THE SENSES. 



545 



accommodation varies for every change of distance within fifty feet ; 

 and the organ is thus adjusted by an instantaneous and unconscious 

 movement, for the most delicate variations of refractive power. But 

 an eye-glass, the curvatures of which are invariable, can give perfect 

 correction only for a single distance. A glass is, therefore, usually 

 selected of such curvature as to serve for the most convenient dis- 

 tance in ordinary manipulations. 



Myopic Eye. In many instances, where the eye is otherwise normal, 

 its antero-posterior diameter is longer than usual, thus placing the retina 

 at a greater distance behind the lens. Consequently, although the 

 rays are brought to a focus at the usual distance behind the cornea, this 

 focus is situated in the vitreous body ; and the rays reach the retina only 

 after their crossing and partial dispersion (Fig. 143). This produces 

 indistinct vision for remote objects. But for those at shorter distances, 



FIG. 142. 



EMMETROPIC EYE, in vision at long distances. (Wundt.) 



FIG. 143. 



MYOPIC EYE, in vision at long distances. (Wundt.) 



the rays enter the pupil under such a divergence, that their focus falls 

 at the retina, and the object is distinctly seen. Such an eye is said to 

 be myopic, or, in ordinary language, "near sighted," because its range 

 of distinct vision is confined to comparatively near objects. The flexi- 

 bility of the lens, and its capacity for increased convexity, may be, in 

 the myopic eye, fully up to the normal standard, and consequently its 

 power of accommodation may be as great as that of the normal eye. 

 In the emme.tropic condition, a certain variation in the curvature of 

 the lens produces the requisite accommodation for all distances be- 

 tween 15 centimetres and infinity. In the myopic eye the same accom- 

 modating power may be exercised between the distances of 8 and 20 

 centimetres. The myopic eye consequently has distinct vision at 



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