990 THE SENSES 



eye is therefore very small. The defect may be corrected by con- 

 cave glasses, which render the rays more divergent. It is to be 

 noted that many cases of internal squint in children are connected 

 with myopia, the eyes necessarily rotating inwards as they are made 

 to fix an abnormally near object. The treatment both of the squint 

 and the myopia in these cases is the use of concave spectacles 

 (Fig. 408). Myopia, although a condition that shows a distinct 



Fig. 408. Myopic Eye. The image P' of a distant point P falls in front of the retina 

 even without accommodation. By means of a concave lens L the image may be 

 made to fall on the retina (dotted lines). 



hereditary tendency, is rarely present at birth; the elongation of the 

 antero-posterior diameter of the eyeball develops gradually as the 

 child grows. 



In hyperrfietropia, or long-sightedness, the eye is, as a rule, too 

 short in relation to its converging power; and with the lens in the 

 position of rest, parallel rays would be focussed behind the retina. 

 Accordingly, the hypermetropic eye must accommodate even for 



Fig. 409.- Hypermetropic Eye. The knage P' of a point P falls behind the retina 

 in the unaccommodated eye. By means of a convex lens L it may be focussed 

 on the retina without accommodation (dotted lines). 



distant objects, while even with maximum accommodation an 

 object cannot be distinctly seen unless it is farther away than the 

 near-point of the emmetropic eye. The far-point of distinct vision 

 is at the same distance as in the emmetropic eye viz., at infinity 

 the near-point is farther from the eye. The defect is corrected by 

 convex glasses (Fig. 409) Hypermetropia, unlike myopia, is 

 present at birth. 



