288 RECENT PROGRESS OF THE THEORY OF VISION. 



We must first make sure that we are really able to 

 distinguish between the two flat images offered us by 

 our eyes. If I hold my finger up and look towards the 

 opposite wall, it covers a different part of the wall to 

 each eye, as I mentioned above. Accordingly I see the 

 finger twice, in front of two different places on the wall ; 

 and if I see a single image of the wall I must see a double 

 image of the finger. 



Now in ordinary vision we try to recognise the solid 

 form of surrounding objects, and either do not notice this 

 double image at all, or only when it is unusually striking. 

 In order to see it we must look at the field of vision 

 in another way — in the way that an artist does who 

 intends to draw it. He tries to forget the actual shape, 

 size, and distance of the objects that he represents. One 

 would think that this is the more simple and original 

 way of seeing things ; and hitherto most physiologists 

 have regarded it as the kind of vision which results 

 most directly from sensation, while they have looked on 

 ordinary solid vision as a secondary way of seeing things, 

 which has to be learned as the result of experience. But 

 every draughtsman knows liow much harder it is to 

 appreciate the apparent form in which objects appear 

 in the field of vision, and to measure the angular 

 distance between them, than to recognise what is their 

 actual form and comparative size. In fact, the knowledge 

 of the true relations of surrounding objects of which the 

 artist cannot divest himself, is his greatest difficulty in 

 drawing from nature. 



Accordingly, if we look at the field of vision with both 

 eyes, in the way an artist does, fixing our attention upon the 

 outlines, as they would appear if projected on a pane 

 of glass between us and them, we then become at once 

 aware of the difference between the two retinal images. 

 We see those objects double which lie further off or 



