1898.] 



on the Theory of Colour Vision, &c. 



803 



colours that we can use, for evidently simplicity is a great desidera- 

 tum, and the number agreed upon must settle the number of separate 

 photographs required. 



This brings us to the question of how we see colour, and how 

 many sensations of colour we have. I am not going into the de- 

 batable ground of rival colour-vision theories, but I am going to 

 adopt for to-night that one which will answer every practical purpose, 

 and that is the Young theory, in which it is held, and held correctly, 

 that a red, a green, and a blue sensation are alone needful to produce 

 the sensation of any other hue by admixture one with another. The 

 fundamental colour sensations are not necessarily identical with 

 any particular colour, but as a matter of fact, at all events one of 

 these sensations is to be found excited singly in the spectrum, 

 viz. that which is excited by the extreme red. The extreme violet 

 seems to be a compound of two sensations, one a deep blue and the 

 other red, so that the pure blue and also the green sensations can 

 never be singly stimulated in the normal eye. The diagram shows 

 these sensations as curves representing the stimulations by the 

 spectrum colours of the seeing apparatus in the retina (Fig. 1). The 

 scales on each curve are so adapted that when the ordinates of the 

 sensation curves are equal we get white. To get a yellow, the red 

 sensations and green sensations are equally stimulated, for there the 

 curves cut. It will be seen that the purest green sensation is largely 

 mixed with white, for at one point, where the red and blue curves 

 cut, the green curve is above them. At that point, then, the red and 



m 



Fig. 1. 



blue, and a certain portion of green, go to form white, and the balance 

 is green, so here the pure green sensation is diluted with white. At 

 any other point it is mixed with some other sensation, either red or 

 blue, and also up to certain points with white. Of course, if we 

 could get three colours which only stimulated respectively the three 

 fundamental sensations, we should take three appropriate photo- 

 graphs of the spectrum and illuminate them with those three colours. 



