. XX. ADAPTATION OF THE EYE. 369 



which becomes invisible, corresponds, in these different 

 positions, to the base of the optic nerve. 



Adaptation of the Eye to different Distances. It is, then, 

 clearly proved, that for vision to be perfectly distinct, our 

 eye should be placed in such a manner that the image 

 should be formed upon the sensible points of the retina, in 

 the smallest possible dimensions, and with sufficient inten- 

 sity. 



This being admitted, let us now see how these conditions 

 can be constantly fulfilled, the distance at which we can see 

 objects being variable. We perceive a star as distinctly as 

 an object placed but a few centimetres off': all that is re- 

 quired for the object to be distinctly seen is, that its. size, 

 and, consequently, the intensity of its light, should increase 

 in proportion to its distance. The image of a luminous 

 object recedes further from, or advances nearer to, a lens, 

 according as the body is brought nearer to, or is carried 

 further away from, the opposite side of the lens. It is, 

 therefore, certain, that the eye, by an act of volition, be- 

 comes so modified as to see at different distances. In fact, 

 if we look at a body, a black spot for instance, made upon 

 a glass, placing it at different distances from the eye, we 

 shall have a confused image of the objects more or less 

 distant from the spot, whilst the latter will be very distinctly 

 seen in every position, and however remote. With a 

 healthy eye, vision is effected without either effort or fatigue 

 at a distance of about 30 centimetres [= 11-8 inches,] but 

 not for greater or less distances. This is the reason why it 

 is no longer distinct after the eye has been looking at an 

 object very near to it for several hours. 



In order to explain the property which this organ pos- 

 sesses, of seeing objects placed at variable distances, we 

 must adopt one or the other of the following two hypotheses ; 

 either we must assume that the retina communicates to the 

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