112 SENSATIONS OF COLOUR. 



of being placed in linear series, in the order of the refran- 

 gibility of the rays which produce them. Those occasioned 

 by the simultaneous action of rays of different refrangi- 

 bilities are much more various, and cannot be arranged in 

 line. If, however, on a plane surface a central position is 

 assigned to the sensation " white," the other sensations may 

 be arranged round it in such a way, that the sensation 

 which results from the " blending " of any two or more 

 others has its place between them, at distances from each 

 which express their relative preponderances. In this 

 arrangement (called the colour circle) spectral colour sensa- 

 tions form an incomplete ring round white, between the red 

 and violet ends of which is placed purple. The relations 

 of colour sensations exhibited in the colour circle can be 

 most simply explained on the following hypothesis. (Young, 

 Helmholtz.) There are three fundamental colour-sensations, 

 viz. : red, green and indigo, from which all others are derived 

 by blending. Every element of the retina (every cone) 

 contains three terminal elements, one of which is most 

 excited by the less refrangible rays (red element), one by 

 the most refrangible (blue element), the other by those of 

 medium refrangibility (green element), but all more or less 

 by all. 



The theory that red, green, and indigo, are the fundamental sensations is 

 supported by the following observations, among others: I. If the spectrum 

 is contemplated while its colours are gradually weakened, until they cease to 

 be visible, the colours named are the last to disappear. (Briicke.) 2. If a 

 white surface is contemplated by a retina of which the excitability for a par- 

 ticular kind of homogeneous light has been weakened by excitation, it appears 

 to be coloured ; the colour sensation produced occupies a place in the colour 

 circle exactly opposite to that immediately occasioned by the excitation, and is 

 therefore said to be " complementary " to it. In like manner, if a coloured 

 surface is contemplated by a retina partially dulled by the same mode of 

 excitation, its hue is provided that its colour is a blended one altered, by the 

 weakening of one of its constituent sensations. But if after dulling the excit- 

 ability of the retina for red, an indigo surface is contemplated, its hue 

 remains unchanged, for inasmuch as, according to the theory, indigo rays 

 scarcely affect the red elements, the quality of the sensation is not affected by 

 their not taking part in its production ; if the experiment is repeated with a 



