Chap, xxvi.] THE YOUNG-HELMHOLTZ THEORY. 343 



produce the sensation of colour, which is, therefore, 

 not an objective fact. It is the mixture, in varying 

 intensities, of three fundamental sensations, that of 

 red, that of green, and that of violet, that gives the 

 sensation of varying colours. 



Another error must be guarded against. The 

 mixture of red, green, and violet pigments will not 

 give white, but this is not a mixture of colours. If a 

 beam of white light be passed through a plate of red 

 glass, and then through one of blue, the result will be 

 almost darkness, because we have absorption of rays. 

 The red glass keeps back almost all rays, and transmits 

 only red to any extent ; but then these rays passing 

 through the green glass are retained, because it trans- 

 mits only green. Similarly with a mixture of powders, 

 the reflected light which gives the powder its colour 

 is that which comes from the deeper layers of the 

 powder after it has travelled through the upper layers, 

 and has, therefore, undergone absorption. 



Thus, in a mixture of two simple coloured pigments, 

 the apparent colour will be that due to the light which 

 has escaped the united absorption of the two, rather 

 than the colour due to a mixture of the two corre- 

 sponding colours of the spectrum. 



Young-If elmholtz theory. The view of three 

 fundamental colours is specially that of Thomas 

 Young, and has been advocated by Helmholtz. It 

 gave a means of explaining the facts of colour sen- 

 sations. Young supposed that in the retina there 

 were nerve-fibres readily affected by vibrations of the 

 rapidity of the red rays of the spectrum, another set 

 sensitive to the vibrations of the green, and a third set 

 sensitive to the vibrations of the violet. If the first 

 set was chiefly affected, the sensation was of red, if 

 the second set chiefly, the sensation was of green, and 

 so on. The vibrations corresponding to the yellow 

 of the spectrum, according to this theory, excite 



