36 COLOUR VISION 



must be equal in order that white may be perceived. This law is not 

 true, but Krarup points out that the ratio of the energies of \i to \o at 

 the retina is independent of the intensities of illumination. The ratio 

 is not constant, still less equal to unity, but if the energy ratios calculated 

 from Angier and Trendelenburg's quantities (slit-widths) be plotted as 

 ordinates, against wave-lengths as abscissae a symmetrical curve re- 

 sembling a parabola and having its apex at about 608 //./x results. 



Ebbinghaus^ states that the brightness of the resultant white is 

 equal to the sum of the brightnesses of the constituent complementary 

 colours. Most observers have found that the white is brighter. 



Complementary spectral colours have seldom the same luminosity, 

 i.e. they do not look equally bright. The nearest approach to equal 

 luminosity is orange, 607'7 ^/x, and blue, 489"7 fifji. Colour mixing shows 

 that the order of saturation of spectral colours diminishes from violet 

 successively through indigo-blue, red and cyan-blue, orange and green, 

 yellow (v. Helmholtz). 



It will at once be observed that there is a range from about 560 to 

 492 ju/i, i.e. green, which possesses no spectral complementary. White 

 can only be obtained from green by mixing it with both red and violet, 

 i.e. purple. 



There are hues with which we are familiar in pigments, etc., that 

 do not at first sight fall into any of the categories mentioned. Of these 

 the most striking is brown. Brown, olive green, and greys possessing 

 some coloured hue are obtained by mixing black with a spectral colour 

 or mixture of colours. Further evidence will accumulate in the course 

 of our discussions in favour of the view that black is an actual and 

 effective stimulus. 



We can now return to the unfinished colour table or diagram, of 

 which only two rectilinear portions have as yet been mapped out. It 

 has been pointed out that the graph must be a closed figure in one plane 

 and that the various points upon it follow the law of the centre of 

 inertia of masses. 



If three colours, neither of which can be obtained from a mixture 

 of the other two, are represented by three points on a plane, then 

 assigning to them values in terms of any unit, the situations and 

 quantitative values of their mixtures can be ascertained. The problem 

 is well stated by Greenwood^. " In order to establish the correctness 

 of this method it is necessary to prove that, given the experimental 



^ Ztsch.f. Psychol, u. Physiol, d. Sinnesorg v. 176. 1893. 

 ^ Physiology of the Special Senses, London, 1910, p. 131. 



