and the Achromatism of Microscopes. 241 
(10) As an example, let us take from the table of M. Frauen- 
hofer, given in the Edinburgh Philosophical Journal, Vol. X, 
the refractive indices in the kinds of glass which he calls Crown 9, 
Flint 13, and Flint 23, for the rays B, E, G. They are as follows. 
B E G 
1,525832 | 1,533005 | 1,541657 
1,627749 | 1,642024 | 1,660285 
1,626580 | 1,640 570] 1,658849 
Let E be the mean ray: then the values of —— , for B and G, are 
P én 
—,013457, +,016232; those of “rq are —,02223, +,02844: those 
” 
of nai are — ,021840, +,028536. Making the assumption above, 
we find c=1,516, e=1,41; c =1,5039, & =2,363. Solving the 
equations above, 
1— 1- 
Page nesih Pb inl gyi 
—1,616+2,645 - 3964 — 2,001 as 
(F, f, f’ beig the focal lengths for the ray £). Since the com- 
pound lens is to niake the rays converge after refraction, we must 
have 
8. 
a 
3348 — 1,356 F 
or 
1 
et FQ-p) 
