COLOUR PERCEPTION IN PISH 23 



yellow and blue. He assumes the existence 

 of three visual substances which undergo 

 metabolic changes when subjected to the 

 action of light. These are the red-green, 

 the yellow-blue, and the white-black sub- 

 stances. The white-black substance is 

 influenced by all the rays of the spectrum, 

 while the red-green and yellow-blue sub- 

 stances are differently influenced by rays 

 of different wave-lengths. When all the 

 rays together fall upon the retina, no 

 metabolism takes place in the red-green 

 and yellow-blue substances, but only the 

 white-black substance is affected. Thus 

 the white-black substance is the most 

 active. 



Any discussion as to the relative value 

 of these theories would in this work be 

 out of place and unnecessary. 



The ordinary form of colour-blindness in 

 human beings is the inability to discrimi^ 

 nate between red and green. This shows 

 that the visual power of these people is 

 dichromatic and not trichromatic, as their 

 power is limited to two colours, or pairs of 

 colours, and does not extend to three. 



The individuals who belong to this class 



