280 RECENT PROGRESS OF THE THEORY OF VISION. 



which has been rectified by comparison with the false 

 retinal images, but, on the contrary, the perception of 

 sight which has been corrected by that of touch. But, 

 again, if, after trying this experiment several times, we 

 take off the spectacles and then look at any object, taking 

 care not to bring our hands into the field of vision, and 

 now try to touch it with our eyes shut, the hand will pass 

 beyond it on the opposite side —that is, to the left. The 

 new harmony which was established between the percep- 

 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 

 microscope, we must learn to harmonise the inverted mi- 

 croscopical image with our muscular sense ; and we have 

 to get over a similar difficulty in shaving before a look- 

 ing-glass, which changes right 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 oppo- 

 site 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 between our retinal 

 images on the one hand, and, on the other, the actual 

 condition of the objects as well as the correct impression 

 of them which we receive. Here we are compelled to 

 choose between the two opposite theories, and accordingly 

 this department of our subject — the explanation of our 

 Perception of Solidity or Depth in the field of 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 no won- 



