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.
SMITHSONIAN
CONTRIBUTIONS TO KNOWLEDGE.
VOL. XIV.
[Illustration]
EVERY MAN IS A VALUABLE MEMBER OF SOCIETY, WHO, BY HIS OBSERVATIONS,
RESEARCHES, AND EXPERIMENTS, PROCURES KNOWLEDGE FOR MEN.--SMITHSON.
CITY OF WASHINGTON:
PUBLISHED BY THE SMITHSONIAN INSTITUTION.
MDCCCLXV.
ADVERTISEMENT.
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
“SMITHSONIAN INSTITUTION FOR THE INCREASE AND DIFFUSION OF KNOWLEDGE
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.
DETAILS OF THE FIRST PART OF THE PLAN.
I. TO INCREASE KNOWLEDGE.--_It is proposed to stimulate research,
by offering rewards for original memoirs on all subjects of
investigation._
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
Congress.
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
reports:--_
I. PHYSICAL CLASS.
1. Physics, including astronomy, natural philosophy, chemistry, and
meteorology.
2. Natural history, including botany, zoology, geology, &c.
3. Agriculture.
4. Application of science to arts.
II. MORAL AND POLITICAL CLASS.
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.
III. LITERATURE AND THE FINE ARTS.
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.
DETAILS OF THE SECOND PART OF THE PLAN OF ORGANIZATION.
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
societies.
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.
OFFICERS
OF THE
SMITHSONIAN INSTITUTION.
THE PRESIDENT OF THE UNITED STATES,
_Ex-officio_ PRESIDING OFFICER OF THE INSTITUTION.
THE VICE-PRESIDENT OF THE UNITED STATES,
_Ex officio_ SECOND PRESIDING OFFICER.
SALMON P. CHASE,
CHANCELLOR OF THE INSTITUTION.
JOSEPH HENRY,
SECRETARY OF THE INSTITUTION.
SPENCER F. BAIRD,
ASSISTANT SECRETARY.
W. W. SEATON, TREASURER.
ALEXANDER D. BACHE, }
RICHARD WALLACH, } EXECUTIVE COMMITTEE.
RICHARD DELAFIELD, }
REGENTS.
---- ---- _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_.
MEMBERS EX-OFFICIO OF THE INSTITUTION.
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_.
HONORARY MEMBER.
JAMES HARLAN. _The Secretary of the Interior_.
TABLE OF CONTENTS.[1]
[1] Each memoir is separately paged and indexed.
PAGE
ARTICLE I. INTRODUCTION. Pp. 16.
Advertisement iii
List of Officers of the Smithsonian Institution ix
ARTICLE II. DISCUSSION OF THE MAGNETIC AND METEOROLOGICAL
OBSERVATIONS MADE AT THE GIRARD COLLEGE OBSERVATORY,
PHILADELPHIA, IN 1840, 1841, 1842, 1843, 1844, AND 1845.
Third Section, comprising Parts VII, VIII, AND IX.
VERTICAL FORCE. INVESTIGATION OF THE ELEVEN (OR TEN) YEAR
PERIOD AND OF THE DISTURBANCES OF THE VERTICAL COMPONENT
OF THE MAGNETIC FORCE, AND APPENDIX ON THE MAGNETIC EFFECT
OF THE AURORA BOREALIS; WITH AN INVESTIGATION OF THE SOLAR
DIURNAL VARIATION, AND OF THE ANNUAL INEQUALITY OF THE
VERTICAL FORCE; AND OF THE LUNAR EFFECT OR THE VERTICAL
FORCE, THE INCLINATION, AND TOTAL FORCE. By A. D. BACHE,
LL. D., F. R. S., Mem. Corr. Acad. Sc. Paris; Prest. Nat.
Acad. Sciences; Superintendent U. S. Coast Survey. Pp. 72.
(Published May, 1864.)
ARTICLE III. DISCUSSION OF THE MAGNETIC AND METEOROLOGICAL
OBSERVATIONS MADE AT THE GIRARD COLLEGE OBSERVATORY,
PHILADELPHIA, IN 1840, 1841, 1842, 1843, 1844, AND 1845.
Fourth Section, comprising Parts X, XI, AND XII. DIP AND
TOTAL FORCE. ANALYSIS OF THE DISTURBANCES OF THE DIP AND
TOTAL FORCE; DISCUSSION OF THE SOLAR DIURNAL VARIATION
AND ANNUAL INEQUALITY OF THE DIP AND TOTAL FORCE; AND
DISCUSSION OF THE ABSOLUTE DIP, WITH THE FINAL VALUES
FOR DECLINATION, DIP AND FORCE BETWEEN 1841 AND 1845. By
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.)
ARTICLE IV. ON THE CONSTRUCTION OF A SILVERED GLASS TELESCOPE,
FIFTEEN AND A HALF INCHES IN APERTURE, AND ITS USE IN
CELESTIAL PHOTOGRAPHY. By HENRY DRAPER, M. D., Professor
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
ARTICLE V. PALÆONTOLOGY OF THE UPPER MISSOURI: A REPORT UPON
COLLECTIONS MADE PRINCIPALLY BY THE EXPEDITIONS UNDER
COMMAND OF LIEUT. G. K. WARREN, U. S. Top. Engrs., IN 1855
AND 1856. INVERTEBRATES. By F. B. MEEK AND F. V. HAYDEN,
M. D. Part I. Pp. 158, and five Plates. (Published April,
1865.)
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.
ARTICLE VI. CRETACEOUS REPTILES OF THE UNITED STATES.
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
SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE.
----180----
ON THE CONSTRUCTION
OF A
SILVERED GLASS TELESCOPE,
FIFTEEN AND A HALF INCHES IN APERTURE,
AND
ITS USE IN CELESTIAL PHOTOGRAPHY.
BY
HENRY DRAPER, M. D.,
PROFESSOR OF NATURAL SCIENCE IN THE UNIVERSITY OF NEW YORK.
[ACCEPTED FOR PUBLICATION, JANUARY, 1864.]
COMMISSION
TO WHICH THIS PAPER HAS BEEN REFERRED.
Prof. WOLCOTT GIBBS.
Com. J. M. GILLISS, U. S. N.
JOSEPH HENRY,
_Secretary S. I._
COLLINS, PRINTER,
PHILADELPHIA.
CONTENTS.
HISTORICAL SKETCH OF THE TELESCOPE. MEMOIR DIVIDED INTO SIX SECTIONS:--
§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
OF
THE CONSTRUCTION AND USE OF A SILVERED GLASS TELESCOPE.
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.
GRINDING AND POLISHING THE MIRRORS; §2. THE TELESCOPE MOUNTING; §3. THE
CLOCK MOVEMENT; §4. THE OBSERVATORY; §5. THE PHOTOGRAPHIC LABORATORY;
§6. THE PHOTOGRAPHIC ENLARGER.
§1. GRINDING AND POLISHING THE MIRRORS.
(1.) EXPERIMENTS ON A METAL SPECULUM.
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
time.
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
alcohol.
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.
(3.) GRINDING AND POLISHING GLASS.
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.