6 ELEMENTS OF APPLIED MICROSCOPY. 
A biconvex lens works roughly, as if it were made 
up of an infinite number of prisms arranged about a 
central axis. Rays parallel to the principal axis of such 
a lens (the line joining the centers of curvature of its 
two surfaces,—will be bent from all directions toward 
that axis, and the rays nearest the outer edge or 
periphery will be most bent, so that all will meet at one 
point, known as the Principal Focus or burning-point of 
the lens (0, Fig. 5). The greater the curvature of the 
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Fic. 5.—PLANO-CONVEX LENS AND ITS PRINCIPAL Focus. 
(After Hager-Mez.) 
lens the nearer to its optical center this principal focus 
will lie. 
5. Formation of Images by the Convex Lens.—As 
the principal focus of a biconvex lens is defined as the 
point at which rays parallel to the principal axis meet, 
it is obvious that rays proceeding from a radiant point 
placed at the principal focus will be sent off on the other 
side of the lens as parallel rays, and will meet to form an 
image only at an infinite distance (B, Fig. 6). Rays. 
from a point between the lens and its principal focus, 
being still more divergent, will not even be made parallel,’ 
but will still continue to diverge after passing through 
