282 



IMAGES FORMED BY LENSES 



[Ch. IX 



thin in the middle and thick on the edge they make the light rays 

 entering them more divergent. In a word, then, thin edge lenses 

 are called convergent, and thick edge ones, divergent lenses. This 

 follows inevitably from the rule that on entering a denser medium 



Fig. 165-166. To Show the Formation of a Reduced Virtual Image by 

 a Concave Lens, and that the Image is Larger the Nearer the Object 

 Is to the Principal (Virtual) Focus. (Compare Fig. 86-87). 



any oblique ray of light is bent toward the normal, and on leaving 

 it for a rarer medium, it is bent from the normal (§ 446.) 



§ 457. Principal features of spherical lenses — (1) Principal 

 axis. This is the straight line passing through the lens and joining 

 the centers of the two spheres contributing to the formation of the 

 lens (fig. 167, \c c'). 



(2) Optic center. The point in a lens or near it through which 

 light rays pass without angular deviation. That is, the ray 



