546 



t Simultaneous Colour Contrast. 



By F. W. Edridge-Green, M.D., F.R.C.S., Beit Memorial Research Fellow. 



(Communicated by Prof. E. Starling, F.R.S. Received December 4, 1911, — 

 Read February 1, 1912.) 



(From the Institute of Physiology, University College.) 



The subject of colour contrast presents exceptional difficulties because of 

 the number of factors to be taken into consideration. It is necessary to 

 eliminate the effects of successive contrast. Many of the results which have 

 been put down to simultaneous contrast are really due to successive contrast. 

 The surfaces to be compared should either be viewed by a flash of light of 

 very short duration or by one eye which is kept rigidly fixed upon a definite 

 point. 



In a series of important papers Hering* has shown that the explanation 

 of contrast given by Helmholtz is not tenable. I hope to show that another 

 explanation is possible which is even more in accordance with the facts. 

 I propose to review some of the experiments of Hering and to show that in 

 conditions in which, according to the requirements stated by him, colour- 

 should be visible no colour is to be seen. 



In experiments on simultaneous contrast it is necessary in order to avoid 

 ■effects of luminosity contrast to have the two surfaces as nearly as possible 

 of the same luminosity. It is also necessary in dealing with mixed colours 

 such as those formed by the light reflected from pigments to take into 

 consideration the effects of chromatic aberration. When a surface reflects 

 lights of different wave-lengths these lights are not all brought to the same 

 focus on the retina. Diffusion circles will extend on both sides of the image 

 of the coloured object on the retina and will influence the colour of another 

 image immediately adjacent. I have made a mosaic of small pieces of 

 coloured cardboard and the effect of the mixture of lights is very noticeable. 

 In general each colour differs as it would do if the other colour had been 

 objectively added to it. These colour changes have been mistaken for effects 

 of simultaneous contrast. 



Coloured Shadows. 



In the classical experiment which has been the subject of so much 

 discussion an opaque object is placed upon a white surface and illuminated 

 ■on one side by daylight and on the other by a candle or petroleum lamp. 

 * ' Pfluger's Arch.,' 1886, 1887, 1888. 



