THE SUBJECTIVE PHENOMENA OF VISION 



573 



coloured blue-green by its after image, because it has left its impression on 

 the visual mechanism, although that impulse has not been conveyed to 

 consciousness. If therefore a series of such red and white impulses be caused 

 to fall on the retina, each white one will be tinted blue-green, and each 

 red one will be suppressed, the result being that the complementary 

 colour is alone seen. This interesting phenomenon was discovered by 

 Bidwell. He took a disc of tin plate about 8 inches in diameter, and 

 arranged so that it could be rotated by an electric motor 6 to 8 times a second. 

 From this disc a sector was cut of approximately 60 degrees ; half the 

 remainder was covered with black velvet and the other half with white 

 paper. Behind the disc were mounted two pieces of silk, one red, the other 

 blue-green. The order in which the images were presented to the eye on 

 rotation of the disc were : (1) coloured silks, (2) white sector, (3) black sector, 

 and so on. The result was found to be that the red silk appeared pale blue- 

 green, and the other pink ; that is in both cases the complementary colour 

 was alone seen. This experiment brings out very clearly the fact that the 

 after image process is entirely subconscious. The following observations 

 confirm this conclusion. 



BURCH'S EXPERIMENT. 

 It has been much discussed in 

 the past whether contrast phe- 

 nomena are due to errors of judg- 

 ment as Hclmholtz supposed, or 

 due to physical or physiological 

 changes taking place in the retina 

 or in the conducting paths leading 

 from it to consciousness. Burch 

 disproved Helmholtz' view by the 

 following simple experiment. A 

 box (Fig. 295) is divided into 

 two long compartments, a 6 and 

 c d. At a the compartment is 

 closed by a red glass -plate and 

 at c by a blue glass -plate. Aper- 

 tures are provided at b and d for 

 the observer's eyes. At + and + 

 two small grey crosses are fixed 

 about the middle of the compart- 

 ment on sheets of transparent glass. On looking through the openings 6 and d and con- 

 verging the eyeballs, so as to fix the line o, we get a fusion more or less complete of the 

 two colours red and blue, so that the background appears purple ; or there may be a 

 struggle between the colours, at one time blue, at another red predominating. To 

 the judgment however, there is one background and not two, and therefore, accord- 

 ing to the theory of Helmholtz, the grey crosses should by contrast both acquire the 

 same induced colour, which would be complementary for purple. But it is found that 

 the two crosses are perfectly distinct in colour, that which is seen by the eye against 

 the blue ground being yellow, while that on the red ground is green, showing that the 

 phenomena of simultaneous contrast are not due to an error of judgment. 



SHERRINGTON'S EXPERIMENT. The same fact is very definitely established 

 by the following experiment devised by Sherrington. The disc (Fig. 296) presents two 

 rings, each half-blue and half-black. The outer ring is intensified when at rest by 



Purple 



Purple Yellow 



Green Purple 



Purple 

 FIG. 295. 



