356 SIR DAVID BREWSTER ON THE LAW OF VISIBLE POSITION 



Hence, we conclude, that when the pyramid is placed in a position of sym- 

 metry between the two eyes, binocular is not superior to monocular vision. 



But if the pyramid is so placed that the left eye sees only four faces of it, 

 while the right eye sees all the six, then the monocular vision of the pyramid is 

 more distinct than the binocular one. The vision of faces 1, 2, 3, and 4 is suffi- 

 ciently distinct with two eyes ; but the faces 5, 6, being seen only with one eye, 

 are less luminous than the other faces, and as the optic axes do not perform 

 their functions with the same accuracy when the object to which they are directed 

 is visible only to one eye, the part of the object seen by single vision will not 

 unite with that seen by double vision ; and, in the case of the pyramid, we shall 

 observe its apex actually projecting upon the faces 5, 6, of the pyramid, and de- 

 stroying the symmetry of the picture. When all the faces but No. 6 are seen by 

 the left eye, vision is still unsatisfactory with both eyes, and yet more so when 

 only three of the faces are seen by the left eye. 



Hence we conclude that, in these cases, binocular is inferior to monocular 

 vision. 



Let us next suppose that the object viewed is a table knife, so placed that, 

 when the back of it is towards the observer, the left side of the blade is seen by 

 the left eye, and the right side of the blade by the right eye. As the back is seen 

 by both eyes, the picture presented to the mind is a compound of one double and 

 two single sensations, and, consequently, a very unsatisfactory representation of 

 the object. 



Hence we conclude that, in this case, binocular is still more inferior to mono- 

 cular vision. 



These results stand in direct opposition to those given by Professor WHEAT- 

 STONE, who considers it an established fact, " that the most vivid belief of the soli- 

 dity of an object of three dimensions arises from two different perspective projections 

 of it being simultaneously presented to the mind.'" Before entering, however, upon 

 this branch of the subject, I must examine Mr WHEATSTONE'S views respecting 

 the binocular vision of figures of different magnitudes. 



4. On the Binocular Vision of Figures of Different Magnitudes* 



Mr WHEATSTONE seems to have been the first person who made experiments 

 on the binocular vision of unequal figures. Having drawn on separate pieces 

 of paper " two squares or circles, differing obviously, but not extravagantly, in 

 size ;" he placed them in the stereoscope, and concluded from his observations 

 that the two unequal pictures " coalesced, and occasioned a single resultant per- 

 ception ;" and that the binocular image thus perceived was apparently intermediate 



