VISION. 473 



is a corresponding focal distance ; and when the 

 eye is in a state adapted for distinct vision at 

 one distance, it will have confused images of 

 objects at another distance ; because the exact 

 foci of the rays will be situated either before or 

 behind the retina. It is evident that if the 

 retina be not placed exactly at the point where 

 the focus is situated, it will either intercept the 

 pencil of rays before they are united into a point, 

 or receive them after they have crossed one 

 another in passing through the focus : in either 

 of which cases, each pencil will throw upon the 

 retina a small circle of light, brighter at the 

 middle and fainter at the edges, which will mix 

 itself with the adjacent pencils, and create con- 

 fusion in the image. 



It is found, however, that the eye has a power 

 of accommodating itself to the distinct vision of 

 objects at a great variety of distances, according 

 as the attention of the mind is directed to the 

 particular object to be viewed. The mode in 

 which this change in the state of the eye is 

 effected has been the subject of much contro- 

 versy. The increase of the refracting power of 

 the eye necessary to adapt it to the vision of near 

 objects is evidently the result of a muscular 

 effort, of which we are distinctly conscious when 



The second shows that divergent rays unite at a more distant 

 focus; and the third, that the focus is more distant the greater 

 the divergence. 



