664 TEXT-BOOK OF PHYSIOLOGY. 



a diverging power in the first instance of 0.5 dioptry and the second of 10 

 dioptrics. (Fig 322.) 



Hypermetropia. Hypermetropia may be defined as a condition of the 

 eye characterized by decrease of the normal antero-posterior diameter or 

 by a subnormal refracting power of the lens. The former is the usual con- 

 dition. Parallel rays of light do not, therefore, come to a focus when the 

 accommodation is suspended. Falling on the retina previous to focalization, 

 they give rise to diffusion-circles and indistinctness of the image. As no 

 object can be seen distinctly no matter how remote, there is no positive far 



FIG. 323. 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. 



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. 323, 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 that only converging rays 



FIG. 324. HYPERMETROPIA. PAR- FIG. 325. CORRECTION OF HYPER- 



ALLEL RAYS FOCUSED BEHIND THE METROPIA BY A CONVEX LENS. 



RETINA. 



can be focalized by a passive hypermetropic eye. 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 hypermetropic person attempts to focalize the rays by increasing the 

 convexity of the lens though an increased accommodative effort which often 

 gives rise to accomodation fatigue and headache. The convergence of the 

 rays of light before they enter the hypermetropic eye is accomplished by 

 the placing before the eye convex lenses the converging power of which is 

 proportional to the degree of hypermetropia. (Figs. 324, 325). 



