determining the dispersive ratio of glass ^ 
263 
APPROXIMATE FORMULA 
For the Construction of Object-glasses. 
Index plate = i a. Index flint = 1 ^ a', 
dispersive ratio 1 : d, focal length = f". 
f =if" (^1 — d) = focal length plate, 
f'=f" = focal length flint. 
Assumed ratio of flint surfaces 1 : q', 
~ inside radius 
= fa‘ 
= /' a' (1 -i- ?') 
Find b 
p^ + 
iius ) 
itto J 
-J* I 
outside ditto 
— («+ org' 
— a'f" — r"' 
flint, 
c* + 2 •— 6 ad q' 
■ -- X 
{be' + 1)2 ^ c' 
and the corresponding value of q in the following table. 
p 
P 
g 
P 
g 
P 
? 
I-I5 
•374 
I -40 
•683 
I'65 
•972 
1-go 
I'zg 
1'20 
•446 
*•45 
•739 
170 
1*03 
*•95 
1-36 
1-25 
•506 
1-50 
798 
175 
I’Op 
zoo 
*•43 
i' 3 ° 
•568 
155 
•855 
i‘8o 
i’i6 
2‘05 
I'SO 
1-35 
•62^ 
v6o 
•913 
i-8s 
I'22 
2-10 
1-58 
Then r =/^ -f. 1) = radius 1st surface ^ 
r‘=.fa ^ ~ radius 2nd surface 1 
