TRANSACTIONS OF SECTION D. 735 



practical importance in relation to the correct otservation of coloured signals. In 

 1855 the late Professor George Wilson/ of this city, called attention to the grow- 

 ing importance of the subject. Some years ago Professor Holmgren made an 

 elaborate statistical inquiry regarding it at the instance of the Swedish Govern- 

 ment, and lately it has been investigated by a committee of the Koyal Society 

 of London, who have quite recently published their report.'- 



Although colour-blindness occasionally results from disease of the brain, retina, 

 or optic nerve, it is usually congenital. Total colour-blindness is extremely rare, 

 but partial colour-blindness is not uncommon. It occurs in about 4 per cent, of 

 males, but in less than 1 per 1,000 of females. Its most common form is termed 

 red-o-reen blindness, in which red and green sensations appear to be absent. So 

 far as I can find, the first fidl and reliable account of the state of vision in red- 

 green blindness is that given in 1859 by Mr. Pole,^ of London, from an examination 

 of his own case, which appears to be a typical one. The state of his vision is 

 dichromic ; his two-colour sensations are yellow and blue. The red, orange, 

 and yellowish-green parts of the spectrum appear to him yellow of different 

 shade's. . Greenish-blue and violet appear blue, and between the yellow and 

 blue portions of the spectrum, as it appears to him, there is a colourless grey 

 band in the position of the full green of the ordinary spectrum. This neutral band 

 is seen in the spectrum in all cases of dichromic vision. It may appear white or 

 orey according to the intensity of the light, and it apparently results from an 

 equilibrium of the two sensations : no such band is seen in the spectrum by a 

 normal eye. Mr. Pole, in the account of his case given now three and thirty years 

 ago, considered it impossible to explain his dichromic vision on the commonly 

 received theory that his sense of red is alone defective, and that his sense of yellow 

 is a compound of blue and green. He believed his green quite as defective as his- 

 red sensation, and that yellow and blue are quite as much entitled to be considered 

 fundamental sensations as red and green. He suggested that in normal colour 

 vision there are at least four primary sensations — red and green, yellow and blue. 

 Professor Hering is commonly accredited with the four-colour theory, but it was 

 previously suggested by Pole.^ 



A year after Pole's paper appeared, Clerk-Maxwell ° published his celebratod 

 paper on the theory of compound colours, to which he appended an account of his 

 observations on a case of what he believed to be red-blindness, but which we now 

 Imow must have been red-green blindness. The spectrum appeared dichromic, 

 its only colours being yellow and blue. His description of the case does not 

 materially ditter from that given by Pole ; but Clerk-Maxwell believed in the tri- 

 chromic theory of normal vision, and that red-green and blue are the three 

 primary sensations ; consequently he supposed that the yellow sensation of a red 

 blind person is not yellow, but green. 



It is evident that much depends on the question. Is the yellow sensation of a 

 red-green blind person the same as that of normal vision ? For many years it was- 

 impossible to give a definite answer to that question, but the answer can now be 

 given, as we shall immediately see. Colour-blindness is frequently hereditary, and 

 two or three cases are known in which the defective colour sense was limited to- 

 one eye, while in the other eye colour vision was normal. In such a case observed 

 by Professor Hippel, of Giessen, there was red-green bhndness in one eye. Holm- 

 gren, who examined Ilippel's case, has published an account of it.'^ With one eye 

 all the colours of the spectrum were seen, but to the other eye the spectrum had 

 only two colours with a narrow grey band between them at the junction of the- 

 blue and yellow. The yellow seen "by the eye with the red-green defect had a 



' Wilson, Resmrohcs on Colour-Uinclness, Edinburgh, 185.5. 



- ' Report of the Committee on Colour Vision,' Proc. Ruy. Soc. Zotid., Ju}j 1802.. 



•■• W. Pole, ' On Colour-blindness,' Phil. Trails., 18i>d, vol. cslix. p. 323. 



^ Ibid., p. 331. 



» Clerk-Maxwell, ' On the Theory of Compound Colours,' &c., PJnl. Tram., 1860, 

 vol. cl. p. 57. 



« F. Holmgren, 'How do the Colour-blind See the different Colours?' Proc. Poy. 

 Soc. Land., 1881, vol. xxxi. p. 302. 



