364 Sir D. Brewster on the Law of Position in Vision. 



only apparent imperfection is, that when we see one point very 

 distinctly we do not see the other parts with equal distinctness ; 

 but this imperfection is unavoidable in vision, whether with 

 one or two eyes; and, in place of being a defect, is the very 

 means by which we judge of the relative distance of its parts. 

 If we saw all its lines and parts with equal distinctness, with- 

 out moving the eyeball, or without altering the mechanism for 

 its adjustment, we should not have been able to distinguish the 

 pyramid from its projection upon a plane surface. 



Hence we draw the conclusion that the vision of bodies of 

 three dimensions with one eye is perfect. 



(2.) By Binocular Vision. — If we now place the pyramid 

 before both eyes, so that the pictures of it on each retina are 

 nearly similar, the one being the reflected image of the other, 

 we shall see the pyramid with great distinctness. It will ap- 

 pear more luminous with the two eyes, and if the observer 

 wished to estimate the distance of its apex, or any other point 

 of it, from himself, the convergency of both eyes to that point 

 would enable him to form a more correct judgement than with 

 a single eye. These, doubtless, are advantages, but they do 

 not in the least degree improve our vision of the pyramid, 

 which is independent of them. More light may injure vision 

 as well as improve it; and if we could project a foot-rule from 

 each eye, and read upon it the distance of every part of the 

 pyramid, the vision of it would not in the slightest degree be 

 affected. May we not add also, that the intromission of scat- 

 tered light through two eyes in place of one, and the possible 

 dissimilarity, however small, between the curvatures and den- 

 sities of their humours, which would give rise to two pictures 

 of different magnitudes, would entitle us to give the preference 

 to single vision, in reference to its power of giving us a distinct 

 view of objects of three dimensions ? 



Hence, we conclude, that when the pyramid is placed in a 

 position of symmetry 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 bi- 

 nocular one. The vision of faces 1, 2, 3, and 4 is sufficiently 

 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 accu- 

 racy 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 



