2 ON THE CONSTRUCTION AND USE OF 



diameter. I have also finished two mirrors 15| inches in aperture, suitable for a 

 Hcrschelian 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 pro- 

 fessional 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 specidum 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 Avhiteness. 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 Avas never obtained — the principal obstacle to success being a tendency to 

 polish in rings of dift'erent focal length. It must, however, be borne in mind that 

 Lord Rosse had so thoroughly mastered the peculiarities of his machine as to pro- 

 duce with it the largest specula ever made and of very fine figure. 



During these experiments there was occasion to grind out some imperfections, 

 j^^ 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-hydro' 

 chloric 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 Avith the acid in graduated depths. 



An attempt also was made to assist tlie 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 sufiicient intensity 

 was given to the current, speculum metal was transferred Avithout oxidation to the 

 grinder, and deposited in thin layers upon it. It Avas proposed at one time to make 

 use of this fact, and coat a mirror of brass Avith a layer of speculum metal by 

 electrotyping. The gain in lightness Avould be considerable. 



During the Avinter of 1860 the speculum was split into tAvo pieces, by the 

 expansion in freezing of a fcAV drops of Avater that had found their Avay into the 

 supporting case. 



(2.) Silvering Glass. 

 At Sir John Herschel's suggestion (giA'cn on the occasion of a visit that my 

 father paid him in 1860), experiments Avere next commenced Avith silvered glass 



