798 



THE EYE AND VISION 



[CH. LV1. 



Join the three angles red, green, and violet, and one gets white 

 light ; or join the blue and orange, which comes to the same thing, 

 and one also gets white. 



Blue and orange on Maxwell's disc give white ; but it is well known that .1 

 mixture of blue and orange paint gives green ; how can on* explain this ? Suppose 

 the paint is laid on white paper ; the white light from the paper on its way to the 

 eye passes through transparent particles of blue ami orange pigment ; the blue 

 particles only let the green and violet sensations reach the eye, and cut off the red ; 

 the yellow particles only let the red and green through, and cut off the violet. The 

 red and violet being thus cut off, the green sensation is the only one which reaches 

 the eye. 



The experiments which led Helmholtz and others to the selection 

 of green, red, and violet as the three fundamental colour sensations 

 were performed in this way : the eye undergoes exhaustion to a 

 colour when exposed to it for some time ; suppose, for instance, the 



eye is fatigued for red, and is 

 then exposed to a pure yellow 

 light, such as that given off by 

 the sodium flame, the yellow 

 then appears greenish ; or fatigue 

 the eye for green and then expose 

 it to blue, the blue will have a 

 violet tint. By the repetition 

 of numerous experiments of this 

 kind, it was found that the 

 fatigue experienced manifested 

 itself in three colours, red, green, 

 and violet, which were therefore 

 selected as the three fundamental 



colour sensations. It was also found that these three colour sensa- 

 tions could not be produced by any combination of other colour 

 sensations, and further that all other colour sensations can be 

 obtained by mixing these three in various proportions. 



The theory of colour vision constructed on these data was 

 originated by Thomas Young, and independently discovered and 

 elaborated by Helmholtz. It is consequently known as the Young- 

 Helmholtz theory. This theory teaches that there are in the retina 

 certain elements (? cones) which answer to each of these primary 

 colours, whereas the innumerable intermediate shades of colour are 

 produced by stimulation of the three primary colour terminals in 

 different degrees, the sensation of white being produced when the 

 three elements are equally excited. Thus, if the retina is stimulated 

 by rays of certain wave length, at the red end of the spectrum, the 

 terminals of the other colours, green and violet, are hardly stimulated 

 at all, but the red terminals are strongly stimulated, the resulting 

 sensation being red. The orange rays excite the red terminals con- 



FIG. 593. Colour triangle. 



