OPTICS 



523 



The image of the round illuminated spot on the ground glass seen on 

 the card has a violet center and a red margin. Double the distance of 

 the card-screen from the lens, and the colors change places. The reason 



FIG. 290 



Refraction. This diagram shows that a double convex lens is optically two prisms base to 

 base and that a double concave lens is two prisms apex to apex. The + lens refracts toward 

 the central ray (g h), the lens away from it. 



FIG. 291 



L' 



Spherical aberration arises in the natural fact that the rays nearer the lens' center are refracted 

 less than those passing through its peripheral parts. The result is an indeterminate image or 

 focus, F f F. Diaphragms (as at D D'~) are therefore employed (e. g., the iris in the eye) to cut 

 off the peripheral rays, leaving a relatively clear focus, as at F. 



