126 SPECIAL SENSES. 



factory explanation of the mechanism of the eye in the ap- 

 preciation 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 di- 

 recting 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 defini- 

 tively 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 recog- 

 nized 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. 1 Under these conditions, 

 displacement of the line of vision would seem to be impos- 

 sible. 



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, 

 though they have no very important bearing upon the phys- 

 iology 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 other eye, in the stereoscope, the 

 impression is not of a single color resulting from the combi- 

 nation of the two. 2 It is true that there is an imperfect min- 

 gling 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, with- 

 out the complete combination with which we are familiar in 

 ordinary experiments. One additional point of interest, 

 however, is that the binocular fusion of two pictures, une- 

 qually illuminated or of different colors, produces a single 



1 HELMHOLTZ, Les perceptions viwelles. Revue des cours scientifiques, Paris, 

 1868-1869, tome vi., p. 422; and, Optique phijsiologique, Paris, 1867, p. 937. 



2 HELMHOLTZ, Optique physiologique, Paris, 1867, p. 976. 



