IN SINGLE AND BINOCULAR VISION 355 



3. On the Vision of Objects of Three Dimensions. 



(1.) By Monocular Vision. If we look with one eye at a solid body, for example a 

 six-sided pyramid with its apex directed to the eye, and uniformly illuminated, we 

 recognise at a single glance that it is not a drawing of the pyramid. When the eye 

 adjusts itself to distinct vision of its apex, all the more distant parts are seen in- 

 distinctly, but the eye quickly surveys the whole, adjusting itself to distinct vision 

 of its base and of its edges, and by these successive efforts, at one time contract- 

 ing the pupil and the eyebrows to see the near parts, and expanding them to see 

 the more remote ones, it obtains a knowledge of the relative distance of its differ- 

 ent parts. The vision of the pyramid thus obtained is nearly perfect. There is 

 no inequality of illumination produced by the act of single vision ; and there is 

 no flickering in the outlines of the figure. The 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, without 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 judgment 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 scattered light through two eyes in place of one, and the 

 possible dissimilarity, however small, between the curvatures and densities 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. 



