230 COLOUR VISION 



colour which is represented by the point of intersection with the curve. 



That this should be the case shows that the blue element is practically 



in abeyance. 



In experiments in matching white with mixtures of spectral rays 



it was found that about 2-8 per cent, of red or green could be added 



without being perceived. 



If we examine the sensation curves shown in the previous diagrams 



we shall find that, besides the maxima of each sensation, there are 



important points where the curves intersect. We will call the point 



where the R and G curves intersect a, R and B, b, and B and G, c. 



These points have been determined by Konig, Abney, and Exner 1 , and 



their values are as follows : 



abed 



Konig (sunlight) 573 503 496 450 



Abney (arc light) 577-2 515 500 



Exner (arc light) 577 508 494 475 



d is the complementary colour of a. From these data Konig and Exner 

 deduced the fundamental colour sensations : R, a purplish red (com- 

 plementary to 494 /z/z (Exner) ; G, 505 /z/z (Konig), 508 //,/z (Exner) ; 

 B, 470 /z/z (Konig), 475 /z/z (Exner); all, however, more saturated than 

 the spectra] colours. 



We have here the explanation of the variations in the discrimination 

 sensibility for hues in the spectrum (v. p. 30). In the equal area curves, 

 e.g. Fig. 64, equal ordinates at any point make white. If we subtract the 

 white in those parts of the spectrum in which all three sensations are 

 stimulated, we can obtain the ratios of the ordinates of the sensation 

 curves to each other throughout the spectrum. These ratios will vary 

 in different parts. In regions where the variation occurs most rapidly 

 we should expect the hues to change most rapidly. Calculations made 

 by Steindler on this basis from Exner's values show that the positions 

 of maximum change are at 500 /z/z, 570 590 /z/z, and 635 /z/z, of minimum 

 change at 470 /z/z, 530 ML, and 625 /i/z. These calculations agree remark- 

 ably well with the results of direct observation, which show that the 

 maxima of discrimination sensibility for hues were at II, 492 /z/z, 

 III, 581 /z/z, and IV, 635-5 /z/z, the minima being at 458 /z/z, 533 /z/z, and 

 627 /z/z (v. p. 31). 



This very striking confirmation of the Young-Helmholtz theory was 

 first pointed out by Konig 2 . 



The curves in Fig. 65 are derived by calculation from the same 



1 Site, d. Wiener Akad. cxi. iio, 857 1902. 2 Konig, p. 106. 



