VISION 989 



While the defects hitherto mentioned are shared in greater or 

 less degree by every normal eye, there are certain other defects 

 which either occur in such a comparatively small number of eyes, 

 or lead to such grave disturbances of vision when they do occur, 

 that they must be reckoned as abnormal conditions. In the normal 

 or emmetropic eye, parallel rays and for this purpose all rays 

 coming from an object at a distance greater than 65 metres may be 

 considered parallel are brought to a focus on the retina without 

 any effort of accommodation. The distance at which objects can 

 be distinctly seen is only limited by their size, the clearness of the 

 atmosphere, and the curvature of the earth; in other words, the 

 punctum remotum, or far-point of vision, the most distant point at 

 which it is possible to see with distinctness, is practically at an 

 infinite distance. When accommodation is paralyzed by atropine, 

 only remote objects can be clearly seen. On the other hand, the 

 normal eye, or, to be more precise, the normal eye of a middle-aged 



Fig. 407. Refraction in the (Normal) Emmetropic Eye. The image P' of a distant 

 point P falls on the retina when the eye is not accommodated. To save space, 

 P is placed much too near the eye in Figs. 407, 408. 



adult, can be adjusted for an object at a distance of not more than 

 12 cm. (or 5 inches). Nearer than this it is not possible to see 

 distinctly; this point is accordingly called the punctum proximum 

 or near-point. The range of accommodation for distinct vision 

 in the emmetropic eye is from 12 cm. to infinity. 



Myopia, or short-sightedness, is generally due to the excessive 

 length of the antero-posterior diameter of the eyeball in relation 

 to the converging power of the cornea and the lens. Even in 

 the absence of accommodation, parallel rays are not focussed on 

 the retina, but in front of it ; and in order that a sharp image may 

 be formed on the retina 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 3 cm. from the eye. The range of vision in the myopic 



