THE SEKSES. 719 



The normal (emmetropic) eye is so adjusted that parallel rays are 

 brought exactly to a focus on the retina without any effort of accommo- 

 dation (1, fig. 428). Hence all objects except near .ones (practically all 

 objects more than twenty feet off) are seen without any effort of accom- 

 modation; in other words, the far-point of the normal eye is at an infinite 

 distance. In viewing near objects we are conscious of the effort (the con- 

 traction of the ciliary muscle) by which the anterior surf ace 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. 428). 



Fig:. 428. Diagram showing 1, normal (emmetropic) 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 meet- 

 ing of the two dotted lines) ; 3, hypermetropic eye, in this case the axis of the eye is shorter, 

 and the lens flatter, than normal ; parallel rays are focussed behind the retina ; 4, myopic eye ; 

 in this case the axis of the eye is abnormally long and the lens too convex ; parallel rays are 

 focussed in front of the retina. 



1. Myopia (short-sight) (4, fig. 428). This defect is due to an abnor- 

 mal elongation of the eyeball. The eye is usually larger than normal 

 and is always longer than normal; the lens is also probably too convex. 

 The retina is too far from the lens and consequently parallel rays are 



