SENSE OF SIGHT. 



571 



Ametropia. Whenever the permanent condition of an eye is 

 not as described above, it is one of ametropia. Of this condition, 

 there are several varieties. 



Myopia (Fig. 349, B). A myopic eye is one that is abnormally 

 elongated, and some au- 

 thorities regard an in- 

 creased convexity of the 

 lens as constituting an 

 essential part of this con- 

 dition. The retina is so 

 far from the lens that 

 parallel rays are focussed 

 in front of it, and, cross- 

 ing, do not form distinct 

 images on the retina, the 

 images being blurred. To 

 correct this,concave glasses 

 are used, which cause these 

 rays to diverge as they en- 

 ter the eye, and by adjust- 

 ing the concavity to the 

 amount of myopia, par- 

 allel rays are brought to 

 a focus on the retina as 

 they are in the emme- 

 tropic eye without glasses. 

 A myopic eye is commonly 

 said to be a " near-sighted " 

 one. The near-point in 

 myopia may be 5 or 6 cm. 

 from the eye, while the far- 

 point is comparatively near 

 the eye, never at an infinite distance, so that the range of accom- 

 modation is extremely limited. 



Hypermetropia (Fig. 349, C). In this condition the eye is 

 shorter than normal, and the retina is too near the lens, so that 

 parallel rays are brought to a focus behind the retina and indis- 

 tinct vision is produced, as in the myopic eye. In the endeavor 

 to overcome this defect the ciliary muscle is liable to overstrain 

 in order to converge the rays to a focus upon the retina, and the 

 constant effort is painful and injurious. The condition is corrected 

 by the use of convex glasses. The near-point in the hypermetropic 

 eye is farther than in the normal eye. The far-point does not 

 exist, for objects would of necessity have to be removed to a 

 greater distance than infinity, which is, of course, impossible, in 

 order that the rays coming from them might be converging, and 



FIG. 349. Diagram showing the difference 

 between (A] emmetropic, (B) myopic, and (C) 

 hypermetropic eyes. 



