;;, COLOUR VISION. 



three colour* b, as you see, nearly white. Let us try the effect of mixing 

 two of those colours. Red and blue form a fine purple or crimson, green and 

 blue form a sea-green or sky-blue, red and green form a yellow. 



Here Again we have a fact not universally known. No painter, wishing to 

 produce a fine yellow, mixes his red with his green. The result would be a 

 TT dirty drab colour. He is furnished by nature with brilliant yellow pigments, 

 and he takes advantage of these. When he mixes red and green paint, the 

 red light scattered by the red paint is robbed of nearly all its brightnee 

 getting among particles of green, and the green light fares no better, fur it 

 is sure to fall in with particles of red paint. But when the pencil with which 

 we paint is composed of the rays of light, the effect of two coats of colour 

 is very different. The red and the green form a yellow of great splendour, 

 which may be shewn to be as intense as the purest yellow of the spectrum. 



I have now arranged the slits to transmit the yellow of the spectrum. 

 You see it is similar in colour to the yellow formed by mixing red and gi 

 It differs from the mixture, however, in being strictly homogeneous in a ]hy 

 point of view. The prism, as you see, does not divide it into two por 

 as it did the mixture. Let us now combine this yellow with the blue of the 

 spectrum. The result is certainly not green ; we may make it pink if our 

 yellow is of a warm hue, but if we choose a greenish yellow we can produce 

 a good white. 



You have now seen the most remarkable of the combinations of colours 

 the others differ from them in degree, not in kind. I must now ask you to 

 think no more of the physical arrangements by which you were enabled to see 

 these colours, and to concentrate your attention upon the colours you saw, that 

 is to say on certain sensations of which you were conscious. We are here 

 surrounded by difficulties of a kind which we do not meet with in purely 

 physical inquiries. We can all feel these sensations, but none of us can des 

 them. They are not only private property, but they are incommunicable. \\ > 

 have names for the external objects which excite our sensations, but not for 

 the sensations themselves. 



When we look at a broad field of uniform colour, whether it is really 

 simple or compound, we find that the sensation of colour appears to our 

 sciousness as one and indivisible. We cannot directly recognise the elemen 

 sensations of which it is composed, as we can distinguish the component notes 



