DIOPTRIC MECHANISMS OF THE EYEBALL 



595 



normal person, has its far point at infinite distance and its near point 

 at a distance from the eye which depends on the elasticity of the 

 lens and the range through which this can alter its curvature. 



The normality of an eye is determined by the fact that parallel 

 rays come to a focus on the retina when the apparatus of accommoda- 

 tion is at rest. Two classes of deviation from this normal, or emme- 

 tropic, eye are common. In the first class parallel rays come to a 

 focus in front of the retina. In such eyes, which are designated 

 myopic, it is impossible to get a clear image on the retina of distant 

 objects. In order that the rays may 

 be focused on the retina they must be 

 divergent, so that even when accom- 

 modation is paralysed the far point 

 of distant vision lies at some finite 

 distance from the eye, varying with 

 the extent of the disorder. If in such 

 eyes the range of accommodation is 

 normal, the near point will be much 

 nearer to the eye than in the em- 

 metropic eye (Fig. 262). 



The second class of abnormal 

 eyes are known as hypermetropic. 

 These eyes can be regarded as too 

 short for their refractive media. 

 Parallel rays are brought to a 

 focus at a point behind the retina. 

 Persons so affected can see objects 

 at a distance, but always with 

 some effort of accommodation. If 

 accommodation be paralysed by 

 means of atropine everything will appear blurred. Since accommoda- 

 tion is required even for infinite distance, the greatest possible effort 

 will be insufficient to bring near objects into accurate focus, and the 

 near point of such eyes will be greater than normal. Persons with 

 hypermetropia, or long-sightedness, can therefore see objects at a 

 distance perfectly well, but are unable to read small print, since 

 the point of near vision is too far from the eye to allow the small 

 letters to subtend a sufficiently large angle. 



Both these disorders can be corrected by suitable spectacles. 

 In the case of the myopic eye we need lenses which will convert the 

 parallel rays into divergent rays ; such cases are treated therefore with 

 concave lenses. Conversely, hypermetropia is treated with convex 

 lenses, which will aid the too feeble refractive power of the eye, and so 

 bring parallel rays to a focus on the retina without any effort of accom- 



FIG. 262. Diagrams of course taken 

 by parallel rays in entering normal 

 (emmetropic) eye (A), hyper- 

 metropic eye (B), and] myopic 

 eye (o). 



