REFRACTION IN THE EYE 



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retina is normally at such a distance from the lens that the lays are 

 brought to a focus exactly at its surface. Inasmuch as the rays cross 

 before they reach the retina, the image is inverted. 



Supposing the crystalline lens to be free from spherical and chro- 

 matic aberration, the formation of a perfect image depends on the fol- 

 lowing conditions : 



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

 object is too near, the rays as they strike the lens are too divergent and 

 are brought to a focus beyond the plane F H D, or behind the retina ; 

 and as a consequence the image is confused. In optical instruments 

 the adjustment is made for objects at different distances by moving the 

 lens itself. In the eye, however, the adjustment is effected by increas- 

 ing or diminishing the curvatures of the lens, so that the rays are 



A 



II 



Fig. 174. Refraction by convex lenses. 



brought to a focus on the visual surface of the retina. The faculty of 

 thus changing the curvatures of the crystalline lens is called accommo- 

 dation. 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 lens. Such persons are said to be near-sighted (myopic), and objects 

 are seen distinctly only when very near the eye. This defect may be 

 remedied for distant objects, by placing concave lenses before the eyes, 

 by which the rays falling upon the lens are diverged. The opposite 

 condition, in which the antero-posterior diameter is too short (hyperme- 

 tropia), is such that the rays are brought to a focus behind the retina. 

 This is corrected by converging the rays of incidence by placing convex 



