334 THE SENSORIAL FUNCTIONS. 



Parallax. When the distance of the object is very great, 

 the rays proceeding from each point arrive at the eye with 

 so little divergence, that each pencil may be considered as 

 composed of rays which are parallel to each other; the ac- 

 tual deviation from parallelism being quite insensible. But 

 if the same object be brought nearer to the eye, the diver- 

 gence of the rays becomes more perceptible; and the effect 

 of the same degree of refraction is to collect them into a 

 focus more remote than before.* For every distance of the 

 object there 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 w r ill be situated either be- 

 fore 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 

 confusion in the image. 



It is found, however, that the eye has a power of accommo- 

 dating itself to the distinct vision of objects at a great variety 

 of distances, according as the attention of the mind is di- 

 rected 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 controversy. The increase of the re- 

 fracting 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 we accurately at- 

 tend to the accompanying sensations. The researches of 



* This is illustrated by Fig. 412, 413, and 414; the first of which shows 

 the rapid convergence of rays proceeding from a very distant object, and 

 which may be considered as parallel. 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. 



