246 RECENT PROGRESS OF THE THEORY OF VISION. 



tions of sight and of touch continues its effects, and thus leads 

 to fresh mistakes when the normal conditions are restored. 



In preparing objects with needles under a compound micro- 

 scope, we must learn to harmonise the inverted microscopical 

 image with our muscular sense; and we have to get over a 

 similar difficulty in shaving before a looking-glass, which changes 

 light to left. 



-. These instances, in which the image presented in the two 

 dimensions of the field of vision is essentially of the same kind 

 as the retinal images, and resembles them, can be equally well 

 explained (or nearly so) by the two opposite theories of vision 

 to which I have referred. But it is quite another matter when 

 we pass to the observation of near objects of three dimensions. 

 In this case there is a thorough and complete incongruity be- 

 tween our retinal images on the one hand, and, on the other, 

 the actual condition of the objects as well as the correct impres- 

 sion of them which we receive. Here we are compelled to choose 

 between the two opposite theories, and accordingly this depart- 

 ment of our subject the explanation of our Perception of 

 Solidity or Depth, in the field o" vision, and that of binocular 

 vision on which the former chiefly depends has for many years 

 become the field of much investigation and no little controversy. 

 And 110 wonder, for we have already learned enough to see 

 that the questions which have here to be decided are of funda- 

 mental importance, not only for the physiology of sight, but for a 

 correct understanding of the true nature and limits of human 

 knowledge generally. 



Each of our eyes projects a plane image upon its own retina. 

 However we may suppose the conducting nerves to be arranged, 

 the two retinal images when united in the brain can only 

 reappear as a plane image. But instead of the two plane 

 retinal images, we find that the actual impression on our mind 

 is a solid image of three dimensions. Here, again, as in the 

 system of colours, the outer world is richer than our sensation 

 by one dimension ; but in this case the conception formed by 

 the mind completely represents the reality of the outer world. 



