568 MANUAL OF PHYSIOLOGY. 



fixed, moving the object nearer to or further from the eye would 

 have the same effect as changing the relation of the retina to m n 

 or p q in Fig. 224, by means of which we may explain the follow- 

 ing observations: (1) The eye being fixed upon the object (of 

 which only one image is seen), move the pin rapidly away ; two 

 objects now appear, showing that the rays coming through the 

 holes have met before they reach the retina as at p q. (2) Move 

 the pin near the eye ; again two very blurred objects are seen, for 

 the rays have not met when they strike the retina, as at m n. (3) 

 Keeping the object in the same position, alter the gaze, as if to 

 look first at distant and then at near objects ; in both extremes 

 two images are seen. (4) When the object is in exact focus as 

 at c, the closure of one of the holes does not affect the single 

 image. (5) When two images are seen, closing the right hand 

 hole at g causes the right or left image to disappear, according as 

 the focus c falls short of m n, or is beyond p q, the retina. (6) 

 By moving the pin's-head nearer the eye a point is reached at 

 which the object cannot be brought to a focus as a single image. 

 This limit of near accommodation marks the near point. A little 

 attention teaches us that looking at the near object requires an 

 effort which looking at the distant one does not ; in fact, we have 

 to do something to see things near us distinctly. This act is the 

 voluntary adjustment of the eye which we call its accommoda- 

 tion for near vision. 



ACCOMMODATION. 



The difference of distance for which we can adjust our eyes is 

 great, so that our range of distinct vision is very extensive. As 

 already stated, the normal eye is considered to be constructed so 

 that parallel rays of light, i.e., those coming from practically in- 

 finite distance, are brought to a focus on the retina. This is why 

 we see the stars which are practically infinitely remote from us 

 as mere luminous points. It is therefore impossible to fix a 

 " far limit " to our power of distant vision. The nearer an object 

 is brought to our eyes, however, the more effort is required to see 

 it distinctly, until at last a point is reached where we cannot get 

 a clear outline, no matter how we "strain our eyes." For a nor- 



