716 SPECIAL SENSES. 



in the second century that in looking at any solid object not so far re- 

 moved as to render the visual axes practically parallel, a portion of the sur- 

 face, seen with the right eye, 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 and produce the impression of a single image, when 

 vision is perfectly normal ; and this gives the idea of relief or solidity, and 

 an exact appreciation of 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 consequent 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, but a single 

 image is seen, 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. 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 occa- 

 sjons little or no inconvenience. 



Although an opposite opinion is held by some experimenters, Helmholtz, 

 with many others, has stated that when one color is seen with one eye and 

 another color, with the other eye, in the stereoscope, the impression is not of a 

 single color resulting from the combination of the two. It is true that there 

 is an imperfect mingling of the two colors, but this is very different from the 

 resulting color produced by the actual fusion of the two. There is, in other 

 words, a sort of confusion of colors, without the complete combination ob- 

 served in ordinary experiments. One additional point of importance, how- 

 ever, is that the binocular fusion of two pictures, unequally illuminated or of 

 different colors, produces a single image of a peculiar lustre, even when both 

 surfaces are dull. This may be shown by making a stereoscopic combination 

 of images of crystals, one with black lines on a white ground, and the other 

 with white lines on a black ground. The resulting image has then the ap- 

 pearance of dark, brilliant crystals, like graphite. 



Duration of Luminous Impressions (After-images). The time re- 

 quired for a single visual stimulation of the retina is exceedingly short. The 

 letters on a printed page are distinctly seen when illuminated by an electric 

 spark, the duration of which is not more than forty billionths of a second 

 (Rood). An impression made upon the retina, however, endures for a length 

 of time that bears a certain relation to the intensity of the luminous excita- 



