636 



THE SENSES. 



of accommodation for near objects begins to interfere with the ordinary 

 occupations of life. When this condition is established the eye is said 

 to be presbyopia. Its vision is still perfect for distant objects, but it 

 can no longer adapt itself for the examination of those in close prox- 

 imity to the eye. To remedy this 'defect the patient employs a convex 

 eye-glass, which replaces for him the increased convexity of the crys- 

 talline lens, in accommodation for near objects ; and by the aid of such 

 a glass he is able to read or write at ordinary distances and in characters 

 of the ordinary size. 



The use of a convex eye-glass does not restore the perfection of sight 

 as it existed beforehand. In the normal eye, the degree of accommoda- 

 tion varies for every change of distance within fifty feet ; and the organ 

 is thus adjusted, by an instantaneous and unconscious movement, for 

 the most delicate variations of refractive power. But an eye-glass, the 

 curvatures of which are invariable, can give perfect correction only for 

 a single distance. A glass is, therefore, usually selected of such a 

 strength as to serve for the most convenient distance in the ordinary 

 manipulation of near objects. 



Fig. 203. 



EMMETEOPIC EYE, iii vision at long distances. (Wundt.) 

 Fig. 204. 



MYOPIC EYE, in vision at long distances. (Wundt.) 



Myopic Eye. In many instances, where the eye is otherwise of nor- 

 mal configuration, its antero-posterior diameter is longer than usual, 

 thus placing the retina at a greater distance behind the lens. The con- 

 sequence of this peculiarity is that while the luminous ra}^s are brought 

 to a focus at the usual distance from their point of entrance into the eye, 

 this focus is situated within the vitreous body; and the rays reach the 

 retina only after they have crossed and suffered a partial dispersion. 



