300 RECENT PROGRESS OF THE THEORY OF VISION. 



mitted from the two eyes, and reach the consciousness 

 at the same time and without coalescing ; that accordingly 

 the combination of these two sensations into the single 

 picture of the external world of which we are conscious 

 in ordinary vision is not produced by any anatomical 

 mechanism of sensation, but by a mental act. 



IV. Further, we find that there is, on the whole, com- 

 plete, or at least nearly complete, coincidence as to 

 localisation in the field of vision of impressions of sight 

 received from corresponding points of the retinae ; but 

 that when we refer both impressions to the same object, 

 their coincidence of localisation is much disturbed. 



If this coincidence were the result of a direct function 

 of sensation, it could not be disturbed by the mental 

 operation which refers the two impressions to the same 

 object. But we avoid the difficulty, if we suppose that 

 the coincidence in localisation of the corresponding 

 pictures received from the two eyes depends upon the 

 power of measuring distances at sight which we gain by 

 experience, that is, on an acquired knowledge of the 

 meaning of the ' signs of localisation.' In this case it is 

 simply one kind of experience opposing another ; and 

 we can then understand how the conclusion that two 

 images belong to the same object should influence our 

 estimation of their relative position by the measuring 

 power of the eye, and how in consequence the distance 

 of the two images from the fixed point in the field of 

 vision should be regarded as the same, although it is not 

 exactly so in reality. 



But if the practical coincidence of corresponding points 

 as to localisation in the two fields of vision does not 

 depend upon sensation, it follows that the origioal power 

 of comparing different distances in each separate field of 

 vision cannot depend upon direct sensation. For, if it 

 were so, it would follow that the coincidence of the two 



