564 



PHYSIOLOGY 



spectrum can be matched. As a rule the mixed colour is not so pure 

 as the corresponding spectral colour, being less saturated (that is diluted with 

 a certain amount of white light). The mixture of red and green is an excep- 

 tion because it is found that accurate matches with spectral yellow can be 



\ 



u 



COLOV* *,XTVRt PATCH 



SCXCEN 



*HIT C U6HT PATCH 



FIG. 290. Colour patch apparatus for mixture and comparison of pure spectral 



colours. (ABNEY-.) 



made if the green component be not shorter in wavelength than 5400 A.U. 

 These facts can be expressed diagrammatically in the form of a 

 geometrical figure, the colour triangle, in which the three fundamental 

 colours occupy the corners and white the centre (see Fig. 243). 

 Matches made by light of one intensity require readjustment if 

 the intensity be changed, and matches made by one observer are 

 different to those made by another. The variation with intensity is 

 readily explained by the shifting of the centre of the luminosity curve 

 from the yellow towards the green, as the intensity is lowered. 

 The amount of red required in a match will become increasingly greater, and 

 that of the blue less, as the intensity is lowered. The variation with the 

 observer, when small, is explained by individual peculiarity in the pigmenta- 

 tion of the eye media or the f ovea centralis ; but when considerable, by abnor- 



