116 



The Discrimination of Colour. 

 By F. W. Edridge-Geeen, M.D., F.R.C.S., Beit Medical Eesearch Fellow. 



(Communicated by Prof. W. M. Bayliss, F.R.S. Received April 24,— 

 Read May 18, 1911.) 



(From the Institute of Physiology, University College.) 



In a paper on the relation of light perception to colour perception,* and in 

 previous writings,! I have stated that if a portion of the spectrum be 

 isolated, it will appear monochromatic, the length of the monochromatic 

 region varying with the intensity and wave-length of the light and the colour 

 perception of the observer. Most normal sighted persons make about 

 eighteen such divisions in a bright spectrum. 



In a paper in the ' Proceedings of the Royal Society,'! Lord Rayleigh, 

 whilst agreeing that the facts were as I stated in the conditions described by 

 me, expressed the opinion that he could distinguish between the wave- 

 lengths included in a monochromatic division to the extent of discriminating 

 between the colours of the two D lines. Lord Rayleigh kindly lent me the 

 colour box with which he had made the experiments, and, on repeating 

 them in the manner described by him, I arrived at similar results. I hope, 

 however, to be able to show that the results obtained by Lord Rayleigh were 

 due to the admixture of small quantities of white and coloured light and to 

 certain physiological influences which had not been taken into consideration,, 

 and which prevented him from arriving at a correct interpretation of the 

 colours. 



If a prism, even of the finest polish, be examined with a strong light 

 against a dark background, numerous small particles and irregularities of the 

 surface, which irregularly disperse the light, will be seen. The reflections 

 from the sides of the prisms, lenses, and sides of the box have also to be 

 taken into consideration. The amount of this irregularly dispersed light is 

 small, but is a very important factor taken in conjunction with other facts. 

 It is necessary, therefore, in order to get rid of the greater part of this 

 irregularly dispersed light, to allow the light included in a monochromatic 

 region to pass through a second aperture, such as that in my spectrometer. 

 When this is done, I have found it impossible by any method which I have 

 adopted to distinguish between the various waves included in the mono- 



* ' Roy. Soc. Proc.,' B, 1910, vol. 82, p. 458. 



t ' Colour Blindness and Colour Perception,' International Scientific Series. 

 X December, 1910. 



