REFRACTION BY LENSES. 79 



the rays C D and C E, from the point C, will be refracted 

 and brought to a focus at the point F. In the same way, the 

 rays from the point K will be brought to a focus at the point 

 L, the two sets of rays crossing at G. The same is true for 

 all of the rays from the object C K, which strike the lens at 

 an angle ; but the ray H I, which is perpendicular to the lens, 

 is not deviated. The line H I is called the axis of the lens. 

 These facts may be applied to the crystalline lens. The 

 rays from an object C K fall upon the lens and are brought 

 to a focus so as to produce the image L F. The retina is 

 supposed to be at such a distance from the lens that the rays 

 are brought to a focus exactly at its surface. Inasmuch as 

 the rays cross each other at the point Gr, the image is always 

 inverted. 



Supposing the crystalline lens to be free from spherical 

 and chromatic aberration, the formation of a perfect image 

 depends upon the following conditions : 



The object must be at a certain distance from the lens. 

 If the object be too near, the rays, as they strike the lens, 

 are too divergent, and are brought to a focus beyond the 

 plane L I F, or behind the retina ; and, as a consequence, 

 the image is confused. In optical instruments, the adjust- 

 ment is made for objects at different distances by moving 

 the lens itself. In the eye, however, the adjustment is ef- 

 fected by increasing or diminishing the curvatures of the 

 lens, so that the rays are always brought to a focus at the 

 visual surface of the retina. The faculty of thus changing 

 the curvatures of the crystalline lens is called accommoda- 

 tion. This power, however, is restricted within certain well- 

 defined limits. 



In some individuals, the antero-posterior diameter of the 

 eye is too long, and the rays, for most objects, come to a focus 

 before they reach the retina. This defect may be remedied 

 by placing the object very near the eye, so as to increase the 

 divergence of the rays as they strike the crystalline. Such 

 persons are said to be near-sighted (myopic), and objects are 

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