THE MIXTURE OF PURE-COLOUR STIMULI 35 



Complications arise when we pass beyond these limits. If a yellow 

 is mixed with a blue-green the resultant mixture, though resembling 

 a pure intermediate colour, does not match it perfectly. The match 

 is made perfect by adding a certain proportion of white light to the pure 

 spectral intermediate. In other words the mixture is paler, or less 

 saturated, than the spectral match. As the distance between the mixing 

 colours is increased the saturation becomes continuously less, until 

 finally at one distance two colours are obtained which, when mixed, 

 yield a sensation of white, free from all trace of colour sensation. Such 

 colours are called complementary colours. 



The following are v. Helmholtz's estimates of the wave-lengths for 

 certain complementary colours : 



Observations have also been published by v. Frey and v. Kries^, 

 Konig and Dieterici^, Angier and Trendelenburg^ and others^. If these 

 results are plotted as curves with wave-lengths from 400-500 /x^u, as 

 abscissae and wave-lengths from 560-680 [ifx as ordinates the curves 

 nearly resemble hyperbolae, but differ slightly from each other. The 

 differences have been attributed in part to macular pigmentation. 

 Krarup"^ has re-investigated the subject and finds that the comple- 

 mentary colours change somewhat as the intensity of the illumination 

 is altered. There is no change due to this cause from 460 to 480 i^fM, but 

 a gradual increase from that point up to 512 /x/x. With a suitable, 

 relatively low, illumination the curve is a rectangular hyperbola. 



The ratio of the quantities of Aj to X.,, where Xj and A.^ are the wave- 

 lengths of two complementary colours, is approximately constant and 

 independent of the intensities of illumination. Glan^ came to the con- 

 clusion that the energies of \ and X. at the percipient retinal structures 



^ Arch. f. Anat. u. Physiol. 336, 1881. 



^ Wied. Ann. xxxiii. 1887; in Konig, p. 2()1. i 



^ Ztsch. f. Psychol, u. Physiol, d. Sinncsorg. xxxix. 284, 1905. i 



1 Hclmholtz, 3rd ed., ii. p. 107. 



^ Physisch-ophlhalmologische Grenzprvbleme, p. 100, Leipzig, 1906. i 



" Arch. J. d. ges. Physiol, xxix. 53, 1886; Wied. Ann. xlviii. 1893. ! 



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