THE SENSE OF SIGHT 701 



remote, there is no positive far point. The near point is abnormally distant 

 sometimes as far as 200 cm. For this reason the condition is termed far 

 sight. A hypermetropic eye without accommodative effort can focalize only 

 converging rays on the retina. If rays of light were to come from the retina 

 of such an eye, they would, on emerging, take a divergent direction, as shown 

 in Fig. 306, dotted line C and D. If these same rays were to be prolonged 

 backward, they would meet at the point K, which is the punctum remotum; 

 and as it is behind the eye, it is termed negative. Since rays coming from 

 the retina take a divergent direction on emerging from the eye, it is evident 



FIG. 306. THE HYPERMETROPIC EYE. Parallel rays (A, B) can be focused only at a point 

 behind the eye, as at /; rays of light coming from the retina take, on emerging from the eye, a 

 divergent direction, C, D. K. The negative punctum remotum. 



that only converging rays can be focalized by a passive hypermetropic eye. 

 The hypermetropic person attempts, and partially succeeds, in focalizing the 

 rays by increasing the convexity of the lens through an increased accommoda- 

 tive effort which often gives rise to accommodation fatigue and headache. 

 As there are no convergent rays in nature, it is necessary for distinct vision 

 that all rays, parallel and divergent, shall be given a convergent direction 

 before entering the eye. The decrease in the length of the antero-posterior 

 diameter may range from a fraction of a millimeter up to 2.78 mm. Normal 

 vision may be established by placing before the hypermetropic eye convex 



FlG 30? _ HYPERMETROPIA. PAR- FIG. 308. CORRECTION OF HYPER- 



ALLEL RAYS FOCUSED BEHIND THE METROPIA BY A CONVEX LENS. 



RETINA. 



lenses with a converging power, in the first instance, of 0.5 dioptry and, in the 

 second instance, of 10 dioptrics (Figs. 307 and 308). 



Astigmatism. Astigmatism may be defined as a condition of the eye 

 characterized by an inequality of curvature of its refracting surfaces in con- 

 sequence of which not all of a homocentric bundle of rays are brought to 

 the same focus. The inequality may be either in the cornea or lens, or 

 both, though usually in the cornea. 



In the normal cornea the radius of curvature in the vertical meridian 

 is a trifle shorter, 7.6 mm., than that of the horizontal, 7.8 mm., and hence 

 its focal distance is slightly shorter. The difference, however, in the focal 



