122 HUMAN PHYSIOLOGY 



Theories of Colour Vision. 1. From consideration of the 

 peripheral colour blind zone of the retina and of the more 

 limited area giving sensations only of blue and yellow when 

 stimulated, and of the most limited central part giving also 

 sensations of red and green, it would seem that some special 

 substance or substances must exist in each of these areas 

 which by its or their stimulation give rise to the various 

 sensations. 



2. The phenomenon of complemental colours suggests the 

 possibility of there being one substance which when under- 

 going one change, say breaking down, produces blue, and 

 when undergoing another change, say building up, produces 

 yellow, and another substance which when undergoing one 

 change produces red, and another change produces green; 

 or that there are four different substances, one when changed 

 giving rise to yellow, another to blue, another to red, and 

 another to green. When the substance giving the sensation 

 of yellow is used up, then the parts stimulated by the rest of 

 the spectrum would react to white light and give a com- 

 plemental colour, and so on through the other substances. 

 If such a view be correct, it becomes almost necessary to 

 postulate the existence of another substance which when 

 stimulated gives rise to sensations which we call white. 



It has also been suggested that the facts may be explained 

 on the assumption that there are three substances in the 

 retina, one more especially stimulated by the red rays but 

 also acted on by the others, one chiefly stimulated by the 

 green rays, and one chiefly acted on by the blue rays. Such 

 theories, however, do not call for consideration from the 

 ordinary student. 



Colour-blindness. While every one is colour blind at the 

 periphery of the retina, a certain proportion of people about 

 5 per cent. are unable to distinguish reds and green, even 

 at the centre of the retina. Individuals who manifest this 

 condition are often able to name colours fairly accurately, 

 but when asked to match a piece of red wool from a number 

 of others, they tend to put beside it green wools. The con- 

 dition is of great importance to engine drivers and seamen. 

 (Experiment.) 



Colour blindness for yellow and blue is very rare. 



