79 8 A MANUAL OF PHYSIOLOGY 



the object must be so near that the rays proceeding from it 

 to the eye are sensibly divergent that is to say, it must be 

 at least nearer than 65 metres ; but as a rule an object at 

 a distance of more than 2 to 3 metres cannot be distinctly 

 seen. With the strongest accommodation the near point 

 may be as little as 5 cm. from the eye. The range of vision 

 in the myopic eye is therefore very small. The defect may 

 be corrected by concave 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 spec- 



FIG. 299. HYPERMETROPIC EYE. 



The image P of a point P falls behind the retina in the unaccommodated eye. By 

 means of a convex lens it may be focussed on the retina without accommodation (dotted 

 lines). 



tacles (Fig. 298). Myopia, although a condition that shows 

 a distinct hereditary tendency, is rarely present at birth ; 

 the elongation of the antero-posterior diameter of the eye- 

 ball develops gradually as the child grows. 



In hypermetropia, 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 h) permetropic eye must 

 accommodate even for 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; 



