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 ab lies out- 
side the principal focus F. The image of the point a is 
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Fic. 7.—FoORMATION 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 0 is similarly fixed, and 
between the two will be formed the enlarged image ab’. 
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 @ will not meet at all on the opposite 
side of the lens and therefore no real image can be 
