570 MANUAL OF PHYSIOLOGY. 



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 G 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- 

 mal eye, called the emmetropie eye, this " near limit" is about 12 

 cm., or 5 inches, but it varies in different individuals. 



For objects that are over 10 metres distant very little change 

 in the eye is required in order to see them distinctly, and the 

 nearer the object approaches the more frequently the adjustment 

 of the eye has to be altered in order to see it clearly. But at 

 every part of the range of distinct vision objects at different dis- 

 tances can be seen without moving the adjustment. The range 

 of this power is measured on the line of vision, and called the 

 focal depth. In the distance we can take in a greater depth of 

 landscape, and this without effort or fatigue ; but when looking 



