570 THE NERVOUS SYSTEM. 



experiment (Fig. 348). In a card two pinholes are made whose 

 distance from each other must not exceed the diameter of the 

 pupil, about 2 mm. Through these holes, with the card held close 

 to the eye, a needle is looked at. It will appear single ; if the 

 needle is brought near to the eye, a point will be reached at which 

 it appears double, or it may be blurred ; this is the near-point for 

 the eye in question. 



Far-point. This is also known as punctum remotum, or p. r. 

 It is the farthest point at which objects can be seen distinctly by a 

 normal eye, and is infinitely distant ; so that the range of accom- 

 modation in the normal eye is from about 12 cm. to infinity. As 

 a matter of fact, rays of light which reach the retina from any 

 object not more than two meters distant from the eye are practi- 

 cally parallel, so that in looking at objects at any distance greater 

 than this there is no change in the accommodative apparatus 

 i. e., the eye is in a state of accommodative rest ; while as objects 

 are brought nearer than two meters there is necessitated an increase 

 in the convexity of the lens in order that they may be seen dis- 

 tinctly until the near-point is reached, within which their images 

 become blurred by reason of the fact that the lens has reached 

 the limit to which its convexity can be increased. 



Convergence of the Eyes during Accommodation. If, as an object 

 is brought nearer to the eyes of an observer, his eyes are inspected 

 by another, it will be seen that, as the accommodative apparatus is 

 brought into action to focus the image on the retina, the eyes are 

 at the same time turned inward i. e., made to converge. When 

 it is remembered that in order to produce distinct vision the image 

 must be formed in the fovea centralis, it will be seen that as the 

 object is brought nearer to the eyes each eye must be turned in- 

 ward, otherwise the image would not fall upon the fovea. 



Contraction of the Pupil during Accommodation. When a dis- 

 tant object is observed, the pupil is relatively large ; but when the 

 eye is accommodated for near objects, the pupil becomes smaller. 

 By this contraction of the iris the very divergent rays which come 

 from the object, and which, by reason of their extreme divergence, 

 would not be brought to a focus on the retina, are excluded, and 

 thus a sharpness of the image is secured which would not be 

 the case if the pupil was large enough to admit these rays. 



Defects in the Visual Apparatus. Emmetropia (Fig. 349, A). 

 The emmetropic or normal eye is one in which parallel rays are 

 brought to a focus upon the retina when the eye is in a state of 

 accommodative rest. In such an eye the near-point is about 

 12 cm. distant from the eye, the far-point at infinity, and from 

 about two meters distant to an infinite distance objects can 

 be distinctly seen without any change in the accommodative 

 apparatus. 



