92 



THEORIES OF COLOUR VISION. [BOOK in. 



for only a sensation corresponding to green would give white 

 when mixed with the other two sensations. Or again, choosing 

 green in the first instance as one of the primary sensations for the 

 reason that it stands apart from the others in its complement, 

 purple, not being a spectral colour, we may decide that the two 

 other primary sensations ought to differ as much as possible from 

 each other, and therefore choose red and blue rather than red and 

 violet since violet is obviously more allied to red than is blue , 

 indeed we may perhaps regard violet, on account of its relations 

 to red, as the beginning of a second spectrum the greater part of 

 which is invisible. The decision between these two forms of the 

 same theory rests on a number of considerations, into the dis- 

 cussion of which we cannot enter here. Unless we specially call 

 attention to the difference between them, which acquires import- 

 ance on certain occasions only, we shall treat them as identical, 

 and use the words blue and violet in this connection indifferently. 

 Such a view of three primary colour sensations is represented 

 in the diagram (Fig. 146). Thus the red primary sensation, 

 excited to a certain extent by the rays at the extreme red end, 



FIG. 146. 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. The 

 diagram illustrates, by the height of the curve in each case, how the several 

 primary colour sensations are respectively excited to different extents by 

 vibrations of different wave-lengths. But, in this, and also in Fig 147, the 

 curves are to be understood not as careful ,curves of actual variations in the 

 intensity of the several changes, but as simply serving to illustrate roughly 

 the nature of the theory. 



is most powerfully affected by the rays at a little distance from 

 that 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 primary sensation though to 

 a different degree, and the total result of each ray, or of each 

 group of rays, is determined by the proportionate amount of 



