Chapter II: TO DIFFUSE KNOWLEDGE.--It is proposed to publish occasionally (1)

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.

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.

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.

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:--

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.

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.

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 pressure.

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 mirror.

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.

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 plane.