398 



THE BODY AT WORK 



with one. If the eyes are normal, the impression received 

 through the two is precisely the same as the impression re- 

 ceived through either singly. But when we are looking at 

 solid objects, the image on one retina is not the same as the 

 image on the other. One eye sees farther round the object on 

 the one side, the other on the other ; and it is just this dis- 

 parity in the pictures, aided by the feeling that the eyes are 

 converging, that gives the impression of solidity. Correspon- 

 dence of points, on the other hand, is not necessarily sufficient 

 by itself to convince the mind that the pictures presented by 

 the two eyes are identical. When a flat triangle such as this 



is regarded with the two eyes, its black lines fall on correspond- 

 ing points ; but the figure is associated in the mind with other 

 sensations sensations of movement and touch. Notwith- 

 standing the identity of the retinal images, the mind tries to 

 see them as disparate. The figure troubles the eyes. At one 

 moment the meeting-point of the three central lines projects 

 forwards, at the next it recedes. That similarity of retinal 

 images counts for something is shown by closing one eye. 

 The uncertainty of shape of the figure is rendered more trouble- 

 some. It changes still more rapidly from convex to concave. 

 When the point seems to be in front of the page, the accommo- 

 dation of the eyes is adjusted for nearness ; when behind the 

 page, for greater distance. But the illusion that the object 

 occupies three dimensions is not dependent upon the sense of 

 contraction of the ciliary muscle. When the paper is moved 

 towards the eye, its centre recedes ; it is left behind until the 

 ciliary muscle has had time to contract. When it is moved 

 away from the eye, it projects until the ciliary muscle has had 

 time to relax. Accommodation follows judgment, not judg- 



