314 
Transactions of the Society. 
First lens, a meniscus, diameter 1*7". 
r = + *964". 
s = 4- 1-375”. 
Second lens, double convex crossed, diameter 2 • 1 
r' = 4* 1-816” 
s' = 
Distance between tbe lenses 
P = 1*6" F = 2 
Spherical aberration Bf = 
SF= — -168”. 
Fig. 34. 
- 12-07”. 
05". 
•425”. n P' = 2-725”. 
•335”. 
/= nV + Bf = 3-06”. 
Fig. 35. 
Fig. 35 is a drawing of a triple of flint /a = 1 • 62 ; 2 in. of 
clear aperture ; angle 7 0°. The first lens is a meniscus, diameter, 
1 ' 65 "' r = + • 958" 
s = — f— 1 35 . 
The second lens is a meniscus, diameter 2 * 0”. 
r' = + 1 • 67" 
s' = + 2-55". 
The third lens is a plano-convex, diameter 2-1”. 
r" = + 2-914”. 
Distance between the lenses * 05”. 
P = 1 • 55”. P' = 2*51”. Q = 2*7”. Q' = 4*37”. 
n Q' = 4-53”. Bf = - -17". BF = - *0226”. 
f=n Q' + S/=4-7”. 
A still better doublet than that in fig. 34 could be made by com- 
bining with the plate glass meniscus there shown a plano-convex of 
flint, y. = 1*62; diameter 2 * 1”. 
*•’ = + 1-83". 
nV = 2-675”. Bf = - -272". 
SF = — -132". /= 2-95”. 
The spherical aberration of this form is therefore -109" greater 
than that of the flint triple, and *036” less than that of the plate glass 
double. 
