THE PERCEPTION OF SIGHT. 253 



and original way of seeing things ; and hitherto most physio- 

 logists 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 how 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 gize. 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 nearer than the point at which 

 we are looking, and are not too far removed from it laterally to 

 admit of their position being sufficiently seen. At first we can 

 only recognise double images of objects at very different dis- 

 tances from the eye, but by practice they will be seen with 

 objects at nearly the same distance. 



All these phenomena, and others like them, of double images 

 of objects seen with both eyes, may be reduced to a simple rule 

 which was laid down by Johannes Miiller : ' For each point of 

 one retina there is on the other a corresponding point.' In the 

 ordinary flat field of vision presented by the two eyes, the images 

 received by corresponding points as a rule coincide, while images 

 received by those which do not correspond do not coincide. The 

 corresponding points in each retina (without noticing slight de- 

 viations) are those which are situated at the same lateral and 

 vertical distance from the point of the retina at which rays of 

 light come to a focus when we fix the eye for exact vision, namely 

 the yellow spot. 



The reader will remember that the intuitive theory of vision 

 of necessity assumes a complete combination of those sensations 



