from tremor was essential, a condition not attained in
the equatorial when driven by a clock, and in addition that in the case
of the moon rotation upon a polar axis does not suffice to counteract
the motion in declination, I was led to adopt the other form.

A great many modifications of the original idea have been made. For
instance, instead of counterpoising the end of the tube containing
the mirror by extending the tube to a distance beyond the altitude or
horizontal axis, I introduced a system of counterpoise levers which
allows the telescope to work in a space little more than its own focal
length across. This construction permits both ends of the tube to be
supported, the lower one on a wire rope, and gives the greatest freedom
from tremor, the parts coming quickly to rest after a movement. In the
use of the telescope for photography, as we shall see, the system of
bringing the mass of the instrument to complete rest before exposing
the sensitive plate, and only driving that plate itself by a clock, is
always adopted.

The obvious disadvantage connected with the alt-azimuth mounting--the
difficulty of finding some objects--has not been a source of
embarrassment. In fact the instability of the optical axis in
reflecting instruments, if the mirror is unconstrainedly supported, as
it should be, renders them unsuitable for determinations of position. A
little patience will enable an observer to find all necessary tests, or
curious objects.

The mounting is divided into: a. The Tube; and b. The supporting frame.

a. _The Tube._

The telescope tube is a sixteen sided prism of walnut wood, 18 inches
in diameter, and 12 feet long. The staves are 3/8 of an inch thick,
and are hooped together with four bands of brass, capable of being
tightened by screws. Inside the tube are placed two rings of iron, half
an inch thick, reducing the internal diameter to about 16 inches. At
opposite sides of the upper end of the tube are screwed the perforated
trunnions _a_, Fig. 28 (of which only one is shown), upon which it
swings. Surrounding the other end is a wire rope _b b′ b″_, the ends of
which go over the pulleys _c_ (_c′_ not shown) on friction rollers, and
terminate in disks of lead _d d′_. These counterpoises are fastened on
the ends of levers _e e′_, which turn below on a fixed axle _f_.

By this arrangement as the tube assumes a horizontal position and
becomes, so to speak, heavier, the counterpoises do the same, while
when the tube becomes perpendicular, and most of its weight falls upon
the trunnions, the counterpoises are carried mostly by their axle. A
continual condition of equilibrium is thus reached, the tube being
easily raised or depressed to any altitude desired. It is necessary,
however, to constrain the wire rope _b b′ b″_, to move in the arc of
the circle described by the end of the tube and ends of the levers
and hence the twelve rollers or guide pulleys _g g′ g″_. Over some of
the same pulleys a thin wire rope _h h′_ runs, but while its ends are
fastened to the lower part of the tube at _b_, the central parts go
twice around a roller connected with the winch _i_, near the eye-piece,
thus enabling the observer to move the telescope in altitude, without
taking the eye from the eye-piece.

[Illustration: Fig. 28. Sectional View of Observatory.]

The iron wire rope required to be carefully made, so as to avoid
rigidity. It contains 2-1/3 miles of wire, 1/100 of an inch in
diameter, and has 300 strands. Each single wire will support 7 pounds.
It is, however, more flexible than a hempen rope of the same size,
owing to its loose twisting.

[Illustration: Fig. 29. The Mirror Support.]

At the lower end of the tube, at the distance of a foot, and crossing
it at right angles, held by three bars of iron _i i′ i″_, Fig. 29, is a
circular table of oak _e_, which carries an India-rubber air sac _d_,
and upon this the mirror _f_ is placed. The edge support of the mirror
is furnished by a semicircular band of tin-plate _a_, lined inside with
cotton, and fastened at the ends by links of chain _b_, (_b′_ not seen)
to two screws _c c′_; _g_ and _h_ are the wire ropes, marked _b_ and
_h_ in Fig. 28.

Instead of the blanket support which Herschel found so advantageous,
M. Foucault has suggested this use of an air sac. In his instrument
there is a tube going up to the observer, by which he may adjust its
degree of inflation. It requires that there should be three bearings
_c c′ c″_, in front of the mirror, against which it may press when
the sac behind is inflated, otherwise the optical axis is altogether
too instable, and objects cannot be found. The arrangement certainly
gives beautiful definition, bringing stars to a disk when the glass
just floats, without touching its front bearings. The first sac that I
made was composed of two circular sheets of India-rubber cloth, joined
around the edges. But this could not be used while photographing,
because the image was kept in a state of continuous oscillation if
there was a breeze, and even under more favorable circumstances took a
long time to come to rest. It was not advisable to blow the mirror hard
up against its three front bearings, in order to avoid the instability,
for then every point in of an object became triple. To the eye the
oscillations were not offensive, because the swaying image was sharp.

Subsequently, however, an air chair cushion was procured, and as the
surface was flat instead of convex the difficulty became so much less,
that the blanket support was definitely abandoned. It is necessary that
the mirror should have free play in the direction of the length of the
tube when this kind of support is used, and that is the reason why the
tin edge hoop must terminate in links of chain.

The interval, eight or ten inches, which separates the face of the
mirror from the tube, is occupied by a curtain of black velvet,
confined below by a drawing cord and tacked above to the tube. This
permits access to the mirror to put a glass cover on it, and when shut
down stops the current of air rushing up. When the instrument is not
being used this curtain is left open, because the mirror and tube are
in that case kept more uniform in temperature with the surrounding air.

In spite of such contrivances there is still sometimes a strong
residual current in the tube. I have tried to overcome it by covering
the mouth of the tube with a sheet of flat glass, but have been obliged
to abandon that because the images were injured. At one time, too, when
it was supposed that the current was partly from the observer’s body,
heated streams of air going out around the tube, the aperture in the
dome was closed by a conical bag of muslin, which fitted the mouth of
the telescope tightly. The only advantages resulting were mere bodily
comfort and a capability of perceiving fainter objects than before,
because the sky-light was shut off.

[Illustration: Fig. 30. Section of Azimuth Axis.]

b. _The Supporting Frame._

The frame which carries the preceding parts is of wood, and rests on
a vertical axis _a_, Fig. 30, turning below in a gun-metal cup _b_,
supported by a marble block resting on the solid rock. The upper end of
the axis is sustained by two collars, one _c c′_ above, and the other
below an intermediate triangular box _e e′_ from the sides of which
three long beams _f f f_ 12 × 3 inches diverge, gradually declining
till they meet the solid rock at the limits of the excavation in
which the observatory is placed. These beams are fastened together
by cross-pieces _g g g_, Fig. 31, and go through the floor in spaces
_h h h_, so contrived that the floor does not touch them. At the ends
they are cased with a thick leaden sheathing, to deaden vibration and
prevent the access of moisture.

[Illustration: Fig. 31. Plan of Observatory (lower floor).]

This tripod support in connection with the sustaining of the telescope
by the wire rope, gives that steadiness which is so essential in
photography. Only a slight amount of force, about two pounds, is
required to move the instrument in azimuth, though it weighs almost a
thousand pounds.

The plan of the frame centrally carried by the axis _a_ is as follows:
From the corners of a parallelogram _i i_ (2 × 13 feet) of wooden
beams, eight inches thick and three inches broad, perpendiculars _n
n′_, Fig. 28, rise. At the top they are connected by lighter pieces to
form a parallelogram, similar to that below, and just large enough to
contain the tube of the telescope. At right angles to the parallelogram
below, and close upon it, a braced bar _o o′_, Fig. 28, crosses.
From its extremities four slanting braces as at _p p′_, Fig. 28, go
to the corners of the upper parallelogram, and combine to give it
lateral support. At the top of one close pair of the perpendiculars
_n′_, Fig. 28, are bronze frames carrying friction rollers upon which
the trunnions move, while similarly upon the other pair _n_ are two
pulleys, also on friction rollers, for the wire rope coming from the

Movement in altitude is very easily accomplished, and with the left
hand upon the winch _i_, under high powers, both altitude and azimuth
motions are controlled, and the right hand left free. The whole
apparatus works so well, that in ordinary observation the want of a
clock movement has not been felt. Of course for photography that is


The apparatus for following celestial bodies is divided into two parts;
a. The Sliding Plate-holder; and b. The Clepsydra. In addition a short
description of the Sun-Camera, c, is necessary.

a. _The Sliding Plate-holder._

Mr. De La Rue, who has done so much for celestial photography, was the
first to suggest photographing the moon on a sensitive plate, carried
by a frame moving in the apparent direction of her path. He never,
however, applied an automatic driving mechanism, but was eventually
led to use a clock which caused the whole telescope to revolve upon a
polar axis, and thus compensate for the rotation of the earth, and on
certain occasions for the motion of the moon herself. In this way he
has produced the best results that have been obtained in Europe. Lord
Rosse, too, employed a similar sliding plate-holder, but provided with
clock-work to move it at an appropriate rate. I have not been able as
yet to procure any precise account of either of these instruments.

The first photographic representations of the moon ever made, were
taken by my father, Professor John W. Draper, and a notice of them
published in his quarto work “On the Forces that Organize Plants,”
and also in the September number, 1840, of the London, Edinburgh, and
Dublin Philosophical Magazine. He presented the specimens to the New
York Lyceum of Natural History. The Secretary of that Association has
sent me the following extract from their minutes:--

  “_March 23d, 1840._ Dr. Draper announced that he had succeeded
  in getting a representation of the moon’s surface by the
  Daguerreotype.... The time occupied was 20 minutes, and the size of
  the figure about 1 inch in diameter. Daguerre had attempted the same
  thing, but did not succeed. This is the first time that anything like
  a distinct representation of the moon’s surface has been obtained.

  “ROBT. H. BROWNNE, _Secretary_.”

As my father was at that time however much occupied with experiments
on the Chemical Action of Light, the Influence of Light on the
Decomposition of Carbonic Acid by Plants, the Fixed Lines of the
Spectrum, Spectrum Analysis, &c., the results of which are to be found
scattered through the Philosophical Magazine, Silliman’s Journal, and
the Journal of the Franklin Institute, he never pursued this very
promising subject. Some of the pictures were taken with a three inch,
and some with a five inch lens, driven by a heliostat.

In 1850, Mr. Bond, taking advantage of the refractor of 15 inches
aperture at Cambridge, obtained some fine pictures of the moon, and
subsequently of double stars, more particularly Mizar in Ursa Major.
The driving power, in this instance, was also applied to move the
telescope upon a polar axis.

Besides these, several English and continental observers, Messrs.
Hartnup, Phillips, Crookes, Father Secchi, and others, have worked at
this branch of astronomy, and, since 1857, Mr. Lewis M. Rutherfurd, of
New York, has taken many exquisite lunar photographs, which compare
favorably with foreign ones.

But in none of these instances has the use of the sliding plate-holder
been persisted in, and its advantages brought into view. In the first
place it gets rid completely of the difficulties arising from the
moon’s motion in declination, and in the second, instead of injuring
the photograph by the tremors produced in moving the whole heavy mass
of a telescope weighing a ton or more, it only necessitates the driving
of an arrangement weighing scarcely an ounce.

My first trials were with a frame to contain the sensitive plate, held
only at three points. Two of these were at the ends of screws to be
turned by the hands, and the third was on a spring so as to maintain
firm contact. This apparatus worked well in many respects, but it was
found that however much care might be taken, the hands always caused
some tremor in the instrument. It was evident then that the difficulty
from friction which besets the movements of all such delicate
machinery, and causes jerking and starts, would have to be avoided in
some other way.

I next constructed a metal slide to run between two parallel strips,
and ground it into position with the greatest care. This, when set in
the direction of the moon’s apparent path, and moved by one screw,
worked better than the preceding. But it was soon perceived that
although the strips fitted the frame as tightly as practicable, an
adhesion of the slide took place first to one strip and then to the
other, and a sort of undulatory or vermicular progression resulted.
The amount of deviation from a rectilinear motion, though small, was
enough to injure the photographs. At this stage of the investigation
the regiment of volunteers to which I belonged was called into active
service, and I spent several months in Virginia.

My brother, Mr. Daniel Draper, to whose mechanical ingenuity I have
on several occasions been indebted for assistance in the manifold
difficulties that have arisen while constructing this telescope,
continued these experiments at intervals. He presented me on my return
with a slide and sand-clock, with which some excellent photographs have
been taken. He had found that unless the slide above mentioned was made
ungovernably long, the same trouble continued. He then ceased catching
the sliding frame _h_, Fig. 32, by two opposite sides, and made it run
along a single steel rod _a_, being attached by means of two perforated
plates of brass _b_, _b′_. The cord _i_ going to the sand-clock, was
applied so as to pull as nearly as possible in the direction of the
rod. A piece of cork _c_, gave the whole steadiness, and yet softness
of motion. The lower end of the frame was prevented from swinging back
and forward by a steel pin _d_, which played along the glass rod _e_.
All these parts were attached to a frame _k_, fitting on the eyepiece
holder, and permitting the rod _a_ to change from the horizontal
position in which it is here drawn, to any angular one desired. The
thumb-screw _f_ retained it in place; _g_ and _g′_ are pulleys which
permit the cord to change direction.

[Illustration: Fig. 32. Sliding Plate-holder.]

Subsequently, a better method of examining the uniformity of the
rate, than by noticing the sharpness of the photograph produced, was
invented. It consists in arranging a fixed microscope, magnifying
about 40 times, at the back of the ground glass plate, which fits in
the same slide as the sensitive plate. By watching the granulated
appearance pass before the eye, as the slide is moved by the clock, the
slightest variation from uniformity, any pulsatile or jerking movement
is rendered visible. By the aid of this microscopic exaggeration, it
was seen that occasionally, when there had been considerable changes in
temperature, the steadiness of the motion varied. This was traced to
the irregular slipping of _b_, _b′_.

[Illustration: Fig. 33. Frictionless Slide (front view).
  Sectional view.]

A different arrangement was then adopted, by which a lunar crater can
be kept bisected as long as is necessary, and which gives origin to no
irregularities, but pursues a steady course. The principle is, not to
allow a slipping friction anywhere, but to substitute rolling friction,
upon wheels turning on points at the ends of their axles. The following
wood-cut is half the real size of this arrangement.

A glass rod _a_, _a′_, Fig. 33, is sustained by two wheels _b_, _b′_,
and kept in contact, with them by a third friction roller _c_, pressed
downward by a spring. This rod carries a circular frame _d_, _d′_, upon
which at _e_, _e′_, _e″_, are three glass holders and platinum catches.
A spring _f_ holds the sensitive plate in position, by pressing
against its back. The circular frame _d_ is kept in one plane by a
fourth friction roller _g_, which runs on a glass rod _h_, and is kept
against it by the inward pressure of the overhanging frame _d_. The
cord _i_ is attached to the arm _k_, and pulls in the direction of the
glass rod _a_. From _m_ to a fixed point near _b_, a strip of elastic
India-rubber is stretched, to keep the cord tight. The ring of brass
_n_, _n′_ carries the whole, serving as a basis for the stationary
parts, and in its turn being fastened to the eyepiece holder, so as
to allow the glass rod _a_ to change direction, and be brought into
coincidence with the apparent path of the moon. At _o_ is a thumb-screw
or clamp. Through the ring _n_, _n′_, a groove _p_ is cut, into which
a piece of yellow glass may be placed, when the actinic rays are to be
shut off from the plate.

Since this contrivance has been completed, all the previous
difficulties have vanished. The moving of a plate can be accomplished
with such precision, that when the atmosphere was steady, negatives
were taken which have been enlarged to three feet in diameter.

The length of time that such a slide can be made to run is indefinite,
depending in my case on the size of the diagonal flat mirror, and
aperture of the eyepiece holder. I can follow the moon for nearly four
minutes, but have never required to do so for more than fifty seconds.
At the mouth of the instrument, where no secondary mirror is necessary,
the time of running could be increased.

The setting of the frictionless slide in angular position is
accomplished as follows: A ground glass plate is put into it, with
the ground face toward the mirror. Upon this face a black line must
have been traced, precisely parallel to the rod _a_. This may be
accomplished by firmly fixing a pencil point against the ground side,
and then drawing the frame d and glass past it, while the rest of the
slide is held fast. As the moon passes across the field, the position
of the apparatus must be changed, until one of the craters runs along
the line from end to end. A cross line drawn perpendicular to the
other, serves to adjust the rate of the clepsydra as we shall see, and
when a crater is kept steadily on the intersection for twice or three
times the time demanded to secure an impression, the adjustment may be
regarded as complete.

It is necessary of course to expose the sensitive plate soon after,
or the apparent path of the moon will have changed direction, unless
indeed the slide is set to suit a future moment.

b. _The Clepsydra._

My prime mover was a weight supported by a column of sand, which, when
the sand was allowed to run out through a variable orifice below, could
be made to descend with any desired velocity and yet with uniformity.
In addition, by these means an unlimited power could be brought to
bear, depending on the size of the weight. Previously it was proposed
to use water, and compensate for the decrease in flow, as the column
shortened, by a conical vessel; but it was soon perceived that as
each drop of water escaped from the funnel-shaped vessel, only a
corresponding weight would be brought into play. This is not the case
with sand, for in this instance every grain that passes out causes the
whole weight that is supported by the column to come into action. In
the former instance a movement consisting of a series of periods of
rest and periods of motion occurs, because power has to accumulate by
floating weight lagging behind the descending water, and then suddenly
overtaking it. In the latter case, on the contrary, there is a regular
descent, all minor resistances in the slide being overcome by the
steady application of the whole mass of the weight.

When these advantages in the flow of sand were ascertained, all the
other prime movers were abandoned. Mercury-clocks, on the principle of
the hydrostatic paradox, air-clocks, &c., in great variety, had been

[Illustration: Fig. 34. The Sand-Clock.]

The sand-clock consisted of a tube _a_ (Fig. 34), eighteen inches
long and one and a half in diameter, nearly filled with sand that had
been raised to a bright red heat and sifted. Upon the top of the sand
a leaden weight _b_ was placed. At the bottom of the tube a peculiar
stopcock, seen at (2) enlarged, regulated the flow, the amount passing
depending on the size of the aperture _d_. This stopcock consisted
of two thin plates, fixed at one end and free at the other. The one
marked _e_ is the adjusting lever, and its aperture moves past that in
the plate _g_. The lever _f_ serves to turn the sand off altogether,
without disturbing the size of the other aperture, which, once set
to the moon’s rate, varies but slightly in short times. A movable
cover _h_, perforated to allow the cord _i_ to pass through, closed
the top, while the vessel _k_ retained the escaped sand, which at
suitable times was returned into the tube _a_, the weight _b_ being
temporarily lifted out. From the clock the cord _i_ communicated motion
to the frictionless slide, as shown in Fig. 33. This cord should be as
inelastic as possible, consistent with pliability, and well waxed.

One who has not investigated the matter would naturally suppose that
the flow of sand in such a long tube would be much quicker when the
tube was full than when nearly empty, and that certainly that result
would occur when a heavy weight was put on the shifting mass. But in
neither case have I been able to detect the slightest variation, for,
although by shaking the tube a diminution of the space occupied by the
sand may be caused, yet no increase of weight tried could accomplish
the same reduction. These peculiarities seem to result from the sand
arching as it were across the vessel, like shot in a narrow tube, and
only yielding when the under supports are removed. In blasting, a heavy
charge of gunpowder can be retained at the bottom of a hole, and made
to split large masses of rock, by filling the rest of the hole with dry

I believe that no prime mover is more suitable than a sand-clock
for purposes where steady motion and a large amount of power are
demanded. The simplicity, for instance, of a heliostat on this plan,
the large size it might assume, and its small cost, would be great
recommendations. In these respects its advantages over wheelwork are
very apparent. The precision with which such a sand-clock goes may be
appreciated when it is stated, that under a power of 300 a lunar crater
can be kept bisected for many times the period required to photograph
it. To secure the greatest accuracy in the rate of a sand-clock, some
precautions must be taken. The tube should be free from dents, of
uniform diameter, and very smooth or polished inside. Water must not be
permitted to find access to the sand, and hygrometric varieties of that
substance should be avoided, or their salts washed out. The sand should
be burned to destroy organic matter, and so sifted as to retain grains
nearly equal in size. The weight, which may be of lead, must be turned
so as to go easily down the tube, and must be covered with writing
paper or some other hard and smooth material, to avoid the proneness to
adhesion of sand. A long bottle filled with mercury answers well as a

I have used in such clocks certain metallic preparations: Fine shot, on
account of its equality of size, might do for a very large clock with
a considerable opening below, but is unsuitable for a tube of the size
stated above. There is, however, a method by which lead can be reduced
to a divided condition, like fine gunpowder, when it may replace the
sand. If that metal is melted with a little antimony, and while cooling
is shaken in a box containing some plumbago, it breaks up at the
instant of solidifying into a fine powder, which is about five times
as heavy as sand. If after being sifted to select the grains of proper
size, it is allowed to run through a small hole, the flow is seen to be
entirely different from that of sand, looking as if a wire or solid rod
were descending, and not an aggregation of particles. It is probable,
therefore, that it would do better than sand for this purpose. I have
not, however, given it a fair trial, because just at the time when the
experiments with the sand-clock had reached this point, I determined to
try a clepsydra as a prime mover.

The reason which led to this change was that it was observed on a
certain occasion when the atmosphere was steady, that the photographs
did not correspond in sharpness, being in fact no better than on other
nights when there was a considerable flickering motion in the air. A
further investigation showed that in these columns of sand there is
apt to be a minute vibrating movement. At the plate-holder above this
is converted into a series of arrests and advances. On some occasions,
however, these slight deviations from continuous motion are entirely
absent, and generally, indeed, they cannot be seen, if the parts of the
image seem to vibrate on account of currents in the air. By the aid
of the microscopic exaggeration described on a former page--which was
subsequently put in practice--they may be observed easily, if present.

When the negative produced at the focus of the great mirror is intended
to be enlarged to two feet or more in size, these movements injure it
sensibly. A variety of expedients was resorted to in order to avoid
them, but none proved on all occasions successful.

It is obvious that in a water-clock, where the mobility of the fluid
is so much greater than that of solid grains, this difficulty would
not arise. The following contrivance in which the fault of the ordinary
clepsydra, in varying rate of flow as the column shortens, is avoided,
was next made. With it the best results are attainable, and it seems to
be practically perfect.

[Illustration: Fig. 35. The Clepsydra.]

It consists of a cylinder _a_, in which a piston _b_ moves watertight.
At the top of the piston rod is a leaden five-pound weight _c_, from
which the cord _i_ goes to the sliding plateholder _g_. The lower end
of the cylinder terminates in a stopcock _d_, the handle of which
carries a strong index rod _e_, moving on a divided arc. At _f_ a tube
with a stopcock is attached. Below, a vessel _h_ receives the waste

In using the clepsydra the stopcock of _f_ is opened, and the piston
being pulled upwards, the cylinder fills with water from _h_. The
stopcock is then closed, and if _d_ also is shut, the weight will
remain motionless. The string _i_ is next connected with the slide, and
the telescope turned on the moon. As soon as the slide is adjusted in
angular position (page 36) the stopcock _d_ is opened, until the weight
_c_ moves downwards, at a rate that matches the moon’s apparent motion.

In order to facilitate the rating of the clepsydra, the index rod
_e_ is pressed by a spring _k_ (2), against an excentric _l_. As the
excentric is turned round, the stopcock _d_ is of course opened,
with great precision and delicacy. The plug of this stopcock (3) is
not perforated by a round hole, but has a slit. This causes equal
movements in the rod _e_, to produce equal changes in the flow. The
rating requires consequently only a few moments.

The object of the side tube _f_ is to avoid disturbing _d_ when it
becomes necessary to refill the cylinder, for when it is once opened
to the right degree, it hardly requires to be touched again during a
night’s work. In order to arrest the downward motion of the piston at
any point, a clamp screws on the piston rod, and can be brought into
contact with the cylinder head, as in the figure.

That this instrument should operate in the best manner, it is essential
to have the interior of the brass cylinder polished from end to end,
and of uniform diameter. If any irregularity should be perceived in the
rate of going, it can be cured completely by taking out the piston,
impregnating its leather stuffing with fine rotten stone and oil, and
then rubbing it up and down for five minutes in the cylinder, so as to
restore the polish. The piston and cylinder must of course be wiped,
and regreased with a mixture of beeswax and olive oil (equal parts)
after such an operation. In replacing the piston, the cylinder must be
first filled with water, to avoid the presence of air, which would act
as a spring.

Although it may be objected that this contrivance seems to be very
troublesome to use, yet that is not the case in practice. Even if it
were, it so far surpasses any prime mover that I have seen, where the
utmost accuracy is needed, that it would be well worth employing.

c. _The Sun Camera._

In taking photographs of the sun with the full aperture of this
telescope, no driving mechanism is necessary. On the contrary, the
difficulty is rather to arrange the apparatus so that an exposure
short enough may be given to the sensitive plate, and solarization of
the picture avoided. It is not desirable to reduce the aperture, for
then the separating power is lessened. The time required to obtain a
negative is a very small fraction of a second, for the wavy appearance
produced by atmospheric disturbance is not unfrequently observed
sharply defined in the photograph, though these aerial motions are so
rapid that they can scarcely be counted. Some kind of shutter that can
admit and cut off the solar image with great quickness is therefore

[Illustration: Fig. 36. The Spring Shutter.]

In front of an ordinary camera _a_, Fig. 36, attached to the eyepiece
holder of the telescope, and from which the lenses have been removed,
a spring shutter is fixed. It consists of a quadrant of thin wood _b_,
fastened by its right angle to one corner of the camera. Over the
hole in this quadrant a plate of tin _d_ can be adjusted, and held
in position by a screw moving in a slot so as to reduce the hole if
desired to a mere slit. It may vary from 1-1/2 inch to less than 1/50
of an inch. The quadrant is drawn downwards by an India-rubber spring
_g_, 1 inch wide, 1/8 of an inch thick, and 8 inches long. This spring
is stretched when in action to about 12 inches, and when released draws
the slit past the aperture _c_ in the camera. Two nicks in the edge of
the quadrant serve with the assistance of a pin _e_, which can easily
be drawn out by a lever (not shown in the cut), to confine the slit
either opposite to or above _c_. A catch at _f_ prevents the shutter
recoiling. The sensitive plate is put inside the box as usual in a
plate-holder. When a photograph is taken, the spring shutter is drawn
up so that the lower nick in the edge of the quadrant is entered by the
pin _e_, and the inside of the camera obscured. The front slide of the
plateholder is then removed in the usual manner, and the solar image
being brought into proper position by the aid of the telescope finder,
the trigger retaining _e_ is touched, the shutter flies past _c_, and
the sensitive plate may then be removed to be developed.

To avoid the very short exposure needed when a silvered mirror of 188
square inches of surface is used, I have taken many solar photographs
with an unsilvered mirror, which only reflects according to Bouguer
2-1/2 per cent. of the light falling upon it, and should permit an
exposure 37 times as long as the silvered mirror. This is the first
time that a plain glass mirror has been used for such a purpose,
although Sir John Herschel suggested it for observation many years ago.
But eventually this application of the unsilvered mirror had to be
abandoned. It has, it is true, the advantage of reducing the light and
heat, but I found that the moment the glass was exposed to the Sun, it
commenced to change in figure, and alter in focal length. This latter
difficulty, which sometimes amounts to half an inch, renders it well
nigh impossible to find the focal plane, and retain it while taking
out the ground glass, and putting in the sensitive plate. If the glass
were supported by a ring around the edge, and the back left more freely
exposed to the air, the difficulty would be lessened but not avoided,
for a glass mirror can be raised to 120° F. on a hot day by putting
it in the sunshine, though only resting on a few points. Other means
of reducing the light and heat, depending on the same principle, can
however be used. By replacing the silvered diagonal mirror with a black
glass or plain unsilvered surface, as suggested by Nasmyth, the trouble
sensibly disappears.

I have in this way secured not only maculæ and their penumbræ, but
also have obtained faculæ almost invisible to observation. On some
occasions, too, the precipitate-like or minute flocculent appearance on
the Sun’s disk was perceptible.

It seems, however, that the best means of acquiring fine results with
solar photography, would be to use the telescope as a Cassegrainian,
and produce an image so much enlarged, that the exposure would not have
to be conducted with such rapidity. Magnifying the image by an eyepiece
would in a general way have the same result, but in that case the
photographic advantages of the reflector would be lost, and it would be
no better than an achromatic.


This section is divided into _a_, The Building; _b_, The Dome; and _c_,
The Observer’s Chair.

a. _The Building._

The Observatory is on the top of a hill, 225 feet above low water mark,
and is in Latitude 40° 59′ 25″ north, and Longitude 73° 52′ 25″ west
from Greenwich, according to the determinations of the Coast Survey.
It is near the village of Hastings-upon-Hudson, and is about 20 miles
north of the city of New York. The surrounding country on the banks
of the North River is occupied by country seats, on the slopes and
summits of ridges of low hills, and no offensive manufactories vitiate
the atmosphere with smoke. Our grounds are sufficiently extensive to
exclude the near passages of vehicles, and to avoid tremor and other

[Illustration: Fig. 37. Dr. Draper’s Observatory.]

An uninterrupted horizon is commanded in every direction, except
where trees near the dwelling house cut off a few degrees toward the
southwest. The advantages of the location are very great, and often
when the valleys round are filled with foggy exhalations, there is a
clear sky over the Observatory, the mist flowing down like a great
stream, and losing itself in the chasm through which the Hudson here

The foundation and lower story of the building are excavated out of the
solid granite, which appears at the edge of the hill. This arrangement
was intended to keep the lower story cool, and avoid, in the case of
the metal reflector, sudden changes of temperature. The eastern side
of the lower story, however, projects over the brow of the hill, and
is therefore freely exposed to the air, furnishing, when desired, both
access and thorough ventilation through the door. The second story or
superstructure is of wood, lined inside with boards like the story
below. They serve to inclose in both cases a non-conducting sheet of

The inside dimensions of both stories taken together are 17-1/2 feet
square, and 22 feet high, to the apex of the dome. This space is
unnecessarily large for the telescope, which only requires a cylinder
13 feet in diameter and 13 feet high. A general idea of the internal
arrangement is gained from Fig. 28. In Fig. 38, _a a′_ is the floor of
the gallery, _b b′ b″_ the circular aperture in which the telescope _c
c′_ turns. The staircase is indicated by _d_. The Enlarger, §6, rests
on the shelf _e_, the heliostat being outside at _f_. The door going
into the photographic room is at _g_, _h h′_ are tables, _i_ the water
tank, _k_ the tap and sink, _l_ the stove, _m_ a heliostat shelf, _n_
the door, _o_ the window.

[Illustration: Fig. 38. Plan of Observatory (upper floor).]

The building is kept ventilated by opening the door in the lower part,
and the dome shutter, seen in Fig. 37, for some time before using the
instrument. On a summer day the upper parts, and especially those close
under the dome, become without this precaution very hot, and this
occurred even before the tin roof was painted. Bright tinplate seems
not to be able to reflect by any means all the heat that falls upon
it, but will become so warm in July that rosin will melt on it, and
insects which have lighted in a few moments dry up, and soon become
pulverizable. A knowledge of these facts led to the abandonment of
wooden sheathing under the tin, for without it when night comes on the
accumulated heat radiates away rapidly, and ceases to cause aerial
currents near the telescope.

The interior of the building is painted and wainscoted, and the roof is
ornamented partly in blue and oak, and partly with panels of tulip-tree

There are only two windows, and they are near the southern angles
of the roof. While they admit sunshine on some occasions, they can
on others be closed, and the interior be reduced to darkness. In
the southeast corner a small opening _e_ may allow a solar beam
three inches in diameter to come in from a heliostat outside. The
greatest facilities are thus presented for optical and photographical
experiments, for in the latter case the whole room can be used as a
camera obscura.

b. _The Dome._

The roof of the observatory is 20 feet square. The angles are filled in
solid, and a circular space 15 feet in diameter is left to be covered
by the revolving dome. Although such a construction is architecturally
weak and liable to lose its level, yet the great advantages of
having the building below square, and the usefulness of the corners,
determined its adoption, the disadvantages being overcome by a very
light dome.

The dome is 16 feet in outside diameter, and rises to a height of 5
feet above its base. It is, therefore, much flatter than usual, in
fact, might have been absolutely flat, with this method of mounting.
It would then have been liable, however, to be crushed in by the deep
winter snows.

It consists of 32 ribs, arcs of a circle, uniting at a common centre
above. Each one is formed of two pieces of thin whitewood, _b_,
Fig. 39, fastened side by side, with the best arrangements of the grain
for strength. They are three inches wide and one inch thick at the
lower end, and taper gradually to 2-1/2 by 1.

[Illustration: Fig. 39. Joints in Tin of Dome.]

Over these ribs tinplate is laid in triangular strips or gores, about
18 inches wide at the base, and 10 feet long. Where the adjacent
triangles of tin _a a′_ meet, they are not soldered, but are bent
together. This allows a certain amount of contraction and expansion,
and is water-proof. It strengthens the roof so much, that if the ribs
below were taken away, this corrugated though thin dome would probably
sustain itself. The tin is fastened to the dome ribs _b_ by extra
pieces _c_ inserted in the joint and doubled with the other parts,
while below they are nailed to the ribs. In the figure the tin is
represented very much thicker than it is in reality.

This dome, although it has 250 square feet of surface, only weighs 250
pounds. That at the Cambridge (Massachusetts) Observatory, 29-1/2 feet
in diameter, weighs 28,000 pounds.

The slit or opening is much shorter than usual, only extending half way
from the base towards the summit. It is in reality an inclined window,
2-1/2 feet wide at the bottom, 1-1/2 wide at the top, and 4 feet long.
It is closed by a single shutter, as seen in Fig. 37, and this when
opened is sustained in position by an iron rod furnished with a hinge
at one end and a hook at the other.

The principal peculiarity of the dome, the means by which it is
rotated, remains to be described. Usually in such structures rollers
or cannon balls are placed at intervals under the edge, and by means
of rack work, a motion of revolution is slowly accomplished. Here,
on the contrary, the whole dome _b b′ b″_ (Fig. 40) is supported on
an arch _h h′ h″_, carrying an axis a at its centre, around which a
slight direct force, a pull with a single finger, will cause movement,
and by a sudden push even a quarter of an entire revolution may be
accomplished. It is desirable, however, to let it rest on the edge
_b b″_, when not in use. At _c_ there is an iron catch on the arch, by
which the lever _e_, that raises the dome, is held down. The fulcrum is
at _d_. The lever is hinged near _c_, so that when by being depressed
it should have come in the way of the telescope below, the lower half
_g_ can be pushed up, the part from _c_ toward _d_ still holding the
dome supported.

[Illustration: Fig. 40. The Dome Arch.]

The arch can be set across the observatory in any direction, north and
south, east and west, or at any intermediate position, because the
abutments where the ends rest, are formed by a ring _l l′ l″_, fastened
round the circular aperture, through the stationary part of the roof.

When the telescope is not in use, and the dome is let down, so that
there is no longer an interval of a quarter of an inch between it and
the rest of the roof, it is confined inside by four clamps and wedges.
Otherwise, owing to its lightness, it would be liable to be blown away.
These clamps _a_, Fig. 41, are three sides of a square, made of iron
one inch square. They catch above by a point in the wooden basis-circle
of the dome _b_, and below are tightened by the wedge _c_.

[Illustration: Fig. 41. A Dome Clamp.]

When the dome is raised it is prevented from moving laterally and
sliding off by three rollers, one of which is seen at _f_, Fig. 40.
These catch against its inner edge, and only allow slight play. At
first it was thought necessary to have a subsidiary half arch at right
angles to the other to hold it up, but that is now removed.

All the parts work very satisfactorily, and owing to the care taken to
get the roof-circle and basis-circle flat and level, no leakage takes
place at the joint, and even snow driven by high winds is unable to

c. _The Observer’s Chair._

This is not a chair in the common acceptation of the word, but is
rather a movable platform three feet square, capable of carrying two
or more persons round the observatory, and maintaining them in an
invariable position with regard to the telescope eyepiece.

[Illustration: Fig. 42. The Observer’s Chair.]

Its general arrangement is better comprehended from the sketch,
Fig. 42, than from a labored description. Below, it runs on a pair of
wheels _a_ (one only is visible) 9 inches in diameter, whose axles
point to the centre of the circle upon which they run. They are
prevented from shifting outwards by a wooden railroad _b_, _b′_, and
inwards by the paling _l_, _l′_. Above, the chair moves on a pair of
small rollers _c_, which press against a circular strip or track _d_,
_d′_, nailed around the lower edge of the dome opening. Access to the
platform is gained by the steps _e_, _e′_. Attached to the railing of
this platform, and near it on the telescope, are two tables (not shown
in the figure) for eyepieces, the sliding plateholder, &c.


This section is divided into _a_, Description of the Apartment; and
_b_, Photographic Processes.

a. _Description of the Apartment._

The room in which the photographical operations are carried on, adjoins
and connects with the observatory on the southeast, as is shown in
Figs. 28 and 38. It is 9 by 10 feet inside, and is supplied with
shelves and tables running nearly all the way round, which have upon
them the principal chemical reagents. It is furnished, too, with an
opening to admit, from a heliostat outside, a solar beam of any size,
up to three inches in diameter.

The supply of water is derived from rain falling on the roof of the
building, and running into a tank _i_, Fig. 38, which will contain
a ton weight. The roof exposes a surface of 532 square feet, and
consequently a fall of rain equal to one inch in depth, completely
fills the tank. During the course of the year the fall at this place
is about 32 inches, so that there is always an abundance. In order
to keep the water free from contamination, the roof is painted with
a ground mineral compound, which hardens to a stony consistence, and
resists atmospheric influences well. The tank is lined with lead, but
having been in use for many years for other purposes, is thoroughly
coated inside with various salts of lead, sulphates, &c. In addition
the precaution is taken of emptying the tank by a large stopcock when a
rainstorm is approaching, so that any accumulation of organic matter,
which can reduce nitrate of silver, may be avoided. It has not been
found feasible to use the well or spring water of the vicinity.

The tank is placed close under the eaves of the building, so as to gain
as much head of water as is desirable. From near its bottom a pipe
terminating in a stopcock _k_, Fig. 38, passes into the Laboratory.
In the northeast corner of the room, and under the tap is a sink for
refuse water and solutions, and over which the negatives are developed.
It is on an average about twelve feet distant from the telescope. In
another corner of the room is a stove, resembling in construction an
open fireplace, but sufficient nevertheless to raise the temperature to
80° F. or higher, if necessary. As a provision against heat in summer,
the walls and roof are double, and a free space with numerous openings
above is left for circulation of air, drawn from the foundations.
The roof is of tinplate, fastened directly to the rafters, without
sheathing, in order that heat may not accumulate to such an extent
during the day as to constitute a source of disturbance when looking
across it at night.

For containing negatives, which from being unvarnished require
particular care, there is at one side of the room a case with twenty
shallow drawers each to hold eighteen. They accumulate very rapidly,
and were it not for frequent reselections the case would soon be
filled. On some nights as many as seventeen negatives have been taken,
most of which were worthy of preservation. Not less than 1500 were made
in 1862 and ’63.

b. _Photographic Processes._

In photographic manipulations I have had the advantage of my father’s
long continued experience. He worked for many years with bromide and
chloride of silver in his photo-chemical researches (Journal of the
Franklin Institute, 1837), and when Daguerre’s beautiful process was
published, was the first to apply it to the taking of portraits (Phil.
Mag., June, 1840) in 1839; the most important of all the applications
of the art. Subsequently he made photographs of the interference
spectrum, and ascertained the existence of great groups of lines _M_,
_N_, _O_, _P_, above _H_, and totally invisible to the naked eye (Phil.
Mag., May, 1843). The importance of these results, and of the study of
the structure of flames containing various elementary bodies, that he
made at the same time, are only now exciting the interest they deserve.

In 1850, when his work on Physiology was in preparation, and the
numerous illustrations had to be produced, I learnt microscopic
photography, and soon after prepared the materials for the collodion
process, then recently invented by Scott Archer. We produced in 1856
many photographs under a power of 700 diameters, by the means described
in the next section.

At first the usual processes for portrait photography were applied to
taking the Moon. But it was soon found necessary to abandon these and
adopt others. When a collodion negative has to be enlarged--and this
is always the case in lunar photography, where the original picture
is taken at the focus of an object glass or mirror--imperfections
invisible to the naked eye assume an importance which causes the
rejection of many otherwise excellent pictures. Some of these
imperfections are pinholes, coarseness of granulation in the reduced
silver, liability to stains and markings, spots produced by dust.

These were all avoided by washing off the free nitrate of silver
from the sensitive plate, before exposing it to the light, and again
submitting it to the action of water, and dipping it back into the
nitrate of silver bath before developing. The quantity of nitrate
of silver necessary to development when pyrogallic acid is used, is
however better procured by mixing a small quantity of a standard
solution of that salt with the acid.

The operation of taking a lunar negative is as follows. The glass
plates 2-3/4 × 3-1/4 inches are kept in nitric acid and water until
wanted. They are then washed under a tap, being well rubbed with
the fingers, which have of course been properly cleaned. They are
wiped with a towel kept for the purpose. Next a few drops of iodized
collodion are poured on each side, and spread with a piece of cotton
flannel. They are then polished with a large piece of this flannel,
and deposited in a close dry plate box. This system of cleaning with
collodion was suggested by Major Russel, to whose skilful experiments
photography is indebted for the tannin process. It certainly is most
effective, the drying pyroxyline removing every injurious impurity.
There is never any trouble from dirty plates.

The stock of plates for the night’s work, a dozen or so, being thus
prepared, one of them is taken, and by movement through the air is
freed from fibres of cotton. It is then coated with filtered collodion
being held near the damp sink. The coated plate, when sufficiently
dry, is immersed in a 40 grain nitrate of silver bath, acidified with
nitric acid until it reddens litmus paper. The exact amount of acid in
the bath makes in this “Washed Plate Process” but little difference.
When the iodide and bromide of silver are thoroughly formed the plate
is removed, drained for a moment, and then held under the tap till all
greasiness, as it is called, disappears. Both front and back receive
the current in turn.

It is then exposed, being carried on a little wooden stand, Fig. 43,
covered with filtering paper to the telescope, and deposited on the
sliding plateholder which has been set to the direction and rate of
the moon, while the plate was in the bath. The time of exposure is
ascertained by counting the beats of a half-second pendulum.

The method by which exposure without causing tremor is accomplished, is
as follows: A yellow glass slides through the eyepiece-holder, Fig. 33,
just in front of the sensitive plate, and is put in before the plate.
The yellow-colored moon is centred on the collodion film, and the
clepsydra and slide are set in motion, the mass of the telescope being
at rest. A pasteboard screen is put in front of the telescope, and the
yellow glass taken out. After 20 seconds the instrument remaining still
untouched and motionless, the screen is withdrawn, and as many seconds
allowed to elapse as desirable. The screen is then replaced and the
plate taken back to the photographic room.

[Illustration: Fig. 43. Plate Carrier.]

After being again put under the tap to remove any dust or impurity,
it is dipped into the nitrate bath for a few seconds. Two drachms
of a solution of protosulphate of iron 20 grains, acetic acid 1
drachm, and water 1 ounce, is poured on it. As soon as the image is
fairly visible this is washed off, and the development continued if
necessary with a weak solution of pyrogallic acid and citro-nitrate of
silver--pyrogallic and citric acids each 1/5 grain, nitrate of silver
1/10 grain, water 1 drachm. In order to measure these small quantities
standard solutions of the substances are made, so that two drops of
each contain the desired amount. They are kept in bottles, through the
corks of which pipettes descend to just below the level of the liquid.
This avoids all necessity of filtering, and yet no blemishes are
produced by particles of floating matter.

[Illustration: Fig. 44. Pipette Bottle.]

During the earlier part of the development, when the protosulphate
of iron is on the film, an accurate judgment can be formed as to the
proper length of time for the exposure in the telescope. If the image
appears in 10 seconds, it will acquire an appropriate density for
enlargement in 45 seconds, and will have the minimum of what is called
fogging and the smallest granulations. If it takes longer to make its
first appearance the exposure must be lengthened, and vice versa.

[Illustration: Fig. 45. Developing Stand.]

The latter part of the development, when re-development is practised,
is purposely made slow, so that the gradation of tones may be varied
by changing the proportion of the ingredients. As it would be tiresome
and uncleanly to hold the plates in the hand, a simple stand is used
to keep them level. It consists of a piece of thin wood _a_, Fig. 45,
with an ordinary wood screw, as at _b_, going through each corner. Four
wooden pegs, as at _c_, furnish a support for the plate _d_. By the aid
of this contrivance and the washing system, I seldom get my fingers
marked, and what is much more important, rarely stain a picture.

When the degree of intensity most suitable for subsequent enlargement
is reached, that is, when the picture is like an overdone positive, the
plate is again flooded with water, treated with cyanide of potassium
or hyposulphite of soda, once more washed and set upon an angle on
filtering paper to dry. It is next morning labelled, and put away
unvarnished in the case.

To the remark that this process implies a great deal of extra trouble,
it can only be replied that more negatives can be taken on each night
than can be kept, and that, even were it not so, one good picture is
worth more than any number of bad ones.

Although the above is the method at present adopted, and by which
excellent results have been obtained, it may at any moment give place
to some other, and is indeed being continually modified. The defects
it presents are two--first, the time of exposure is too long, and
second, there is a certain amount of lateral diffusion in the thickness
of the film, and in consequence a degree of sharpness inferior to that
of the image produced by the parabolic mirror. The shortest time in
which the moon has been taken in this observatory has been one-third
of a second, on the twenty-first day, but on that occasion the sky
was singularly clear, and the intrinsic splendor of the light great.
The full moon under the same circumstances would have required a much
shorter exposure. A person, however, who has put his eye at the focus
of such a silvered mirror will not be surprised at the shortness of the
time needed for impressing the bromo-iodide film; the brilliancy is so
great that it impairs vision, and for a long time the exposed eye fails
to distinguish any moderately illuminated object. The light from 188
square inches of an almost total reflecting surface is condensed upon 2
square inches of sensitive plate.

Occasionally a condition of the sky, the reverse of that mentioned
above, occurs. The moon assumes a pale yellow color, and will
continue to be of that non-actinic tint for a month or six weeks.
This phenomenon is not confined to special localities, but may extend
over great tracts of country. In August, 1862, when our regiment was
encamped in Virginia, at Harper’s Ferry, the atmosphere was in this
condition there, and was also similarly affected at the observatory,
more than 200 miles distant. As to the cause, it was not forest or
prairie fires, for none of them of sufficient magnitude and duration
occurred, but was probably dust in a state of minute division. No
continued rain fell for several weeks, and the clay of the Virginia
roads was turned into a fine powder for a depth of many inches. The
Upper Potomac river was so low that it could be crossed dry-shod. On
a subsequent occasion when the same state of things occurred again,
I exposed a series of plates (whose sensitiveness was not less than
usual, as was proved by a standard artificial flame) to the image of
the full moon in the 15-1/2 inch reflector for 20 seconds, and yet
obtained only a moderately intense picture. This was 40 times as long
as common.

Upon all photographic pictures of celestial objects the influence of
the atmosphere is seen, being sometimes greater and sometimes less.
To obtain the best impressions, just as steady a night is necessary
as for critical observations. If the image of Jupiter is allowed to
pass across a sensitive plate, a streak almost as wide as the planet
is left. It is easily seen not to be continuous, as it would have
been were there no atmospheric disturbances, but composed of a set
of partially isolated images. Besides this planet, I have also taken
impressions of Venus, Mars, double stars, &c.

An attempt has been made to overcome lateral diffusion in the thickness
of the film by the use of dry collodion plates, more particularly
those of Major Russel and Dr. Hill Norris. These present, it is true,
a fine and very thin film during exposure, but while developing are
so changed by wetting in their mechanical condition that no advantage
has resulted. It was while trying them, that I ascertained the great
control that hot water exercises over the rapidity of development, and
time of exposure, owing partly no doubt to increase of permeability
in the collodion film, but also partly to the fact that chemical
decompositions go on more rapidly at higher temperatures. I have
attempted in vain to develop a tannin plate when it and the solutions
used were at 32° F., and this though it had had a hundred times the
exposure to light that was demanded when the plate was kept at 140° F.
by warm water.

Protochloride of palladium, which I introduced in 1859, is frequently
employed when it is desired to increase the intensity of a negative
without altering its thickness. This substance will augment the opacity
16 times, without any tendency to injure the image or produce markings.
It is only at present kept out of general use by the scarcity of the


Two distinct arrangements are used for enlarging, _a_, for Low Powers
varying from 1 to 25; and _b_, for High Powers from 50 to 700 diameters.

a. _Low Powers._

The essential feature in this contrivance is an entire novelty in
photographic enlargement, and it is so superior to solar cameras, as
they are called, that they are never used in the observatory now. It
consists in employing instead of an achromatic combination of lenses, a
_mirror_ of appropriate curvature to magnify the original negatives or
objects. The advantages are easily enumerated, perfect coincidence of
visual and chemical foci, flat field, absolute sharpness of definition.
If the negative is a fine one, the enlarged proofs will be as good as

[Illustration: Fig. 46. The Photographic Enlarger.]

The mirror is of 9 inches aperture, and 11-1/2 inches focal length. It
was polished on my machine to an elliptical figure of 8 feet distance
between the conjugate foci, and was intended to magnify 7 times. At
first the whole mirror was allowed to officiate, the object being
illuminated by diffused daylight. But it was soon apparent, that
although a minute object placed in one focus was perfectly reproduced
at the other, seven times as large, yet a large one was not equally
well defined in all its parts.

I determined then to produce the enlarged image by passing a solar-beam
1-1/2 inch in diameter through the original lunar negative--placed in
the focus nearest to the mirror--and allowing it to fall on a portion
of the concave mirror, 1-1/2 inch in diameter, at one side of the
vertex. Being reflected, it returns past the negative, and goes to form
the magnified image at the other focus of the ellipse.

In Fig. 46, _a_ is the heliostat on a stone shelf outside; _b_ a
silvered glass mirror, to direct the parallel rays through _c_, the
negative; _d_ is the elliptical mirror; _e_ an aperture to be partly
closed by diaphragms; _f_ a rackwork movement carried by the tripod
_g_; the curtain _h h′_ shuts out stray light from the interior of the
observatory. The aperture _i_ is also diaphragmed, but is shown open to
indicate the position of the heliostat, the shelf of which joins the
outside of the building at _l_. The dotted line points out the course
of the light, which coming from the sun falls on the heliostat mirror
_a_, then on _b_, through _c_ to _d_, and thence returning through _e_
to the sensitive plate in the plate holder _k_.

The distance of this last can be made to vary, being either two feet
or twenty-eight feet from _d_. In the latter case a magnifying power
of about 25 results, the moon being made three feet in diameter. The
sensitive plate is carried by a frame, which screws to the side wall of
the building, and can be easily changed in position. The focussing is
accomplished by the rack _f_. Where so small a part (1-1/2 inch) of the
surface of the mirror is used, a rigid adherence then to the true foci
of this ellipse is not demanded, the mirror seeming to perform equally
well whether magnifying 7 or 25 times. Theoretically it would seem to
be limited to the former power.

If instead of placing a lunar photograph, which in the nature of the
case is never absolutely sharp, at _c_, some natural object, as for
instance a section of bone, is attached to the frame moved by _f_, then
under a power of 25 times it is as well defined as in any microscope,
while at the same time the amount of its surface seen at once is
much larger than in such instruments, and the field is flat. If the
intention were, however, to make microscopic photographs, a mirror of
much shorter focal length would be desirable, one approaching more to
those of Amici’s microscopes.

By the aid of a concave mirror used thus obliquely, or excentrically,
all the difficulties in the way of enlarging disappear, and pictures of
the greatest size can be produced in perfection. I should long ago have
made lunar photographs of more than 3 feet in diameter, except for the
difficulties of manipulating such large surfaces.

In order to secure a constant beam of sunlight a heliostat is placed
outside the observatory, at its southeast corner _f_, Fig. 38. This
beam, which can be sent for an entire day in the direction of the
earth’s axis, is intercepted as shown at _b_, Fig. 46, and thus if
needed an exposure of many hours could be given. The interior of the
observatory and photographic room being only illuminated by faint
yellow rays, no camera box is required to cut off stray light. The
eye is by these means kept in a most sensitive condition, and the
focussing can be effected with the critical accuracy that the optical
arrangement allows, no correction for chromatic aberration being

I have made all the parts of this apparatus so that they can be easily
separated or changed. The flat mirrors are of silvered glass, and are
used with the silvered side toward the light, to avoid the double image
produced when reflection from both sides of a parallel plate of glass
is permitted. The large concave mirror happens to be of speculum metal,
but it can be repolished if necessary by means of a four inch polisher,
passed in succession over every chord of the face. A yellow film of
tarnish easily accumulates on metal specula if they are not carefully
kept, and decreases their photographic power seriously.

_Of the making of Reverses._--In addition to the use of the Enlarger
for magnifying, it is found to have important advantages in copying by
contact. The picture of the image of the moon produced in the telescope
is negative, that is, the lights and shades are reversed. In enlarging
such a negative reversal again takes place, and a positive results.
This positive cannot, however, be used to make prints on paper, because
in that operation reversing of light and shade once more occurs. It
is necessary then at some stage to introduce still another reversal.
This may be accomplished either by printing from the original negative
a positive, which may be enlarged, or else printing from the enlarged
positive a negative to make the paper proofs from. In either case a
collodion film, properly sensitized, is placed behind the positive or
negative, and the two exposed to light.

If diffused light or lamplight is used, the two plates must be as
closely in contact as possible, or the sharpness of the resulting
proof is greatly less than the original. This is because the light
finds its way through in many various directions. If the two plates,
however, are placed in the cone of sunlight coming from the Enlarger,
and at a distance of fifteen or twenty feet from it, the light passes
in straight lines and only in one direction through the front picture
to the sensitive plate behind. I have not been able to see under these
circumstances any perceptible diminution in sharpness, though the
plates had been 1/16 of an inch apart. It is perfectly feasible to use
wet collodion instead of dry plates, no risk of scratching by contact
is incurred, and the whole operation is easily and quickly performed.
The time of exposure, 5 seconds, is of convenient length, but may be
increased by putting a less reflecting surface or an unsilvered glass
mirror in the heliostat. A diaphragm with an aperture of half an inch
if placed at _e_, Fig. 46, to shut out needless light, and avoid
injuring the sharpness of the reverse by diffusion through the room.
In enlarging other diaphragms are also for the same reason put in the
place of this one. For a half moon for instance, a yellow paper with
a half circular aperture, whose size may be found by trial in a few
minutes, is pinned against _e_.

The enlarged pictures obtained by this apparatus are much better than
can be obtained by any other method known at present. The effect,
for instance, of a portrait, made life-size, is very striking. Some
astronomers have supposed that advantages would arise from taking
original lunar negatives of larger size in the telescope, that is,
from enlarging the image two or three times by a suitable eyepiece or
concave achromatic, before it reached the sensitive plate. But apart
from the fact that a reflector would then have all the disadvantages
of an achromatic, the atmospheric difficulties, which in reality
constitute the great obstacle to success, would not be diminished by
such means. The apparent advantage, that of not magnifying defects
in the collodion, is not of much moment, for when development of the
photographs is properly conducted, and thorough cleanliness practised,
imperfections are not produced, and the size of the silver granules is
not objectionable.

b. _High Powers._

Although negatives of astronomical objects have not as yet been made
which could stand the high powers of the arrangement about to be
described, yet they bear the lower powers well, and give promise of
improvement in the future.

Photography of microscopic objects as usually described, consists
in passing a beam of light through the transparent object into the
compound body of the microscope, and receiving it on its exit from the
eyepiece upon a ground glass or sensitive plate. The difficulty which
besets the instrument generally, and interferes with the production of
fine results, arises from the uncertainty of ascertaining the focus
or place for the sensitive plate. For if the collodion film be put
where the image on ground glass seems best defined, the resulting
photograph will not be sharp, because the actinic rays do not form
their image there, but either farther from or nearer to the lenses,
depending on the amount of the chromatic correction given by the
optician. Practically by repeated trials and variation of the place of
the sensitive compound, an approximation to the focus of the rays of
maximum photographic intensity is reached.

[Illustration: Fig. 47. Microscope for Photography.]

During my father’s experiments on light, and more particularly when
engaged in the invention of portrait photography, he found that the
ammonio-sulphate of copper, a deep blue liquid, will separate the more
refrangible rays of light, the rays concerned in photography, from the
rest. If a beam of sunlight be passed through such a solution, inclosed
between parallel plates of glass, and then condensed upon an object on
the stage of a microscope, a blue colored image will be formed on the
ground glass, above the eyepiece. If the place of best definition be
carefully ascertained, and a sensitive plate put in the stead of the
ground glass, a sharp photograph will always result.

Besides, there is no danger of burning up the object, as there would
be if the unabsorbed sunlight were condensed on it, and hence a much
larger beam of light and much higher powers can be used. The best
results are attained when an image of the sun produced by a short
focussed lens is made to fall upon and coincide with the transparent
object. In 1856 we obtained photographs of frog’s blood disks,
navicula angulata, and several other similar objects under a power of
700 diameters, excellently defined. Since then several hundreds of
microscopic pictures have been taken.

In the figure, _a_ is the heliostat, _b_ a lens of three inches
aperture, _c_ the glass cell for the ammonio-sulphate of copper, _d_
the object on the stage of the microscope _e_, _f_ the camera for the
ground glass or sensitive plate. Above the figure the course of the
rays is shown by dotted lines.

       *       *       *       *       *

In concluding this account of a Silvered Glass Telescope I may answer
an inquiry which doubtless will be made by many of my readers, whether
this kind of reflector can ever rival in size and efficiency such
great metallic specula as those of Sir William Herschel, the Earl of
Rosse, and Mr. Lassell? My experience in the matter, strengthened by
the recent successful attempt of M. Foucault to figure such a surface
more than thirty inches in diameter, assures me that not only can the
four and six feet telescopes of those astronomers be equalled, but even
excelled. It is merely an affair of expense and patience. I hope that
the minute details I have given in this paper may lead some one to make
the effort.

  NEW YORK, 1863.

_Postscript._--Since writing the above I have completed a photograph
of the moon 50 inches in diameter. The original negative from which it
has been made, bears this magnifying well, and the picture has a very
imposing effect.

                           WASHINGTON CITY,
                              JULY, 1864.

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