THE SENSES 915 



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 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 dis- 

 tinctly ; this point is accordingly called the punctum proximum 



FIG. 393. MYOPIC EYE. 



The image P' of a distant point P falls in front of the retina, even without accom- 

 modation. By means of a concave lens L the image may be made to fall on the 

 retina (dotted lines). 



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

 in the emme tropic 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 eye is therefore very small. The defect 



