﻿918 Dr. F. W. Edridge-Green on Colour- Vision 



equation changed for him in the same way as for the 

 normal-sighted*. After fatigue with the red light, lie required 

 only 4 scale divisions instead of 8. It is quite obvious that 

 this was not a case of partial red-blindness from the point of 

 view of the three-sensation theory, though he was not as 

 sensitive to the red end of the spectrum as the normal sighted. 



The White Equation and Colour- Blindness. 



The colour-blind have been classified by some as red or 

 green-blind, in accordance with their white equations — that 

 is, the amount of pure spectral red, green, and violet required 

 to match a simple white ; those who put too much red in the 

 equation being classed as red-blind_, and those who put too 

 much green in the equation being classed as green-blind. 

 There are, however, many who, whilst agreeing with the 

 normal equation, are quite satisfied when a considerable 

 additional amount of green or red is added to the equation. 

 This explains why in certain cases some have been described 

 as red-blind by one observer and green-blind by another. 



A remarkable fact, which does not seem to have been 

 previously observed, is that many colour-blind persons who 

 strongly object to the normal match, but are satisfied with 

 an anomalous equation, will completely agree with the 

 normal equation when the comparison white light is increased 

 in intensity so that it is much too bright to a normal-sighted 

 person. This clearly shows that the normal mixed white 

 produces the same effect as far as colour is concerned, but 

 has a more powerful effect as to luminosity. This is in 

 complete accordance with other observations, and is found in 

 those cases in which there is abrupt and slight shortening 

 of the red end of the spectrum. If there be shortening of 

 the red end of the spectrum which does not affect A, 6 70 ////-, 

 and \670/jL/jL has its normal light value, the mixed light will 

 be more luminous than the simple white in proportion to the 

 shortening. This portion of red light not producing any 

 effect has to be subtracted from the white light. These 

 facts are quite inconsistent with a hypothetical red sensation 

 which is affected by light of all wave-lengths. Another 

 illustration may make this point clear. A man with 

 shortening of the red end of the spectrum and normal 

 colour discrimination will put together as exactly alike a 

 pink and a blue or violet much darker. If, however, the 

 pink and blue be viewed by a normal-sighted person through 

 a blue-green glass which cuts off the red end of the spectrum, 



* Proc. Eoy. Soc, B. vol. xcii. p. 232 (1921). 



