Dr. Draper's Telescope. 369 



obtained ; and I can see no reason why silvered glass instru- 

 ments 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." His 

 first idea was derived from an examination, in 1857, of Lord 

 B-osse's great reflector, and of the machinery by which it was 

 perfected ; and on his return home in the following year, he re- 

 solved to construct a similar, though smaller instrument, larger, 

 however, than any in America, and adapted to celestial photo- 

 graphy. A metal speculum was first completed, but was split 

 in two during the winter of 1860 by the expansion of a few drops 

 of water that became frozen in the supporting case, and his 

 attention was then, at Sir John HerscheFs suggestion, turned 

 to silvered glass mirrors, as reflecting more than 90 per cent, of 

 incident light, with only -i-th of the weight of metal. The year 

 1861 was occupied in overcoming the difficulties of grinding 

 and polishing three 15^-inch discs of glass, as well as a variety 

 of smaller pieces. Three similar mirrors were found almost 

 essential, as two would often be so much alike that a third 

 was necessary to gain a further step in advance. One was 

 made to acquire a parabolic figure (see Intellectual Ob- 

 seevee, hi. 213), and bore a power of 1000. The winter was 

 spent in perfecting the art of silvering, and studying photo- 

 graphic processes. A large portion of 1862 was spent with 

 a regiment in a campaign in Virginia, but in the autumn sand- 

 clocks and clepsydras of various kinds were made, and the 

 driving mechanism attained great excellence. During the 

 winter the art of communicating the parabolic figure by Fou- 

 cauhVs method was acquired, and two 15|-inch mirrors, and 

 two of 9 inches, for enlarging photographs, were completed. 

 The greater part of 1863 was spent in lunar and planetary 

 photography, and the enlargement of negatives, some of which 

 were magnified to three feet in diameter. Two specula of 15-£ 

 inches were also completed, ground to an oblique focus for 

 front view. " This work/-' he adds — and any one with very 

 little experience may judge of the immense amount of toil 

 involved — " has all been accomplished in the intervals of pro- 

 fessional labour." Many of the expedients adopted in the 

 working, which are detailed at full length, are strikingly 

 characteristic of ingenuity as well as perseverance. To avoid 

 the tediousness of grinding out defects in a metal surface, they 

 were " stopped out," after the manner of engravers, and the 

 uncovered space corroded away by the action of nitro-hydro- 

 chloric acid ; by a similar mode, the strength of the acid being 

 graduated in separate zones towards the edge, an increase of 

 15 inches in focal length was gained. The grinder and mirror 

 were at another time included in a voltaic circuit to abridge 



