CHAP. iii.J SIGHT. 105 



according to the Young-Helmholtz theory they ought to be wholly 

 unlike, since red is wholly unlike green. It is true that when 

 the curves of the red and green primary sensations are carefully 

 worked out they lie much closer to each other than does either to 

 that of the primary blue sensation ; but this does not do away 

 with the fact that subjectively the primary sensation of red must 

 be something quite different from that of green. Hence further 

 support is given to the former theory by the fact that while 

 there is no difficulty in finding out whether a person is colour- 

 blind or no, it needs much greater care to determine to which 

 class he belongs ; indeed, as seems easy on the one theory, but 

 difficult on the other, cases occur which by different observers are 

 placed now in the one class, now in the other. 



766. We have treated of the colour-blind as if they were 

 confined to those who confounded red with green. According 

 to the Young-Helmholtz theory, another class of colour-blind is 

 possible, the blue- or violet-blind, those who while possessing 

 red and green sensations lack the third, blue or violet sensation. 

 And indeed cases of such blue-blindness have been described ; but 

 none of them are free from doubt. The drug santonin has also 

 been said to produce violet-blindness ; the effect of the drug is first 

 to excite sensations of violet, and then annul them ; but careful 

 observations shew that vision under the influence of santonin 

 is not truly dichromic and that the effects of the drug therefore 

 are not due to its abolishing a primary sensation. We may 

 add that the peculiar effect of the drug is not due to any 

 coloration of retinal structures, but appears to be the result of 

 cerebral or at least of central changes. 



Lastly we may remark that absolute colour-blindness, a condition 

 in which shades of black and white alone indicate the features of 

 external objects, while possible on Hering's theory, is impossible 

 on the Young-Helmholtz theory. According to the latter a person 

 reduced to one primary sensation must see either red, green or 

 blue ; this one sensation is excited in him both by objects which 

 we called coloured and by objects which we call white. He 

 would probably call it white ; but it would be either red, 

 green, or blue. According to Hering's theory he might still see 

 white and black in the total absence of both the red-green and 

 the yellow-blue substance. A case has been recorded in which 

 only black and white were seen; it would be hazardous to 

 insist much on a single case and that one of obvious disease ; 

 but such a case if indubitably established would seem to afford 

 an almost complete refutation of the Young-Helmholtz theory. 

 It might indeed be reconciled to that theory by help of the 

 supposition that in such a case the primary sensations were not 

 wanting but shifted so to speak in their relative positions along 

 the length of the spectrum, brought into the same position on 

 the spectrum, so that each ray of light affected them all equally ; 



