DEFECTS OF ACCOMMODATION. 573 



DEFECTS OF ACCOMMODATION. 



Myopia. It has been said that the " near limit " of distinct 

 vision differs in many persons from the twelve centimetres of the 

 normal emmetropic eye, and it is further found that the power of 

 accommodation varies very much in different individuals. Thus 

 in "short-sighted" people, who have myopic eyes, i.e., in which 

 distant parallel rays fall short of the retina, the near limit may 

 only be half the normal, i. e., five centimetres, and the far limit, 

 which is normally indefinite, is found to be within a compara- 

 tively short distance of the eye. They, therefore, cannot see 

 distant objects clearly, since the rays are focused before the 

 retina is reached, and then diverging, cause diffusion circles and 



FIG. 226. 



Showing the course of the rays of light from two luminous points to the retina. The 

 rays from the point a, on passing through the cornea, lens, etc., are collected on the 

 retina at b. Those from ' meet at l>', and thus the lower point becomes the upper. 



a blurred picture. The work of their accommodation is also 

 much more laborious, since they can only see in that part of the 

 range of accommodation where the adjustment has to be altered 

 for slight variations of distance. The defect can be made much 

 less distressing by the use of concave glasses, which make parallel 

 rays strike the cornea as divergent ones, and thus allow them to 

 be focused on the retina. 



Hypermetropia. Another abnormality is "long sight." In 

 the hypermetropic eye, parallel rays of light are brought to a 

 focus at a point beyond the retina, so that divergent or parallel 

 rays cause diffusion circles and a blurred image. This may be 

 corrected by means of convex glasses, which make the rays con- 



