THE SENSES 797 



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 con- 

 sidered 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 pos- 

 sible to see with distinctness, is practically at an infinite 

 distance. When accommodation is paralyzed by atropia, 

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

 the normal eye, or, to be more precise, the normal eye of 



FIG. 298. MYOPIC EYE. 



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

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

 (dotted lines). 



a middle-aged adult, can be adjusted for an object at a dis- 

 tance of not more than 12 cm. (or 5 inches). Nearer than 

 this it is not possible to see distinctly ; this point is accord- 

 ingly called the pun&tum 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 eye- 

 ball 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 



