8 ELEMENTS OF APPLIED MICROSCOPY. 



deflected at all, because it will cut surfaces which are 

 parallel to each other. Where these two rays meet the 

 image of the point must be formed. Thus in Fig. 7 

 is shown a biconvex lens the surfaces of which have 

 an equal curvature; its principal foci, therefore, lie at 

 equal distances from its center. The object ah lies out- 

 side the principal focus F. The image of the point a is 



Fig. 7.- 



-Formation of Image by Object outside the Principal 

 Focus. (After Hager-Mez.) 



determined by the straight line aa^ and the broken line 

 aca^. The image of the point h is similarly fixed, and 

 between the two will be formed the enlarged image a'h'. 

 This will be a real image — one, that is, which could be 

 caught upon a screen held in the right plane, — and it will 

 be inverted. These are the characteristics of all images 

 formed by objects lying outside the principal focus. 

 Fig. 8 illustrates the other case, in which the object lies 

 inside the principal focus. Here it is evident that the 

 rays from the point a will not meet at all on the opposite 

 side of . the lens and therefore no real image can be 



