Capt. W. de W. Abney. 



[Feb. in. 



green or the red, the matching of a compound colour with a simple 

 one in the spectrum should possess no difficulties. Taking the trichro- 

 matic theory of three sensations for the normal-eyed person, it is 

 evident that only the following classes of sensations are possible in 

 the normal-eyed, the green colour-blind, and the red colour-blind : 



Normal eye. 



Red 



Green 



Violet 



Mixtures of red 



and green 

 Mixtures of red 



and violet 



Mixtures of green 



and violet 

 Mixtures of red, 



gieen, and violet 



Green colour-blind. 

 Red 



Green. 



Violet. . Violet. 



Red colour-blind. 



Mixtures of 

 and violet. 



red 



Mixtures of green 

 and violet. 



If we take as a type of colour-blindness the green colour-blind 

 person, we see that every colour in the spectrum must be either red, 

 violet, or these colours mixed with more or less white light, since 

 these two sensations when e-xrited in certain proportions give the 

 sensation of white. At one place, which is commonly called th 

 neutral point, these proportions are such that there is the impression 

 of white light ; it follows that, between this neutral point and each end 

 of the spectrum, the rays are mixtures of violet and white or red and 

 white, the dilution of the colours varying from no white to all whit 

 As every compound colour must be a mixture of the same two colours ii 

 certain proportions, it follows that the green colour-blind person 

 match every compound colour with some one ray of the spectrum, 

 and that every colour must to him be either red or violet, diluted 

 with different proportions of white light. 



In the same way, a person who is colour-blind to the red 

 match any colour with a single spectrum colour, and he will see it as 

 green or violet diluted with more or less white light. This can be 

 readily understood, but it is not quite so plain how any colour sensa- 

 tion felt by the normal eye can be referred to the spectrum. 



The following is an outline of the reasoning which leads up to 

 method of registration employed : 



If we take three rays in the spectrum one in the red between 

 and the red lithium line, which we will call R, another in the green 

 between F and b, which we will call G, and a third in the violet neur 

 G, but on the H side of it, and which we may call V then, by vary- 

 ing their intensities (which is equivalent to varying the luminosities) 



