CH. LV. ] 



COLOUR VISION. 



777 



Thus, the orange and yellow between the red and green are 

 mixtures of the red and green sensations ; the blue a mixture of 

 green and violet ; and the purples (which are not represented in 

 the spectrum) of red and violet. 



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 aiid orange on Maxwell's disc give white ; but it is well known that 

 a mixture of blue and orange paint gives green : how can one 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 and 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 selec- 

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

 sensations were performed 

 in this way : the eye under- 

 goes 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 

 sensations could not be produced by any combination of other 

 colour sensations. 



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 rods or cones which answer to each of these primary colours, 

 whereas the innumerable intermediate shades of colour are pro- 

 duced by stimulation of the three primary colour terminals in 



Fig. 592. Diagram of the three primary colour 

 sensations. (Young-Helmholtz theory.) i. is 

 the red ; 2, green, and 3, violet, primary colour 

 sensation. The lettering indicates the colours 

 of the spectrum. The diagram indicates by 

 the height of the curve to what extent the 

 several primary sensations of colour are ex- 

 cited by vibrations of different wave lengths. 



