CHAP, ii.] 



SIGHT. 



529 



sensations of red, green, and blue or violet. Such a view of three 

 primary colour sensations is represented in the diagram (Fig. 72). 

 Thus the red primary sensation, excited to a certain extent by the 

 rays at the extreme red end, is most powerfully affected by the 

 rays at a little distance from the end, the rays from this point 

 onwards towards the blue end producing less and less effect. The 

 curve of the green primary sensation begins later and reaches its 

 maximum in the green of the spectrum, while the blue or violet 

 primary sensation is still later and only reaches its maximum 

 towards the blue end of the spectrum. Each ray calls forth each 

 sensation but to a different degree, and the total result of each ray, 

 or of each group of rays, is determined by the proportionate amount 

 of the three sensations. Thus the sensation of orange (0 in the 

 figure) is brought about by a mixture of a great deal of the primary 

 red with much less of the primary green, and hardly any of the 

 primary blue ; the orange sensation is converted into a yellow 

 sensation by diminishing the primary red and largely increasing 

 the primary green, the primary blue undergoing also some slight 

 increase. And similarly with all the other sensations. When 

 each of the primary sensations is excited to a maximum, as when 

 ordinary light falls on the retina, the result is a sensation of white. 

 According to this theory, black is simply the absence of sensation 

 from the visual apparatus. 



FIG. 72. DIAGRAM OF THREE PRIMARY COLOUR SENSATIONS. 



1 is the so-called ' red,' 2 'green, 'and 3 ' violet ' primary colour sensation. R,0, Y, &c. 

 represent the red, orange, yellow, &c., colour of the spectrum, and the diagram 

 shews, by the height of the curve in each case, to what extent the several 

 primary colour sensations are respectively excited by vibrations of different 

 wave-lengths. 



In the view, as originally put forward by Young, the three 

 primary sensations were supposed to be represented by three sets of 

 fibres, each set of fibres being differently affected by different rays of 

 light, and the impulses passing to the brain along each set awaken- 

 ing a distinct sensation. No such distinction of fibres can be found 

 in the retina ; but an anatomical basis of this kind is not necessary 

 for the theory ; we can easily conceive of the same fibre trans- 

 p. 34 



