2 ONTirECONSTRlTCTIONANDUSEOr 



diiiinetor. I have also finislicd two mirrors 15| inches in aperture, suitable for a 

 Hersehelian telescope, that is, which can only converge oblitpie pencils to a focus 

 free from aberration. This work has all been accomplislied in the inter\al8 of })ro- 

 fessional labor. 



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



AND PoLISniNC THE MiKKOKS ; §'2. TuE TELESCOPE MOUNTING; §3. TlIE ClOCK 



Movement ; § 4. The Observatory ; § 5. The Photographic Laboratory ; § 6. 

 The Photographic Enlarger. 



§1. GRINDING AND POLISHING THE MIRRORS. 



(1.) IVM'ERIAIENTS ON A ISIeTAL SpECULUM. 



My first 1-) incli sjxmuIiiiu was an alloy of copper and tin, in tlie proportions 

 given by Lord Ivossc. His general directions were t-losely followed, and the- 

 casting was v(>ry tine, free from ])ores, and of sihcrN wliiteness. It was '2 incli(-s 

 thick, weighed 110 pounds, and was intended to lie of I '2 feet focal length, 'ilie 

 grinding and polishing were conducted with tin; Rosse machine. Althougli a great 

 amount of time was spent in various trials, extending over more: tlian a year, a tine 

 figure was never obtained — the principal obstacle to success being a tendency to 

 polish in rings of different focal length. It must, however, l)e lioiiie in mind tli;it 

 Lord Rosse had so thorouglily nnistered tlu^ peculiariti(^s of his machine as to pro- 

 duce with it the larg(>st specula ever made and of very fine figur(\ 



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

 Y^g of an inch deep, from the face of the metal. Tliis o])eration was greatly assisted 

 by stopping up tlie defects witli a thick alcoholic solution of Canada balsnni, ami 

 having made a rim of wax around the edge of the mirror, pouring on nitro-liydriH 

 chloric acid, which ([uickly corroded away the unco\-ered spac(\s. Snbsecpiently an 

 increase in focal length of 15 incites Avas accom])lislie(l, by attacking the edge 

 zones of the siu'face with the acid in gi'aduatcd (l('i)tlis. 



An attempt also was made to assist tlu" tedious grinding o])eration by including 

 the grinder and mirror in a Voltaic circuit, making the speculum the positive pole. 

 By decomposing acidulated wati'r between it and the grinder, and thereby oxitlizing 

 the tin and copper of the speculum, the o])eratioii was much facilitated, but the 

 battery surface required was too gn at for common use. If a sufficient intensity 

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

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

 use of this fact, and amt a mirror of brass with a layer of speculum metal by 

 el(;ctrotyping. The gain in lightness would Ix; considerable. 



During the winter of 1860 the speculum was split into two pi(M-(^s, by the 

 expansion in freezing of a few drops of wati'r that had found their way into the 

 supporting case. 



(2 ) Silvering Glass. 



At Sir John Ilerscliers suggestion (gi\en on the occasion of a visit that my 

 father paid him in 18()()), experiments were next commenced with silvered glass 



