BINOCULAR VISION. 805 



we cannot determine with accuracy whether the points in relief be nearer or farther 

 from the eye than the plane surface. This will not fully explain, however, the idea of 

 solidity of objects which we obtain by the use of both eyes ; for the estimation of dis- 

 tance is obtained by bringing the axes of both eyes to bear upon a single object, be it 

 near or remote. The fact is, as was distinctly stated by Galen, in the second century 

 that, when we look at any solid object not so far removed as to render the visual axes, 

 practically parallel, we see with the right eye a portion of the surface which is not seen 

 with the left eye, and vice versa. The two impressions, therefore, are not identical for 

 each retina ; the image upon the left retina including a portion of the left side of the 

 object not seen by the right eye, the right image in the same way including a portion of 

 the right surface not seen by the left eye. These slightly dissimilar impressions are 

 fused, as it were, produce the impression of a single image, when vision is perfectly 

 normal, and this gives the idea of relief or solidity, enabling us to appreciate exactly the 

 form of objects, when they are not too remote. 



The fact just stated is of course a mathematical necessity in binocular vision for near 

 objects ; but the actual demonstration of the fusion of two dissimilar images and the con- 

 sequent formation of a single image giving the impression of solidity was made by the 

 invention of the stereoscope, by Wheatstone. The principle of this instrument is very 

 simple. Two pictures are made, representing a solid object, one viewed slightly from 

 the right side, and the other, slightly from the left, so as to imitate the differences in 

 the images formed upon the two retinae. These pictures are so placed in a box that the 

 image of one is formed upon the right retina, and the other, upon the left. When these 

 conditions are accurately fulfilled, we see but a single image, and this conveys to the 

 mind the perfect illusion of a solid object. Experiments with the stereoscope are so 

 familiar that they need hardly be dwelt upon. With most persons, an apparatus is 

 necessary to shut off disturbing visual impressions ; but some individuals are able to 

 fuse two images in this way, placed in proper position, without the aid of an instrument, 

 by a simple effort of the will. 



The invention of the stereoscope has led to many curious and interesting experiments, 

 especially since the art of photography has enabled us to produce pictures in any position 

 with absolute accuracy ; but a simple statement of the principle upon which the instru- 

 ment is constructed illustrates the mechanism of binocular vision in the appreciation of 

 the form of objects. Experience, the aid of the sense of touch, etc., enable persons with 

 but one eye to get a notion of form, but the impressions are never entirely accurate in 

 this regard, although, from habit, this defect occasions little or no inconvenience. A 

 striking illustration of these points is afforded by the binocular microscope, which, 

 especially with low magnifying powers, produces a startling impression of relief. 



As we have just remarked, the stereoscope affords a satisfactory explanation of the 

 mechanism of the eye in the appreciation of the form of objects ; but, notwithstanding 

 this, a theory has been proposed, and is adopted by some writers, that we obtain an idea 

 of form by rapidly and insensibly directing the eyes successively toward different points 

 on the surface of objects. It is difficult to understand how the eye can make these rapid 

 movements, but the question is definitively settled by a very simple fact demonstrated 

 by Dove, Helmholtz, and others. In an article on visual perception, by Helmholtz, it is 

 stated that stereoscopic effect is recognized when two pictures are seen illuminated by 

 an electric spark, the duration of which does not amount to the four-thousandth part of 

 a second, so short, indeed, that a falling body appears absolutely motionless. Under 

 these conditions, displacement of the line of vision would seem to be impossible. 



We shall conclude our discussion of binocular vision and the stereoscope with a brief 

 account of some experiments upon the binocular fusion of colors, which are very curious, 

 although they have no very important bearing upon the physiology of the eye in ordinary 

 vision. Though an opposite opinion is held by some experimenters, Helmholtz, with 

 many others, states that, when one color is seen with one eye and another color with the 



