CH. LVL] 



ERRORS OF REFRACTION 



785 



rays are brought exactly to a focus on the retina (1, fig. 587). 

 Hence all objects except near ones (practically all objects more than 

 twenty feet off) are seen without any effort of accommodation ; in 

 other words, the far-point of the normal eye is at an infinite distance. 



Flu. 587. Diagram showing 1, normal (emmet ropic) eye bringing parallel rays exactly to a focus on 

 the retina ; 2, normal eye adapted to a near-point ; without accommodation the rays would be 

 focussed behind the retina, but by increasing the curvature of the anterior surface of the lens 

 (shown by a dotted line) the rays are focussed on the retina (as indicated by the meeting of the two 

 dotted lines) ; 3, hypermetropic eye ; in this case the axis of the eye is shorter than normal ; parallel 

 rays are focussed behind the retina ; 4, myopic eye ; in this case the axis of the eye is abnormally 

 long ; parallel rays are focussed in front of the retina. The figure incorrectly represents the 

 refraction as occurring only in the crystalline lens; the principal refraction really occurs at the 

 anterior surface of the cornea. 



In viewing near objects we are conscious of the effort (the contraction 

 of the ciliary muscle) by which the anterior surface of the lens is 

 rendered more convex, and rays which would otherwise be focussed 

 behind the retina are converged upon the retina (see dotted lines, 

 2, fig. 587). 



1. Myopia (short-sight), (4, fig. 587). This defect is clue to an 



3 D 



