The President's Address. By E. M. Nelson. 
167 
is. situated about 2*3 in. to the right of the posterior surface of 
the equi-concave lens. Careful attention is required when the doublet 
combination is combined with the other bi-convex, for you will observe 
that the distance D from the second principal point Q of the doublet 
to the first principal point p of the second bi-convex is measured from 
left to right, it is therefore negative, and its sign has to be changed 
in the expression </> -j- f" — D. 
Fig. 20. 
Q F 0 
I 
j 
r" = *830 r = - *822 / = *822 f = *4 
s” = - *822 s = *822 s' = - *830 t = -1 
y = 1-5179 y! = 1-6202 t’ = -4 
g" = — = • 2635 ; O - \)g" = -1365; 
f 1 
- g" r" = - * 2187; - g" s" = • 2166 ; 
r" s" = — * 6823 ; s" - r" + (y - 1) g" = - 1 ■ 5155 = h" ; 
0-1 )h" = - -7849 = F; p" = = -1443; 
_ n" <' r" t" 
q" = — = - -1429 ; /' = — = -8693. 
g = -, = -06172 ; O' - 1) <7 = '03828; 
f 1 
-gr= -05073; - gs = - -05073; 
n 2 
