THEORIES OF COLOUR-VISION. 



1107 



With predominant decomposition of one substance the sensation red 

 is experienced, of the others green and violet or blue. The colours inter- 

 mediate between these, and the purple series connecting the two ends of 

 the spectrum, depend on the relative amounts of decomposition of the three 

 substances. According to this theory, the white-black series of sensations 

 has no separate physiological basis, but arises when all three chromatic 

 substances are excited in an equal degree. Black is the sensation which 

 accompanies absent or subliminal decomposition of the substances. 



The primary colours. There has been much difference of opinion 

 as to the choice of the three primary colours. Those most commonly 

 adopted have been red, green, and violet. In addition to evidence 

 derived from colour mixture, their adoption has been supported by the 

 fact that, when the intensity of the spectrum is lowered, these three 

 colours remain visible after yellow and blue have disappeared. Don- 

 ders and Konig have attached importance to the existence at each end 



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FIG. 397. The curve c is the elementary curve of the sensation of the short-wave end of 

 the spectrum, and was approximately the same in both groups of red-green blindness. 

 The curve b is that of the sensation from the long- wave end in the scoterythrous group, 

 while the curve a is that of the photerythrous group. Konig and Dieterici. 



of the spectrum of a region showing only change in saturation and 

 no change of colour-tone, and they suppose that at each end only one 

 fundamental sensation is present, namely, red and violet. Maxwell 

 adopted blue as the third primary, and Exner l came to the same con- 

 clusion, on the ground that red, green, and blue were the colours least 

 changed in tone by fatigue. 2 More recently Helmholtz and Konig have 

 arrived at more exact conclusions as to the nature of the fundamental 

 sensations. 



Konig and Dieterici 3 distinguish between elementary and fundamental 

 sensations. By the former Konig means the smallest number of elements to 

 which it is possible to reduce the manifold colour sensations we experience, 



1 Arch.f. d. ges. PhysioL, Bonn, 1868, Bd. i. S. 373. 



2 On the ground of his fatigue experiments, described on p. 1059, Burch regards red, 

 green, blue, and violet as primary colour sensations. 



3 Ztschr.f. Psychol. u. Physiol. d. Sinnes&rg., Hamburg u. Leipzig, 1893, Bd. iv. S. 241. 



