1911.] 



Simultaneous Colour Contrast. 



549 



however, to show by a simple experiment that this cannot be the 

 explanation. 



Let us consider the factors of the experiment when a grey patch upon a 

 whitish-green ground is under observation : the objective light reflected 

 from the grey patch differs from that on the green ground chiefly in 

 containing less green, that is to say, the light is relatively and objectively 

 rose-coloured in comparison with the whitish-green ground. The white 

 light for this purpose may be divided into two portions, one of which 

 is green and the other is a mixture of the remaining constituents of white 

 light, which give rise to a sensation of rose. If a small portion of the 

 common constituent green be deducted from both the whitish-green and 

 the white the green will appear less saturated, and the white will appear 

 rose. In this case the white light will be objectively rose in comparison 

 to the green. It will be seen, therefore, that there are two ways in which 

 the two coloured surfaces may be objectively considered. If the green 

 be considered of less saturation than it really is, that is to say, a whiter 

 colour, then the white will be rose in comparison with it, but if the white 

 be considered white then the green will be objectively of greater saturation 

 in comparison with it. 



A simple experiment which I have devised for the purpose decides this 

 point. If, when the grey square is situated upon the larger green square, 

 another square of white paper of a size midway between the two other 

 squares have a hole corresponding to the size of the small grey square cut 

 in it, on laying this white paper so that the opening corresponds with the 

 grey paper, the grey square will be seen without a trace of colour. A mark 

 should then be made on the extreme left of the grey paper, and whilst 

 one eye is kept rigidly fixed upon this spot the white square is gradually 

 moved to the left until the field is occupied half by the whitish-green 

 ground and half by the grey ground. It will be noticed that the green is 

 greatly increased in saturation, and that not a trace of rose is visible upon 

 the grey ground. If the right eye be kept fixedly upon these two surfaces 

 for ten seconds, and then be directed to a white surface, a brilliant rose- 

 coloured after-image, much brighter and more saturated than the one that 

 was previously visible, and a pale white after-image, without a trace of 

 colour corresponding to the grey region, will be seen. If the colour were 

 really subjectively produced in the retina it should appear in this experi- 

 ment as in the other. It might be thought that the increased saturation 

 noticed in the green was due to the luminosity contrast of the white paper, 

 but exactly the same result may be obtained with black paper, the green, 

 when uncovered, appearing much more saturated, and the subsequent 

 vol. Lxxxrv. — b. 2 s 



