276 ELEMENTARY PHYSIOLOGY. [XXVII. 



of about six inches, a needle, so that it is at 



right angles to one of the horizontal bars of the 



window. 



Look at the window -bar, the needle will appear 



dim and diffuse. 



Look at the needle, the window-bar will appear 



dim and diffuse. 



The eye can accommodate itself for either the 



needle or the window-bar, but not for both at the 



same time. The accommodation for the near 



object is accompanied by a distinct feeling of 



effort. 



0. Facing a window, or a white surface, close one 

 eye and hold a fine needle vertically before the 

 other. At about six inches the needle will be 

 seen distinctly. Bring it nearer the eye, the 

 image will be dim and diffuse, and at the same 

 time larger. The dimness and apparent increase 

 of size are due to diffusion, resulting from 

 imperfect accommodation. 



Prick a small clean hole in a piece of card, hold 

 it before the eye and again bring the needle 

 close to the eye. It will be seen distinctly at a 

 much smaller distance than before, and at the 

 same time will appear magnified. It will be 

 seen more distinctly because the diffusion circles 

 are cut off. It will seem larger because, as it is 

 nearer the eye, the rays which pass through the 

 hole in the card are more divergent. 

 For a similar reason an object at a distance 

 viewed through the same hole will appear 



