134 



Animal Micrology 



Lenses. Each of the two principal forms of lenses is in effect 

 practically two prisms, (1) with the bases placed together 

 (Fig. 43 , convex lens), cr, (2) with the apices together (Fig. 436, 

 concave lens] . 



In the convex lens, since rays of light are refracted toward the 

 bases of the respective prisms, they will converge ; in the concave 

 lens, for the same reason, they will diverge. The terms conver- 

 ging lens and diverging lens, therefore, are used frequently as 

 synonymous with the terms convex lens and concave lens. All 

 lenses are modifications or combinations of these two types. 



FIG. 43. FIG. 44. 



If parallel rays of light pass through a convex lens (Fig. 44) 

 they are so refracted as to meet in one point F, which is termed, 

 in consequence, the focal point or principal focus. If, on the 

 other hand, the source of light be placed at the focal point, then, 

 after traversing the lens, the rays of light will emerge parallel. 

 If parallel rays of light came from the opposite side of the lens, 

 manifestly there would be a second focal point at F' . The two 

 principal foci are termed conjugate foci, and will be equidistant 

 from the center of the lens when both sides of the lens have equal 

 curvature. 



The ray which passes through the center of the lens (Fig. 44 c) 

 and the focal point, traverses what is termed the principal a.ris 

 of the lens. The optical center of the lens is a point on the 

 principal axis at or near the actual center of the lens, through 

 which rays pass without angular deviation. Any line (e <1), other 

 than the principal axis, which passes through the optical center 

 of the lens is termed a secondary axis. 



