DRAPER’'S TELESCOPE. 63 
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 wiater 
the art of communicating the parabolic figure by Foucault's 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 pho- 
tography and the enlargement of negatives, some of which were maguified to 
three feet in diameter. T'wo specula of 154 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 
ail been accomplished in the intervals of professional labor.” 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-hydrochloric 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 the grinding process, and an idea 
Was entertained of saving much weight by electrotyping a brass mirror with 
speculum metal. When he commenced operations with glass he had to polish 
with his own hands more than one hundred mirrors of various sizes, from 19 
inches to 4 inch, and to experience very frequent failures for three years before 
he was able to produce large surfaces certainly and speedily. His labor would 
have been much diminished, inasmuch as he would have been spared the cause- 
less condemnation of many fine mirrors, as well as the working of some square 
ones, had he become earlier aware of an important fact respecting the rigidity 
of the material.* Generally speaking, a sheet of glass,even when very thick, 
can hardly be set on edge without so much flexure as to render it optically 
worthless ; but, fortunately, in every disk that he tried, there was one diameter 
on either end of which it might stand without harm. On turning a disk of 154 
inches, with a thickness of 14 inch, one quarter round, it could hardly be 
realized that the surface was the same: 90° more restored it to its original 
defining power ; and this effect was found to be independent of any irregularity 
on the edge of the disk and of the mode of support. Dr. Draper refers it, with 
great probability, to the structure of the glass, resulting from its having been 
subjected to rolling pressure. A similar irregularity of structure is knewn to 
obtain in many large object-glasses, and Dr. Draper specifies the great achromatic 
by Cauchoix, presented by the late Duke of Northumberland to the University 
of Cambridge, as having had its lenses turned round by Mr. Airy in monnting, 
for this reason. Short’s Gregorian specula, too, were always marked on the 
same account. The strange deformations of image produced by heat, even by 
* In examining and testing last year some fine 2-inch specula of Mr. With’s workmanship, 
Thad independently ascertained this peculiarity, so far as a best position for each was con- 
cerned, but I stopped short of Dr. Draper’s discovery of a regular axis of rigidity. 
