On the Construction of a Colour Map. 



497 



other colours, as they can be indicated in both ways, occur on 

 both sides of the spectrum line. 



In using the term quantity of white light, I mean that a 

 beam of white light is to be obtained in some definite manner 

 from a definite source of light which forms the spectrum, and 

 that the map is to show how much of this beam is used. 

 Captain Abney finds that the positive pole of the electric arc 

 is a source of light of constant quality, and uses it in his 

 measurements ; and he indicates the quantity of white light 

 used by the ratio between its luminosity and that of the 

 spectrum colour. It is a more complicated matter to express 

 such a ratio than to express the amount of white light only, 

 and I failed to work into a map Captain Abney's method 

 of defining the quantity of white light. 



The principle on which this map is founded will come out 

 more clearly by the consideration of fig. 1, which may be con- 

 sidered as a sort of colour staff 3 to borrow a term from music. 



The three horizontal lines represent the three colour sensa- 

 tions — Red, Green, and Violet, with such luminosity that the 

 mixture represented by equal lengths of the three lines repre- 

 sents white light. Thus the vertical lines A, A', wherever 

 they may be placed, will include between them white light, 

 which will be the more intense the farther they are apart. 

 Any colour whatever may be represented by taking the line 

 A as a base and measuring off the quantities of the sensations 

 to points, R, G, and V. The distances included between A' 



Fig. 1. 



R G V 



Red. 

 Green. 

 Violet. 



Red. 

 Green. 



Violet. 



Red. 

 Green. 



Violet. 







{ i 











■ - 9 1 ■ 



1 



1 









A' S' 



Fig. 2. 

 E G V i 



J A 







» ■ 1 1 

 1 1 











J 









A' G 



Fig. 3. 

 V E 



S A 





i 



■t ■ w 



1 











| 







Red. 

 Green. 



Violet. 



Eed. 

 Green. 



Violet. 



Red. 

 Green. 



Violet. 



and R, G, Y give every complementary colour to that repre- 

 sented by R, G, Y, and A. The whole of the colours of this 

 system are related together only by the position with respect 



