§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.