842 THE EYE AND VISION [CH. LVIII. 



left of the red are less refrangible, being the calorific rays which act 

 upon the thermometer, and those to the right of the violet, which 

 are called the actinic rays, have a powerful chemical action. 



White light may be built from its constituents in several ways, 

 for instance, by a second prism reversing the dispersion produced by 

 the first, or by causing the colours of the spectrum to fall on the 

 retina in rapid succession. The best way to study the effects of 

 compounding successive colour stimuli is by means of a rapidly 

 revolving disc to which two or more coloured sectors are fixed. 

 Each colour is viewed in rapid succession, but owing to the per- 

 sistence of retinal impressions, the constituent colour stimuli give a 

 single sensation of colour. 



A colourless sensation can be produced by the mixture of three 

 colours, or even of two colours in certain hues and proportions. 

 These pairs of colours, of which red and greenish-blue, orange and 

 blue, and violet and yellow are examples, are called complementary. 



Thus blue and orange, when rotated on the colour-wheel, produce a colourless 

 sensation ; but it is well known that a mixture of blue and orange paint gives green. 

 This is explained on the supposition that the colours used are not pure and that 

 each contains green ; the true blue and orange present neutralise each other to 

 produce white, and thus green is the only colour sensation obtained. 



Three properly chosen colours will not only produce a colourless 

 sensation, but when combined in appropriate amounts they can be 

 made to yield any other colour sensation. It is on this principle 

 that Thomas Young based his trichromatic theory of colour vision, 

 which was subsequently elaborated by Helmholtz and Clerk-Maxwell. 

 It is known as the Young-Helmholtz theory. The theory selects 

 red, green, and violet as the three primary colour-sensations. These 

 three particular colours are chosen, partly because of their position 

 within the spectrum, partly on account of the phenomena of colour- 

 blindness, and for other reasons. 



The Young-Helmholtz theory teaches that there are in the retina 

 certain elements (? within the 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 spec- 

 trum, 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 considerably, the green rather more, and the violet slightly, 

 the resulting sensation being that of orange, and so on (fig. 536). 



Another theory of colour vision (Hering's) supposes that there are 

 six primary colour-sensations, viz. : three antagonistic (complemen- 



