COLOUR VISION AND COLOUR-VISION THEORIES 4 75 



the red and green cancel each other, and only the green is left 

 (Hering) : that the yellow was the original substance, and has 

 become split up in the course of development into a red and a 

 green substance, and when green and red are mixed they com- 

 bine into the original substance yellow (Ladd Franklin) : that 

 in previous stages of development all men saw the yellow 

 region as red-green, but when a new colour yellow had replaced 

 the red-green of a previous stage of development a mixture of 

 red and green gave rise to the colour yellow which had replaced 

 the red-green of the previous stage (Edridge-Green). Now it 

 will be noticed that these three last hypotheses all explain the 

 facts better than the trichromatic theory, which does not, like 

 the others, explain why red and green should make yellow and 

 not red-green, especially in view of the fact that it does make 

 red-green to some persons. Many physicists confuse the mixing 

 of objective lights with the mixing of physiological sensations. 

 This particularly applies to those who state that the trichromatic 

 theory is not a theory, but a fact ; not only is it not a fact, but 

 only a possible explanation of certain facts, and, as will be 

 shown in the remainder of this article, there is not a single fact 

 which directly supports it, but very many which show that it 

 cannot be true. Those writers who state that three-colour 

 photography: or three-colour printing are based on the tri- 

 chromatic theory ought to state that they are based on the facts 

 of colour mixing. In the case of mixing pigments the primary 

 colours are different, certain yellow and blue pigments when 

 mixed make green instead of white, which is the case when pure 

 spectral light is used. The reason of this can be made plain by 

 examining a yellow and a blue glass with a spectroscope : it will 

 be noticed that the yellow lets through orange and green as 

 well as yellow rays, and the blue lets through green and violet 

 as well as blue rays. If the two glasses be super-imposed, and 

 then placed before the spectroscope, it will be noticed that only 

 the green rays get through, the others being stopped by the 

 combination of the two glasses. 



II. The Simple Character of the Yellow Sensation (3, 4) 



If the trichromatic theory were a fact, it should be possible 

 to show that the sensation excited by pure spectral yellow is a 

 composite sensation, and there should be evidence of its alleged 

 components. Every fact, however, shows that yellow is a 



