TRANSACTIONS OF SECTION D. 737 



these confusions have all their explanation in the fact that the red-green blind 

 liave only two colour sensations — yellow and blue — with a grey band in what 

 should have been the green part of his spectrum. 



We have now to show you another and far more beautiful method of ascer- 

 taining what fundamental colour sensations are absent in the colour-blind. It is 

 the method of testing them by what Cbevreul long ago termed simultaneous contrast. 

 If in a semi-darkened room we throw a beam of coloured light on a white 

 screen and interpose an opaque object in its path, the shadow shows the comple- 

 mentary colour. If the light be red, the shadow appears green-blue; if it be green, 

 the shadow appears purple or red according to the nature of the green light em- 

 ployed. If the light is jellow, the shadow is blue ; if it is blue, the shadow is 

 yellow. We must remember that the part of the screen on which the shadow falls 

 is not entirely dark; a little diffuse light falls on the retina from the shadowed 

 part, so that the retina and vision centre are slightly stimulated, where the 

 image of the shadow falls. We use an oxyhydrogen lantern and transmit the light 

 through plates of coloured glass carefully selected. 



The experiment is rendered still more striking, though at the same time a 

 little more complicated, by using two oxyhydrogen lamps and throwing their light 

 on the same portion of the screen. If a plate of coloured — say ruby — glass is held 

 before one of the lamps, and au opaque object such as the head of a T-square is 

 placed in the path of both lights, the shadow cast by the white light falls on a 

 surface illuminated by a red light, and shows a deep red far more saturated than 

 the surrounding surface of the screen where the red and wliite lights fall. The 

 shadow cast by the red light shows the complementary bluish green ; and the con- 

 trast of the two is exceedingly striking. When we use a plate of pure green glass, 

 the shadow showing the inducing colour is a saturated green, the other shows the 

 complementary purple. With yellow glass one shadow is deep yellow, the other 

 pale blue. With blue glass one shadow is saturated blue, the other pale yellow. 

 AViih pink glass the complementary green is deeper than the shadow showing the 

 inducing pink, and is on that account very striking. 



These experiments which we have shown you point to some subtle physiological 

 relations between complementary colours. A colour sensation produced in one 

 part of the vision apparatus forces, so to speak, the neighbouring part, which is 

 relatively quiescent, to produce the complementary colour subjectively. I 

 say vision-centre rather than retina, because, if one eye is illuminated with 

 coloured light while the other eye is feebly illuminated with white light, the com- 

 plementary colour appears in the centre Ijelonging to that eye. The sense of white 

 appears to be a mysterious unit}' ; if you ohjectively call up one part of the sensa- 

 tion, you call up its counterpart subjectively. If a colour and its complementary 

 counterpart be both displayed objectively at the same time, the action and reaction 

 of effect afford a sensation far more agreeable than is producible by the objective 

 display of only one of them. The agreeableness of the contrast of complementary 

 colours, no doubt, springs from the harmony of effect. There is no harmony of 

 colour effect analogous to that of music, but there is harmony of a different kind, 

 and that harmony is formed by the contrast of complementary colours. 



Now I imagine many of you have already anticipated the question. What infor- 

 mation can simultaneous contrast give regarding the fundamental sensations of 

 the colour-blind ? From an extended series of observations Dr. Stilling,^ of 

 Cassel, has ascertained that if a person cannot distinguish between red and green, 

 no complementary colour appears in the shadow when the inducing light is red or 

 green, but if the inducing light is yellow or blue the proper complementary appears 

 in the shadow. If a person was blind to red, he never found the complementary 

 green appear; if he was blind to green, he never found the complementary red 

 appear. When the inducing light appeared colourless, the shadow was also 

 colourless. Stilling therefore concluded that either the sensations of red and green 

 or of blue and yellow were wanting at the same time or all colour sense was absent. 



' J. Stilling, 'The Present Aspect of the Colour Question,' .4w,'(U-« of O^hthal- 

 molpyy, 1879, viii. p. 164. 



1892. 3 B 



