346 



NATURE 



[August ii, 1892 



which I have already referred, there is a narrow zone in which 

 red and green sensations are entirely wanting, while blue and 

 yellow sensations are normal. Possibly the red-green defect is 

 due to an imperfectly developed colour sense in the portion of the 

 vision centre connected with that zone of the retina, but Hippel's 

 case seems to me to show that such defect might be on the retina. 



It has probably already struck you that red-green blindness is 

 really blindness to red, green, and violet, that Young's three 

 primary sensations appear to be absent, and the two remaining 

 colours are those which he regarded as secondary compounds of 

 his primaries. 



That, however, is not all that is revealed by colour-blindness. 

 There is at least another well-known though rare form in which 

 a sense of yellow, blue, and violet is absent, and the only colour 

 sensations present are red and green. The defect is sometimes 

 termed violet blindness, but the term is somewhat misleading. 

 It is much more in accordance with the fact to term it yellow- 

 blue blindness ; indeed, we would define it precisely by terming 

 it yellow-blue-violet blindness. Holmgren ^ has recorded a uni- 

 lateral case of that defect analogous to Hippel's ca5e of uni- 

 lateral red-green defect ; we therefore know definitely how the 

 spectrum appears to such a person. In the case referred to all 

 the colours of the spectrum were seen with the normal eye, but 

 to the other eye the spectrum had only two colours, red and 

 green. The red colour extended over the whole left side of the 

 spectrum to a neutral band in the yellow-green, a little to the 

 right of Fraunhofer's line D. All the right side of the spectrum 

 was green as far as the beginning of the violet, where it " ended 

 with a sharp limit (about the line G)." 



If you turn to the Report of the Royal Society's Committee - 

 on Colour Vision, you will find the spectrum as it appears to 

 yellow-blue-violet blind persons. The plate agrees with the 

 description of Holmgren's case already given ; but you will not 

 find a representation of the spectrum as it appears to those 

 who are red-green blind, and as described by Pole and 

 others. In place of it you will find two dichromic spectra, 

 one with a red and blue half said to be seen by a green blind, 

 the other with a green and a blue half said to be seen by a red 

 blind person. We have copied the spectra for your inspection, 

 and you will observe that yellow does not appear in either of 

 them. I do not for a moment pretend to criticize these spectra 

 from any observations of my own ; I am aware Holmgren 

 maintains that red-and-green blindness may occur separately ; 

 but, on the other hand. Dr. George Berry, an eminent ophthal- 

 mologist, has assured me that he has always found them associ- 

 ated. That statement was originally made by Hering. 



Of the various methods of testmg colour vision, that suggested 

 by Seebeck is most commonly employed. The individual is 

 mainly tested with regard to his sense of green and red. He is 

 shown skeins of wool, one pale green, another pink or purple, 

 and a third bright red, and he is asked to select from a heap of 

 coloured wools, laid on a white cloth, the colours that appear 

 to him to match those of the several tests. We have arranged 

 such test skeins for your inspection, and have placed beneath 

 each of them the colours which a red-green blind person usually 

 selects as having hues similar to those of the test. It is startling 

 enough to find brown, orange, green, and grey confused with 

 bright red ; pale red, orange, yellow, and grey confused with 

 green ; blue, violet, and green confused with pink ; but these 

 confusions have all their explanation in the fact that the red- 

 green blind have 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 ascertaining what fundamental colour sensations are 

 absent in the colour-blind. It is the method of testing them by 

 what Chevreul long ago termed simultaneous contra%t. 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 complementary 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 employed. If the light is yellow, 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 



' F. Holmgren, " How do the Colour-Blind see the Diflferent Colours ? " 

 Proc. Roy. Soc. Lond., i88r, vol. xxxL p. 306. 

 = See Reference 8, Plate I., No. 4. 



NO. II 89, VOL. 46] 



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

 whereby the image of the shadow. 



The experiment can be rendered still more striking, though 

 at the same time a little more complicated, by using two oxy- 

 hydrogen 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 an 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 sur- 

 rounding surface of the screen where the red and white lights 

 fall. The shadow cast by the red light shows the comple- 

 mentary bluish green ; and the contrast of the two is exceedingly 

 striking. 



These experiments 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 complementary colour appears 

 in the centre belonging to that eye. The sense of white appears 

 to be a mysterious unity ; if you objectively call up one part of 

 the sensation, 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 sensa- 

 tion far more agreeable than is producible by the objective dis- 

 play 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 information can simultaneous contrast give 

 regarding the fundamental sensations of the colour-blind ? 

 From an extended series of observations Dr. Stilling, ^ of 

 Cassel, has entertained 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 complemen- 

 tary 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 comple- 

 mentary 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. It is difficult to see how these results are to be 

 harmonized with the conclusions arrived at by the Committee of 

 the Royal Society. 



Facts such as these are regarded by some as lending support 

 to the theory of colour sense proposed by Prof. Hering, of 

 Prague.- He supposes that the diversity of our visual percep- 

 tions arises from six fundamental sensations constituting three 

 pairs — white and black, red and green, yellow and blue. The 

 three pairs of sensations are supposed to arise from chemical 

 changes in three visual substances not confined to the retina, but 

 contained also in the optic nerve and in the vision centre.^ He 

 imagines that a sense of white results from decomposition induced 

 in a special visual substance by all visible rays, and that the 

 restitution of the same substance produces a sense of black. 

 The sensations of the red and green pair are supposed to arise, 

 the one from decomposition, the other from restitution of a 

 second substance ; while yellow and blue are supposed to result 

 from decomposition and restitution of a third substance. From 

 our knowledge of photo-chemical processes we can readily sup- 

 pose that light induces chemical change in the visual apparatus ; 

 but that the wave-lengths in the red and yellow parts of the 

 spectrum induce decomposition, while the wave-lengths in the 

 green and blue induce restitution of substances, it is difficult to 

 believe. How such a visual mechanism could work it would be 

 difficult to comprehend ; for example, if we look at a bright red 

 light for a few moments and then close our eyes, the sensation 

 remains for a time, but changes from red to green and then 

 slowly fades away. According to Hering's theory, the green 



'J. Stilling, "The Present Aspect of the Colour Qae^sxioTi," Archives of 

 OJihihalmolog^y, 1879, viii. p. 164. 



2E. Hering, ZtirLehrevom Lichtsinne, 2nd ed. Vienna, 1878. 



3 Hering, ibid., p. 75. 



