Sept. 1 8, 1879] 



NATURE 



479 



received, but he has unfortunately vitiated his whole 

 work by relying to a large extent on the naming of 

 colours by the patients. This is the very worst and most 

 fallacious test of all, and Dr. Jeffries quotes, in reference 

 to it, the following forcible remarks by Helmholtz : — 



"As to the examination of the colour-blind, simply 

 asking them to name this or that colour will naturally 

 elicit but very little, since they are then forced to apply 

 the system of names adapted to normal perception to 

 their own perception, for which it is not adapted. It is 

 not only not adapted, because it contains too many 

 names, but in the series of spectral colours we designate 

 differences of tone [hue ?] as such, which to the colour- 

 blind are only variations of saturation or luminosity." 



My own experience enables me thoroughly to corrobo- 

 rate this : if any one asks me to name a colour shown 

 me, I tell him it would be as reasonable to treat me as 

 a clairvoyant and to expect me to read the contents of a 

 sealed envelope. 



But these inquiries, as above stated, always have to do 

 I with the theories of colour-blindness, and a few words 

 must be said on this point, which is one of great diffi- 

 culty, and in regard to which the state of knowledge is 

 at present exceedingly unsatisfactory. When Dalton 

 wrote what may be considered the first good account 

 of the defect, he, notwithstanding his great acuteness and 

 his extraordinary powers of scientific investigation, failed 

 to discover the important point of his case, namely, that 

 he saw two colours only — yellow and blue. This was 

 found out for him at a much later period by the pene- 

 tration of Sir David Brewster and Sir John Herschel, 

 who designated the malady by the term, dichromic vision 

 which has ever since been used. Sir David Brewster, 

 acting on this, framed a very simple explanation of the 

 defect, founded on his own views as to the nature of 

 colours. He had a theory that the solar spectrum was 

 formed from three separate spectra overlapping each 

 other, one giving red light, one yellow, and one blue, 

 which might therefore be considered the thi-ee primitive 

 colours for normal eyes, as it was taken for granted all 

 other colours might be compounded from them. All that 

 was necessary for the explanation of colour-blindness was 

 to assume the eye of the patient insensible to the red rays, 

 and the phenomena followed as a matter of course. This 

 theory was a very plausible one, and is still in favour with 

 many persons who have practically to do with colour. 

 But unluckily on further examination it was found want- 

 ing, inasmuch as one of the main effects in it, namely, 

 the supposed production of green by a mixture of yellow 

 and blue, turned out to be a delusion ; and moreover, as 

 the theory of light became better known, the idea of 

 overlapping spectra was abandoned, it being clear that 

 every hue of colour had its own peculiar generating wave. 

 Hence Brewster's elegant and simple explanation of 

 colour-blindness fell to the ground. 



Some years afterwards came out what is called the 

 "Young-Helmholtz" theory, whichassumes that the normal 

 visual organs are capable of being impressed with three 

 colour sensations, corresponding to red, green, and violet, 

 and that all colour-perception is caused by the combined 

 action of these in varying proportions. It is then assumed 

 that in colour-blind people one of these sensations is 

 wanting, leaving only the other two in action, and thus 

 causing dichromic vision. The most common defect is 



supposed to be blindness to red, and on this hypothesis 

 the colour-blind ought to see only violet and green. At 

 the same time the supporters of this theory fancy they 

 can detect some cases where the green isvifanting, leaving 

 only red and violet, and others where the violet is 

 wanting, leaving visible only the red and the green. 



This theory is in great favour, owing to the eminence 

 of its authors and the support of many distinguished 

 physicists ; and it is adopted implicitly by Dr. Jeffries. 

 But objections have been raised to it on several grounds, 

 one of the most forcible being that it does not accord 

 with the experience of the colour-blind. If there is any 

 one fact more unequivocally deducible from their evidence 

 than another, it is that the less refrangible colour they 

 perceive corresponds to yellow, and not to green. In my 

 own case, which I believe is a typical one, my long-wave 

 colour is most vivid and positive, and it is an absolute 

 certainty that its maximum splendour is excited by the 

 buttercup, or by the pigment chrome-yellow, or by the 

 sodium line ; whereas objects that I hear called green 

 give me no definite impressions at all ; sometimes they 

 assume a debased, dirty, or washed-out buttercup colour ; 

 sometimes they look black or grey ; and sometimes they 

 even give my opposite sensation, blue. How, therefore, 

 it can be argued that my most brilliant buttercup sensation 

 is excited by green objects rather than by yellow ones, is 

 to me unintelligible. 



A theory has lately been started by a Belgian savant, 

 that the colour-blind defect is caused by an undue sensi- 

 tiveness to green, which destroys the proper effect of other 

 colours ; to illustrate which he says that the normal eye, 

 by looking through a certain green solution, will become 

 colour-blind. But he carries his theory to the further 

 length of asserting that if a colour-blind person looks 

 through a certain red solution, he will be restored 

 to normal vision, a conclusion which is so improbable 

 that we may dismiss the theory from consideration, par- 

 ticularly as it has found no supporters. 



There is, however, another hypothesis lately offered, 

 which has a very different aspect. It was laid before the 

 Academy of Sciences of Vienna a few years ago by Herr 

 Ewald Hering, Professor of Physiology at Prague. Its 

 scope is considerably wider than has to do with our present 

 purpose, as it embraces the whole physiological theory of 

 the perception of light, and it would be out of the question 

 to give a complete account of it here. It is, however, 

 of such great importance, and has been so favourably 

 received by some of the highest authorities, that it may be 

 worth while to devote a future article in Nature to its 

 description. Meantime it may be briefly stated that the 

 author assumes, not three fundamental colour-sensations, 

 as in the Young-Helmholtz theory, but (excluding black 

 and white, for which he provides separately)yt^//r, namely, 

 blue, yellow, red, and green.' 



These, however, result Irom only two sources of sensa- 

 tion, each of which is capable of a double, or reversible, 

 mode of excitement (in a manner somewhat analogous to 

 positive and negative in electricity, or plus and minus in 

 algebra), producing the sensation of two colours comple- 

 mentary to each other. Thus, one of the sources of sen- 



* In the description of my own case, published in the Philoiophtca^ 

 Transactions Uir 1859, I ventured to express the view that the a>^t>iiniption 

 of these four colours as fundauieni.tl, was necessary in order to explain satis- 

 factorily the phenomena of coloitr-blindness. 



