VISUAL SENSATIONS. 



773 



reduce the matter to still further simplicity. Starting from the fact men- 

 tioned a short time since, that all color sensations, including the sensation of 

 white, may be obtained by the appropriate mixture of three standard sensa- 

 tions, this theory teaches that our visual apparatus is so constituted as, when 

 excited, to give rise to three primary sensations, and that these primary sen- 

 sations are called forth in different degrees by different rays of light, so that 

 each ray gives rise to a different mixture of the three. Several sets of three 

 such primary sensations might be chosen, which would satisfy the conditions 

 of giving rise, by appropriate mixture, to all sensations of color, including 

 white ; but for reasons into which we cannot enter fully here, the sensations 

 which may thus be taken as primary sensations appear to correspond to our 

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

 color sensations is represented in the diagram (Fig. 176). Thus the red 



FIG. 176. 





ROY Gr. Bl. V 



Diagram of Three Primary Color Sensations, 1, is the so-called " red ;" 2, "green,' and 3, 

 " violet " primary color sensation. R, 0, Y, etc., represent the red, orange, yellow, etc., color of 

 the spectrum, and the diagram shows by the height of the curve in each case, to what extent 

 the several primary color sensations are respectively excited by vibrations of different wave- 

 lengths. 



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 onward toward 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 toward 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 deter- 

 mined by the proportionate amount of the three sensations. Thus the sen- 

 sation of orange ( 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 ex- 

 cited 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. 



In the view, as originally put forward by Young, the three primary sen- 

 sations 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 awakening 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 



