456 ©. A. Young—Color Correction of Achromatic Object Glasses. 
published in the Astro- 
( His num- 
ment, the computation being based upon Fraunhofer’s deter- 
mination of the indices of refraction for specimens of glass like 
that used in the lens. 
Ray of Spectrum. Gauss. Littrow. |Fraunhofer. 
aF dF 
Designa- aF wane = 
tion. i. dF F Ly 
A 7600 0-199 in.) -00145 00172 e 
B 6866 097 69 97 00064? 
0 6561 052 317 49 52 
D 5890 002 I 0 re 
1235 K 5590 000 0 aM ots 
1474 K 5316 004 3 35 te 
E 5269 eee an ee 15 
b 5183 ‘O11 8 oe 25 
c 4956 035 25 Lge se 
F 4860 059 42 129? 72 
2500 K 4530 197 143 ae wie 
f 4471 OS sean f 214 
2796 K 4340 ace es 294 271 
CG 4307 “370 262 we 289 
h 4101 626 437 444? 452 
H 3968 *810in.! -00575 00650 ? 00582 
A glance at the table shows that as to the extreme rays there 
is very little difference between the three objectives, but that 
for the middle rays the Gauss lens is decidedly the best, and 
the Littrow the worst. The same thing is still better shown by 
the diagram of the color curves. The ordinates of these curves 
are taken from the columns(+;) of the table: for abscissas I 
1 
have used the values of Z i. e., the number of wave-lengths to 
a millimeter. If we take the wave lengths themselves as ab- 
scissas, the left-hand portion of the curve is so flattened and 
extended, and the right-hand portion becomes so steep as 
make the figure very inconvenien . 
A ? is appended in the table to a few of the numbers which 
do not seem to fall in well with the general course of the curve 
to which they ought to conform. 
Princeton, N. J., April 14, 1880. 
