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TEXT-BOOK OF PHYSIOLOGY. 



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 point. The near point is abnormally distant 

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

 jar 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 



FIG. 296. THE HYPERMETROPIC EYE. Parallel rays (A, B} can be focused only at a 

 point behind the eye, as at /j rays of light coming from the retina take, on 

 emerging from the eye, a divergent direction, C, D. K. The negative punctum 

 remotum. 



a divergent direction, as shown in Fig. 206, 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 can be focalized by a passive hyperme- 



FIG. 297. HYPERMETROPIA. PAR- 

 ALLEL RAYS FOCUSED BEHIND 

 THE RETINA. (Hansell and 

 Sweet.} 



FIG. 298. CORRECTION OF HYPER- 

 METROPIA BY A CONVEX LENS. 

 (Hansell and Sweet.} 



tropic 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. This is done by 

 placing before the eye convex lenses the converging power of which 

 is proportional to the degree of hypermetropia (Figs. 297, 298). 



Astigmatism. This is a condition of the eye characterized by 

 an inequality of curvature of its refracting surfaces in consequence of 

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



