VISUAL SENSATIONS 577 



wave-lengths, or successively at such short intervals of time that there is complete 

 fusion of the sensations resulting from the individual excitations. In order to deter- 

 mine fine differences in shade a coloured surface is always provided with which a colour 

 produced by the fusion of the different rays under experiment may be compared : 



(1) The most exact method of mixing colours is to employ a couple of spectra 

 and by means of prisms to bring different parts of the spectra on one and the same 

 white surface on which the result of the mixture can be compared with sample colours. 



(2) In Maxwell's ' colour top ' discs with a radial slit are placed one over the other 

 on a disc which can be made to revolve with considerable rapidity. By means of the 

 slit two or three discs of different colours can be slid one into the other, so that the 

 disc is composed of sectors of variable extent of the two or three colours which we wish 

 to mix. These discs are generally mounted on a background of a larger disc, which 

 can be used as a sample colour to compare with the result of the mixture of the colours 

 in the centre. If we are determining the relative amount of different colours necessary 

 to produce white, the outside disc would be partly white and partly black, so that on 

 rotation the effect of grey is produced, i.e. a weak white. Thus in one experiment the 

 small discs in the centre were red, green, and blue. It was found that when the sectors 

 were chosen in the following proportion : 165 red, 122 green, and 73 blue, a grey colour 

 was produced equal to the grey obtained in the outer disc by mixing 100 white with 

 260 black. 



These methods and all others in which coloured surfaces are used suffer 

 from the defect that no pigments give perfectly pure colour-sensations. 

 On looking at a red painted surface, for instance, in a bright light with a 

 spectroscope, it will be found that the spectrum contains yellow and green 

 rays as well as red. On this account no information as to the effect of mixing 

 colour- sensations can be obtained by mixing the pigments themselves. Thus 

 a familiar way of producing a green pigment is to mix a blue and a yellow 

 pigment. A mixture of blue and yellow rays will give a sensation of white. 

 The fact that blue and yellow pigment mixed together give a green pigment 

 is due to the fact that the blue pigment cuts off the red and yellow rays, and 

 reflects, or allows to pass, the blue and green rays, while the yellow pigment 

 retains the blue and violet rays, but reflects the red, yellow, and green rays. 

 When we mix the two we get, not an addition, but a subtraction ; the blue 

 pigment absorbs the red rays which the yellow pigment allows to pass, and 

 the yellow pigment absorbs the blue rays which the blue allows to pass. 

 The only rays left over are the green, and the mixture of pigment has a green 

 colour. 



THEORIES OF COLOUR VISION 



These facts show that in all probability the primitive colour- sensations 

 are few in number, and that the various sensations excited by the different 

 parts of the spectrum are not simple, but are compounded of mixtures of these 

 primary sensations. The two theories which have obtained most vogue, and 

 which enable us to account for a certain number of the phenomena associated 

 with colour- vision, are those known as the Young-Helmholtz theory and the 

 Hering theory. 



(a) THE YOUNG-HELMHOLTZ THEORY. This theory, which was 

 first put forward by Young and elaborated by Helmholtz, assumes that 

 there are three primary colour-sensations red, green, and violet which 

 are represented by three separate sets of elements in the retina, or in each 



19 



