ORGANS OF SPECIAL SENSE THE EYE 



347 



lens, they enter it at different angles, and are consequently 

 bent in different directions and that when they emerge they 

 are bent again, each differently from the others. Now when 

 the surfaces of the spheres are perfect curves, the angles at 

 which the rays enter and leave it are such that after passing 

 through they are bent toward one point where all of the rays 

 meet; Fig. 169. If a 

 sheet of paper is held at 

 this focus, a point of 

 light will show upon it. 

 Figure 170 shows a 

 slightly different con- 

 dition. Here A is a 

 point of light near the 

 lens, and from it light 



rays pass outward in all directions. The rays are not parallel 

 as in the former instance but diverging; yet they are bent as 

 before, as they are passing through the lens and are also 

 brought toward one point a, at which they focus. But it will 

 be noticed that this focus is farther from the lens than the 



FIG. 169. DIAGRAM 



Showing the manner in which a lens bends 

 rays of light so as to bring them to a focus. 



FIG. 170. DIAGRAM 



Showing the relative position of the foci formed by parallel rays of light and by 

 diverging rays of light coming from objects near the lens. 



focus of parallel rays, /. If the point of light is brought nearer 

 the lens, at B, its focus, 6, will be still farther away. 



There is a way in which the image can be brought nearer 

 to the lens and the focal distance not be lengthened. If the 

 lens be replaced by one of greater curvature, i. e. more bulging, 

 it will bring the rays to a focus sooner. If on the other 



