THE SENSES 



is too long, and as a result parallel rays rays from distant 

 objects are focussed in front of the retina, and it is only 

 when the object is brought near to the eye that a perfect 

 image can be formed. In such an eye, no positive accom- 

 modation is needed till the object is well within the normal 

 far point; and the near point is approximated to the eye. 

 To enable distant objects to be seen it is necessary to provide 

 concave glasses by which the parallel rays are rendered 

 divergent (Fig. 52, D). 



(2) Hypermetropia. The eye of a considerable number of 

 people is too short from before backwards, and thus, in the 

 resting state, parallel rays are focussed behind the retina, 

 and to see even a distant object the individual has to use 

 his positive accommodation. As the object is approached 

 to the eye it is focussed with greater and greater difficulty 

 and the near point is further off than in the emmetropic 

 eye (Fig. 52, C). 



The long-sighted eye differs from the slightly presbyopic 

 in the fact that not merely divergent, but also parallel rays, 

 are unfocussed in the resting state. 



The condition is corrected by using convex glasses which 

 render the rays convergent, and, therefore, capable of being 

 focussed upon the retina of the 

 shortened eye. 



(3) Astigmatism is a defect 

 due to unequal curvature of 

 the one or more of the refract- 

 ing surfaces in different planes. 

 If the vertical curvature of the 

 cornea is greater than the 

 horizontal, when a vertical line 

 is looked at, horizontal lines 

 will not be sharply focussed at 

 the same time. To correct 

 this condition, the lesser cur- 

 vature in any particular plane 

 must be made as great as the 

 greater curvature in the other 



O 



plane, and this is done by placing a cylindrical or part 

 cylindrical glass in front of the eye so that its curvature is 



FIG. 53. To show the cause of 

 Astigmatism. A, a slight curva- 

 ture of the cornea in the vertical 

 plane ; B, more marked curvature 

 in the horizontal plane, leading to 

 rays from b a horizontal line being 

 focussed in front of the retina when 

 a a vertical line is looked at. 



