determining the dispersive ratio of glass, &c. 
257 
Experiment 4. 
21. Here our flint lens was one of Guinand's, and we 
assumed for its radii r" = — 40^4, and r'" infinite. Its index 
was found to be 1*630, and its dispersion with our plate *545 ; 
the index of the plate to be matched with it 1*515 ; and the 
proposed, or rather the resulting, compound focal length, 
77 inches ; diameter 5^ inches. 
Here a =*515 ^' = *630, i/==-545,/= 34*95,/'= *641, 
/"'= 77 ; and since r'" and in this case are both infinite, 
we must substitute for q', in the expression for c', viz. 
, __ (a’+ _ (a' + l)/" _ 
^ ~~ a'f'-^r'" r" 
h 
a' 
a! + I 
- 1-90 
•386. 
Since q is infinite, our general equation (14) by rejecting 
all the terms into which q' does not enter, reduces to 
a' 4 2 
c {a' c' a' I) ^ 
^ (c' 4 1 )* ^ c' 4 2 — 
1 
I 
r 
y, ad =/> 
(pc' + I) 
= (— *107 + 5*289) X *2806= 1*46. 
This answers to ^ = *753. 
Whence r =/ a [q1) — 31 '61 
r' —f a = 41 *93. 
J q 
The radii we really employed were 32*5 and 40*4, and the 
result \N as satisfactory in every respect with regard to cor¬ 
rection ; but the flint lens was very veiny, which prevented 
its being a good object-glass. 
MDCCCXXVII. 
LI 
