determining the dispersive ratio of glass, &c. 
25 7 
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 a' = *630, d = *545,/ = 34*95,/' = '641 , 
f ,,, = 77 ; and since r"‘ and q ' in this case are both infinite, 
we must substitute for q', ~-p in the expression for c‘, viz. 
r ' — 0/V — («' + 0/" _ o-t 
— a'f" — r'" r" J 
£_ 
r v 
a! 
o! 1 
*386. 
Since <7 is infinite, our general equation (14) by rejecting 
all the terms into which q' does not enter, reduces to 
a ' 4 2 1 
c (a' c' + a' -f i ) 1 I » , 
v y * a d=zp 
(c' 4 i ) 1 c' 4 2 — ^ * 
+ 
J J 
(5 c' 4 0 c 
= ( — *107 -f- 5*289) x *280 6= 14 6. 
This answers to <7 = *753. 
Whence r =/ (</ + 1 ) = 3 i *57 
r'=fa 11—1 = 41 * 93 - 
The radii we really employed were 32*5 and 40*4, and the 
result w 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 
