SOME CURIOSITIES OF VISION. 207 



A circular aperture was cut iu a wooded board and covered with 

 white paper; a lamp was placed behiud the board, and thus a bright 

 disc was obtained, as in the former experiment. An arrangement was 

 prepared by means of which one-half of this bright disk could be sud- 

 denly covered by a metal shutter, and it was found that when this 

 was done a narrow blue band appeared on the bright ground just 

 beyond aud adjoining the edge of the shutter when it had come to rest. 

 The blue band lasted for about one-tenth of a second, and it seemed to 

 disappear by retreating into the black edge of the shutter. An 

 attempt has been made to illustrate it in figure 9, where the shaded 

 band indicates the blue border. 



We have then to account, if possible, for the two facts that in the 

 formation of these transient borders the red sensation occurs in a 

 portion of the retina which has not been exposed to the direct action of 

 light, while the blue occurs in a portion which is exposed to unchanged 

 illumination. Accepting the Young-Helmholtz theory of color vision, 

 the effects must, I think, be attributed to a sympathetic affection of 

 the red nerve fibers. When the various nerve fibers occupying a lim- 

 ited portion of the retina are suddenly stimulated by white or yellow 

 light of moderate intensity, the immediately surrounding red nerve 

 fibers are for a short period excited sympathetically, while the violet 

 and green fibers are not so excited, or in a much less degree. And 

 again, when light is suddenly cut off from a patch in a bright field, 

 there occurs an insensitive reaction in the red fibers just outside the 

 darkened patch, in virtue of which they cease for a moment to respond 

 to the luminous stimulous ; the green and violet fibers, by continuing 

 to respond uninterruptedly, give rise to the sensation of a blue border. 



Whether or not the hypothesis which I have suggested is correct in 

 all its details, it is, I think, sufficiently obvious that the red and blue 

 colors of Benham's top are due to exactly the same causes as the colors 

 observed in my own experiments, for the essential conditions are the 

 same in both cases. 



I have mentioned only a few among many curious phenomena which 

 have presented themselves in the course of my investigation. It is not 

 improbable that a careful study of the subjective effects produced by 

 intermittent illumination would lead to results tending to clear up 

 many doubtful points in the theory of color vision. 



