Tlie Perception of Colour. 33 



will be all the greater if it throws a clearer light on, or 

 gives a simpler explanation of the phenomena, than the 

 current theory. MM. Landolt and Charpentier have 

 shown {Gazette Medicale, 10, 1878), that before any 

 colour is recognised for what it is, a variety of phases are 

 passed through, the first being a simple luminous sensation ; 

 and that gradually the chromatic character of the light is 

 perceived. It has also been long known that a different 

 length of time is required for the perception of different 

 colours, red requiring the longest time. On the theory 

 of Young, it is not easy to see why this should be the 

 case; why a nerve termination, specially adapted for the 

 perception of one colour, should respond more slowly to the 

 stimulus of that colour than a second nerve termination 

 does to another colour, by which alone it is acted on. On 

 the photo-chemical theory it meets with a simple expla- 

 nation in the varying action of different rays on the 

 pigmentary matter of the retina, red light transforming it 

 most slowly. In the same way when we take the remarkable 

 abnormality of vision, known as Daltonism, the superiority 

 of the photo-chemical hypothesis is apparent. In the vast 

 majority of cases red is the colour which is not seen, there 

 being cases in which very intense red can be detected, but 

 not duller shades. On Young's theory this is to be 

 explained only on the supposition that one of the three 

 new elements, whose existence is postulated, is awanting, 

 or has whoUy or partially lost its excitability ; but no 

 explanation is afforded of the fact, that it is almost always 

 the element susceptible to red which is thus defective. On 

 the hypothesis of photo-chemical action the explanation is 

 much simpler and more easily acceptable. The least refran- 

 gible (red) ra^ys have least action on the pigment of the 

 retina, even when isolated ; they are also normally absorbed 

 in great proportion by the transparent media of the eye ; 

 and it is only necessary to suppose a slight increase of that 

 resistance to their passage to account for their total absorp- 

 tion, the same increase of resistance having a slighter effect 

 on the more refrangible rays. In this way the partial or 

 total blindness to red would be accounted for, the perception 

 of other colours being inappreciably impaired. 



There is another point which at first seemed to throw 

 serious difficulty in the way of this view of the mechanism 

 of the production of impressions of colour. The retinas of 

 most birds and reptiles have none of this retinal colour, and 



P 



