430 



Transactions of the Society. 



respectively, meet the axis in the common point C, we shall have found 

 at C the optical centre ; for € ... C and C ... rj will be proportional to 

 e . . . A and A . . . rj respectively. 



Fig. 102. 



If, now, we apply to this system the rule already established in 

 connection with fig. 101, we shall find that the equation (10) implies 

 the sine law in its ordinary form as applied to the divergence angles 

 lie and iir\. 



Fig. 103. 



For consider fig. 103. Here we have figs. 101 and 102 combined, and 

 we know therefore from (10) that 



1 _ _1 



r e " r v 



(10) 



Also, we see from the diagram that 



sin 6 r e = sin i< ( (e . . . a t ) % 



