234 Tlte Effect of Red Fatigue on the White Equation. 



It will be seen that, after fatigue with red, only about half as much green 

 is required in the equation. In making these observations, it is important 

 that the fatigue should not be too strong, as the blue-green after-effect may 

 obscure the appearance, and prevent correct observations being taken. A 

 man with normal colour vision, who had passed my card test, had a normal 

 white equation, normal complementary of yellow and violet, and normal 

 monochromatic division of yellow, after one minute's red fatigue, was quite 

 satisfied when the green slit was quite closed or open to its full extent, the 

 equation then appearing bright red and green respectively to a normal- 

 sighted person. This is in accordance witli the observations of A. W. Porter 

 and myself on successive contrast,* when we found that the after-image was 

 not affected by subsequent light falling on the retina when this was not of 

 too great intensity. When the fatigue is great, a blue-green after-effect is 

 seen, which is negative in colour but positive in luminosity, appearing bright 

 on the dark ground ; it appears to consist of minute particles, which move 

 towards the centre with a whirlpool movement and then disappear. If the 

 equation be then made, it will be found, as before, that the proportion of red 

 is too high. A colour-blind man who piit too much green in the white 

 equation, and another who put too little, after fatigue with red put a much 

 less amount, thus varying in the same way as the normal. 



It is generally stated that, with regard to the fovea, it is found that all 

 colour matches still remain valid, no matter what kind of light may have 

 previously stimulated the retina. This appears to be true when red of the 

 region A. 780 is used for the fatigue. This was obtained by viewing the light 

 through a deep violet glass combined with a deep red glass. The two glasses 

 only allow red of the region of X 780 to pass. Watsonf stated that colour 

 matches remain valid after stimulation with another light, but that the mean 

 deviation was considerably increased. This does not appear to be the 

 case when the fatigue is not excessive. The blue-green after-effect appears 

 to be due to the decomposition products of a photo-chemical substance (the 

 visual purple) in the retina, and to be distinct from fatigue of the retino- 

 cerebral apparatus. There seems to be considerable variation in the length of 

 time necessary to cause fatigue with different persons. 



These observations are quite inconsistent with the three-sensation theory. 

 The white equation and its match cannot be due to similar physiological 

 processes, or both would change in the same ratio. 



I must express my indebtedness to Captain Fulton and Mr. Isaacs, of the 

 Eoard of Trade, for their help in making these observations. 



* 'Eoy. Soc. Proc.,' B, vol. 85, p. 434 (1912). 

 T 'Roy. Soc. Proc.,' A, vol. 89, p. 235 (1913). 



