- 
C. S. Hastings—Color Correction of Double Objectives. 175 
Professor Vogel, in the Monatsbericht d. k. Preuss. Aka- 
aa d. Wissenschaften for April, 1880, has described three 
arge telescopes in respect to their color correction, one by 
ee of 298™™ aperture, another by Grubb of 207" aper- 
ture, and a third by Fraunhofer of s4gem aperture, as deter- 
mined by observing a stellar image spectroscopically. A fourth 
objective by Steinheil of 135™", Tess c ompletely investigated, is 
of little importance to us here on account of its smaller size. 
e method was to determine the various lengths of the teles- 
cope when a stellar image, observed through the e e-piece 
armed with a prism, was caused to eat the greatest con- 
tractions at recorded ay eaten in its s 
Professor Young, in this Journal for Te. 1880, gives the 
results of his measurements on two telescopes by the Clarks, 
each of nearly nine and a half inches aperture. ‘I'he method 
employed was analogous to that of Vogel except that a very 
powerful spectroscope was used and the limb of the sun taken 
instead of a star. By this means the focal planes for various 
wave lengths of light were determined by the position of the 
slit plate of the spectroscope. 
In the summer of 1879 I determined, by the method last 
described, the color characteristics of Mr. Ed gecomb’s telescope 
at Hartford, Conn., a 9-4 inch objective by the Clarks, and, 
with Professor Van V logis assistance, those of the 12-ineh 
Clark at the Wesleyan University. 
To compare these various successful practical solutions with 
that given by the theory embodied in this paper, I foun 
interpolation, using the first two terms of Cauchy’s formula, 
the wave lengths corresponding to the various values of n,, that 
is, those wave lengths which should have, according to theo 
their focal point coincident with light defined by the ralelcter 
ne C, 
They a 
W. GL 
Crown glass Se 50074 
f 64 in. 499°5 
" 2% of 9-4 in. 498°6 
Flint glass 1237 500°7 
“of 6} in. 499°7 
ie of 9°4 in. 498°9 
Mean, 499°6 
€ corresponding values as derived by measurement from 
various telescopes are as follows: 
