VISION. 



The point from which they diverge and the point to which they 

 converge are called conjugate foci. As one approaches the lens, 

 the other recedes, and vice versa. 



By means of a candle, determine the principal focus of the con- 

 vex lens before you. 



The foci of concave lenses for parallel or divergent rays are 



FIG. 41.- Described in text. 



virtual or negative. They are the points from which rays seem to 

 diverge after passing through the lens (see Fig. 41, F). 



Parallel rays of light, falling upon a concave lens, are diverged. 

 If these rays were traced backward, they would seem to diverge 

 from a point nearer the lens (see Fig. 41, F). 



The conjugate foci of concave lenses are also virtual and found 

 in a similar manner. Find them. 



Formation of images. The image of an object is the collection 

 of the foci of its principal points. 



FIG. 42. Described in text. 



SIMPLE DIOPTRIC SYSTEM. The simplest form of a dioptric 

 apparatus consists of two media of different refractive indices, 



[181] 



