230 



THE SPECIAL SENSES 



axes of the eyes. The farthest point from the eye at which 

 an object can be distinctly seen is called the far point, and the 

 nearest point of distinct vision is the near point, while the dis- 

 tance between near point and far point is the range of accom- 

 modation. The near point is the shortest focus of the crystalline 

 lens, and is usually about five or six inches. 



Emmetropia is the normal eye that is, an eye in which par- 

 allel rays or rays from objects at a distance are focussed upon 

 the retina without an effort of accommodation. Such a distance 

 for practical purposes is considered to be any point beyond 

 twenty feet. Absolutely emmetropic eyes are not common. 



Myopia, or near sight, is the term applied to an eye in which 

 the rays from a distance are focussed in front of the retina and 

 the image is blurred. Such an eye is permanently focussed for 

 near objects (Fig. 15). 



FIG. 15 



Myopic eye. 



Myopia is produced in two ways by the anteroposterior 

 diameter of the eye being too great, or by the convexity of the 

 lens being exaggerated. In either case the focus of the lens will 

 fall in front of the retina. Myopia is corrected by the use of a 

 concave lens, which diverges the rays and in this way prevents 

 their coming to a focus too soon. 



Hypermetropia, or far sight, is the reverse of myopia (Fig. 16). 

 In this case the anteroposterior axis of the eye is too short, or 

 else there is an abnormal flattening of the lens which does not 

 allow accommodation for near vision. The result is that the 

 image of an object near by is focussed behind the retina; but 

 objects at a distance are clearly seen. 



Hypermetropia is corrected by the use of a convex lens, 

 which adds to the refractive power of the eye and the convexity 

 can be increased for near vision. 



