THE SENSATION OF SIGHT. 245 



In this way we may reduce all possible actual (ob- 

 jective) differences in colour, so far as they are appre- 

 ciated by the eye, to three kinds ; difference of hue (tone), 

 difference of fulness (saturation), and difference of amount 

 of illumination (brightness}. It is in this way that we de- 

 scribe the system of colours in ordinary language. But we 

 are able to express this threefold difference in another way. 



I said above that a properly constructed colour-disc 

 approaches a triangle in its outline. Let us suppose for 

 a moment that it is an exact rectilinear triangle, as made 

 by the dotted line in Fig. 34 ; how far this differs from the 

 actual condition we shall have afterwards to point out. 

 Let the colours red, green, and violet be placed at the 

 corners, then we see the law which was mentioned above : 

 namely, that all the colours in the interior and on the 

 sides of the triangle are compounds of the three at its 

 corners. It follows that all differences of hue depend 

 upon combinations in different proportions of the three 

 primary colours. It is best to consider the three just 

 named as primary ; the old ones red, yellow, and blue are 

 inconvenient, and were only chosen from experience of 

 painters' colours. It is impossible to make a green out 

 of blue and yellow light. 



We shall better understand the remarkable fact that we 

 are able to refer all the varieties in the composition of 

 external light to mixtures of three primitive colours, if 

 in this respect we compare the eye with the ear. 



Sound, as I mentioned before, is, like light, an undulat- 

 ing movement, spreading by waves. In the case of sound 

 also, we have to distinguish waves of various length which 

 produce upon our ear impressions of different quality. 

 We recognise the long waves as low notes, the short 

 as high-pitched, and the ear may receive at once many 

 waves of sound that is to say, many notes. But here 

 these do not melt into compound notes, in the same waj 



