of rays ; for the purpose in hand, it is sufficient to 

 consider only those which reach the plane surface of 

 the large lens ; if an equal space is imagined over B', 

 the same number of rays will be contained in this ; 



C A 



Fig. 10. 



therefore, the cone contained in the angle A' B' C' 

 will contain as many rays as that contained in A B C; 

 but as the lens I)' E' is considerably smaller than A 

 C, only as many rays can enter it as are contained in 

 the angle I)' B' E'. As the rays contained in the 

 angle ABC and D' B' E' are carried through the 

 two lenses, which are supposed to be of the same 

 magnifying power, the image -formed by A C will be 

 considerably brighter than that formed by D' E', and 

 will therefore show more of its structure, as will be 

 shown hereafter. 



