386 THE BODY AT WORK 



colours say, red, green, and violet we obtain, when they are 

 duly mixed, not white light only, but light of any other tint, 

 although not of spectral purity, since it is mixed with white. 

 Young considered that all the conditions of colour- vision would 

 be satisfied, all our various sensations provided for, if the 

 retina contain three kinds of apparatus which light, according 

 to its quality, affects in varying degrees ; and with this theory 

 of three kinds of apparatus a, b, and c the theory of three 

 elementary or fundamental colour-sensations is indissolubly 

 linked. The colour x produces its intensest effect when a is 

 stimulated, with the least possible stimulation of b and c ; y is 

 the reaction of b } z of c. Recent studies of the curves of 

 intensity give us the tints of x, y, and z as carmine-red, apple- 

 green, and ultramarine blue. 



The blending of sensations is illustrated with the well-known 

 colour-top. But perhaps the most striking proof that three 

 elementary colour-sensations are adequate to produce our 

 visual world is afforded by photographs taken with the three- 

 colour method. Three plates are exposed (a) behind a red 

 screen, (b) behind a greenish-yellow screen, (c) behind a blue 

 screen. They are fixed in such a way that the portions acted 

 upon by light are rendered insoluble, whereas the rest of the 

 film can be dissolved away ; a is then stained red, b greenish 

 yellow, c blue. The three are superposed, and the result 

 appears to the eye as an exact reproduction of the subject 

 of the photograph in all its hues. It shows every shade of 

 orange and green and violet. It is as bright that is to say, 

 as full of white light as the original. 



Various objections may, however, be brought against 

 Young's theory. Of these, the most weighty are : (1) The retina 

 does not contain three kinds of apparatus, as Young sup- 

 posed; nor can we find three kinds of photochemical substances, 

 as required by the theory in its modern form. If we could 

 find them, a fresh difficulty would arise ; for we have no 

 reasons for supposing that one and the same nerve-ending can 

 receive stimuli of three different kinds. (2) The theory offers 

 no explanation of negative after-images the complementary 

 colours experienced when the eye is closed after staring at a 

 brightly coloured object. (3) It does not adequately account 

 for the various deficiencies of colour-blindness. 



