DIOPTRICS OF THE EYE. 



321 



In hypermetropia the conditions are the opposite of those in 

 myopia. Parallel rays of light after refraction in the eye cut the 

 retina before they come to a focus. The principal focal distance, in 

 other words, is behind the retina. In this case, also, each point 

 of a distant object will make upon the retina, when the eye is not 

 accommodated, a diffusion circle, and the image consequently is 

 blurred. This defect may be caused by a lessened curvature or 

 refractive power in the cornea or lens, but in the majority of cases 

 it is referable to a diminution in the anteroposterior diameter of 

 the eyeball. This condi- 

 tion is usually congenital: 

 the eyeball from birth is 

 smaller than the normal. 

 The path of the parallel 

 rays in this case is repre- 

 sented in the diagram C, 

 Fig. 132. When such an 

 eye looks at a distant ob- 

 ject a clear image may be 

 obtained only by using the 

 ciliary muscle, and to pre- 

 vent this constant strain 

 upon the muscle of accom- 

 modation convex glasses 

 must be worn. Glasses of 

 this kind converge the 

 rays and if properly chosen 

 will bring parallel rays to 

 a focus without the con- 

 stant aid of accommoda- 

 tion. It is obvious that 

 in the hypermetropic eye 

 there is no far point of 

 distinct vision when the 

 eye is at rest, since some 

 accommodation must be 

 used to bring even parallel rays to a focus. The near point of 

 distinct vision will be farther away than in the normal eye, since 

 accommodation begins when the rays are parallel and its limits 

 are reached with a less degree of divergence; hence the name of 

 far-sightedness. 



Presbyopia or old-sigh tedness has been referred to above. It 

 is due to a gradual failure in the effectiveness of accommodation 

 with increasing age, and is attributed usually to a progressive in- 

 crease of rigidity in the lens. The near point of distinct vision 

 21 



Fig. 132. Diagram showing the difference be- 

 tween normal (A), myopic (), and hypermetropic 

 (C) eyes. In B and C the dotted lines represent the 

 path of the rays after correction by glasses. (.B&w- 

 ditch.') 



