of the Eye-pieces of Telescopes. 29 



OM- OM^ 



Now OK = JiKM'-OM^)=KM-^^^=s + RM- ^^^^^ 



••• CO = RM- ^-^^-p^. And i2C= ^^ -a; 



(- - -V ^ c- - ~y 



,. CO = RM-Z' ^^ ^ .- =RM-B\ ^^^ ^ .^: 



S+Z 2 Z + S 2 



and, putting for RM its value, 



CO=Z-^,(a-A) 

 \B~2) ,\C~z) 1 \s'^?)\B~ a)\Z~ c) 





(l^hXl^i) 



(n—l ^^CL n—l\ n— 1 /■! J^r l n+i 



The expression within the brackets, it will be observed, is strictly sym- 

 metrical with respect to the quantities on each side of the lens. 



If, in the expression for CO or z we substitute for 



1-1 1 i. ^ ^ 

 A' B' C Z' r' ~s' 



the values above given, we find 



