STATIC REFRACTION IN THE EYE 



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3. STATIC REFRACTION IN THE EYE 



The laws of refraction in an optical system teach us that for every different 

 position of the object the position of the image changes. For this reason in 

 order to take a picture on a sensitive plate by means of a camera,, the position 

 of the plate must be adapted to the distance of the object. 



But if the plate is immovable,, as is true of the retina,, a nearer object can 

 be focused by using a stronger lens i. e., by increasing the refracting power 

 of its system. This is what happens in the eye. By accommodation (see 6), 

 the refractive power of the crystalline lens can be increased to different de- 



FIG. 215. The static refraction of : A, a hypermetropic eye; B, an emmetropic eye; and C, a 



myopic eye. 



grees, so that objects at widely different distances can be focused sharply 

 on the retina. 



An optical system is characterized by the distance of its posterior focal 

 point; and we can distinguish three kinds of eyes according as the posterior 

 focal point is on the retina, in front of or behind the retina (Bonders). 

 Unaccommodated eyes with the posterior focal point on the retina are to be 

 regarded as normal and are called emmetropic (Fig. 215, B). 



Eyes of the second kind where the focal point of parallel rays falls in 

 front of the retina are called myopic or nearsighted, because they are only 

 able to focus on the retina such light rays as come from objects at a finite 

 distance (Fig. 215, C). 



