308 LL. M. Rutherfurd on Astronomical Photography. 
Having obtained the achromatic correction, I had a most dele 
cate task to produce the correction for figure, since the judg. 
ment of the eye ‘was useless unless entirely protected from the 
influence of all but the actinic rays. A cell of glass inclosing a 
sufficient thickness of the cupro-sulphate of ammonia, held be- 
tween the eye and the eye-piece, enabled me to work for coarse 
corrections upon « Lyre and Sirius, but so darkened the ex- 
panded disk of a star in and out of focus that all the final cor- 
rections were made upon tests by photography, which gave per- 
manent record of all the irregularities of surface to be combated. 
Still, however the process was long and tedious, dependent u 
but three stars as tests, and they too often obscured by 
weather. My mode of correction was almost entirely of a local 
nature, such as practiced by the late Mr. Fitz and Mr. Clark for 
man ' 
oa ae 
