THE SENSES. 601 



on the retina; thus the images of distant objects are blurred and indis- 

 tinct. The eye is, as it were, permanently adjusted for a near-point. 

 Rays from a point near the eye are exactly focussed in the retina. But 

 those which issue from any object beyond a certain distance (far-point] 

 cannot be distinctly focussed. This defect is corrected by concave glasses 

 which cause the rays entering the eye to diverge; hence they do not 

 oome to a focus so soon. Such glasses of course are only needed to give 



FIG. 409. Diagrams 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 meeting 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. 



a clear vision of distant objects. For near objects, except in extreme 

 cases, they are not required. 



2. Hypermetropia (long-sight) (3, Fig. 409). This is the reverse 

 defect. The eye is too short and the lens too flat. Parallel rays are 

 focussed behind the retina; an effort of accommodation is required to 

 focus even parallel ra}'s on the retina; and when they are divergent, as 



