THE EYE AND THE PHYSIOLOGY OF VISION. 



539 



glasses, by means of which the parallel rays of the sun are 

 concentrated at one point sufficiently to start ignition. 



If the lens be concave, either on both sides (double con- 

 cave) or on one side (single concave) the rays of light will 

 be bent apart; that is, scattered. This action of lenses will 

 be more evident by comparison of Figures 165 and 166, 

 where the course of rays of light through such lenses is 

 shown by a series of lines. Some lenses may be concave on 

 one side and convex on the other, called concavo-convex 

 lenses, and such lenses will either converge the light or 

 scatter the light, according as the convexity is greater or less 

 than the concavity. 



Fig. 165. To SHOW THE CONVERGING ACTION OF A CONVEX LENS. 



Fig. 166. TO SHOW THE DISPERSIVE ACTION OF A CONCAVE LENS. 



F, the focus at which the dispersed rays would meet if continued. 



For the purpose of understanding the eye it is especially 

 desirable to bear in mind the fact that the convexity of a 

 lens will determine the extent of the refraction of the light 

 through it and also the position of a focus back of it. The 

 more convex the lens is the closer will be the focus behind 

 the lens, the less convex it is the further removed from the 

 lens will be this focus. Upon these simple considerations 



