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LXXVIII. Colour- Vision Theories in Relation to Colour- 

 Blindness. By F. W. Edridge-Gkeen, C.B.E., M.I)., 



F.R.C C S., Special Examiner and Adviser to the Board of 

 Trade on Colour Vision and Eyesight* . 



THE importance of colour-blindness as a key to any 

 colour-vision theory does not seem to be sufficiently 

 recognized, though the fact was well known to Helmholtz f, 

 who showed that Hering's theory explained the facts of 

 colour mixing quite as well as his own, and stated : " As far 

 as I see, there is no other means of deciding on the 

 elementary colour sensations than the examination of the 

 colour-blind. " This cannot be too widely known, because 

 any other method assumes that the three-sensation theory is 

 correct, and is useless when this theory is denied. As has 

 been shown by Houstoun %\ my non-elemental theory explains 

 the facts of colour mixing quite as well as either of the 

 above mentioned. Recently T have examined about 200 cases 

 of colour-blindness by colour-mixing methods as well as my 

 own, in order to ascertain certain facts of crucial importance 

 in deciding between an elemental and a non-elemental 

 theory. 



Relation of Luminosity to Colour. 



If the sensation of white were compounded of the addition 

 of three elementary processes and one of these processes 

 were subtracted, the position of the apex of the luminosity 

 curve would not be the same in the colour-blind as in the 

 normal. It is, however, well known that there are numerous 

 dichromics who have a luminosity curve similar to the 

 normal. 



A case of colour-blindness regarded from the point of 

 view of a three-sensation theory may, for instance, be one-half 

 red-blind ; the composition of the theoretical white will then 

 be ^R + lGr + lV. As far as luminosity is concerned, this 

 white may be compared with light of various wave-lengths 

 by the colour-blind subject, just as normal white is by the 

 normal sighted. Now, as the apex of the luminosity curve 

 depends upon the point where the aggregate stimulation of 

 the three theoretical sensations is greatest in terms of 

 luminosity, this apex will be displaced towards the point of 

 maximum stimulation of the other sensation, namely green, 

 the luminosity of the blue being so low as to be negligible. 

 Numerous cases can, however, be found in which the apex of 



* Communicated by the Author. 

 t Physio log ische Optik, 2nd edition, p. 377. 

 J Phil. Mag. vol. xxxviii. p. 402 (3919). 



