determining the dispersive ratio of glass , &c. 
263 
APPROXIMATE FORMULA 
For the Construction of Object-glasses. 
Index plate = 1 -J- a. Index flint = 1 -f 
dispersive ratio 1 : d, focal length = f". 
f =/" (1 — d) = focal length plate, 
f =f" ^ 1 = focal length flint. 
lius i 
> flint, 
itto j 
Assumed ratio of flint surfaces 1 : q' 
r" = f* a! • 1 + , 7 ■ = inside radius 
J q 
r'" = f a! (1 -j- q[) = outside ditto 
Find b — — - d = (g ' + 
rmu u — a + t c — a ,jr„ _ jj,, 
a — (c' + i ) 2 c' + 2 — b ad q' 
P ( 6 c' + i ) 2 X c' X q 1 + 1 * 
and the corresponding value of q in the following table. 
p 
1 
P 
1 
P 
1 
P 
f 
1 * 1 5 
•374 
1*40 
•683 
1 -65 
'97 2 
1-90 
1*29 
1 *20 
•446 
r 45 
739 
170 
1-03 
1- 95 
1-36 
125 
VO 
0 
1-50 
798 
175 
1 ’09 
200 
i *43 
i' 3 ° 
•568 
*55 
00 
i’8o 
1*16 
2-05 
1 *50 
1-35 
•625 
i*6o 
* 9*3 
i-8 S 
1*22 
2-10 
1-58 
Then r —f a (q -f- 1) = radius 1st surface 
r'=zf a A?— +_Ll = radius 2nd surface 
j plate. 
