764 



THE EYE AND VISION. 



[CH. LV. 



a near object, the accommodation is insufficient to focus them. 

 Thus in well-marked cases distant objects require an effort of 

 accommodation, and near ones a very powerful effort, and the ciliary 

 muscle is, therefore, constantly acting. This defect is obviated by 



Fig. 585. Diagrams showing i, normal (emmetropic) eye bringing parallel rays exactly 

 to a focus on the retina ; 2, normal eye adapted to a near-point ; without accommo- 

 dation 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. 



the use of convex glasses, which render the pencils of light more 

 convergent. Such glasses are of course especially needed for near 

 objects, as in reading, &c. They rest the eye by relieving the 

 ciliary muscle from excessive work. 



