﻿Colour- Vision in Relation to Colour-rBlindness. 917 



the luminosity curve is at the same point as t ho normal. A 

 striking case of this kind was examined recently— a dichromic 

 with shortening of the red end of the spectrum ; the apex of 

 his luminosity curve for the light of the Pointolite arc was 

 at \bS5fifjb, which is the apex for the normal-sighted. 



Explanation of the facts of Colour-Blindness. 



The facts of colour-blindness are quite inconsistent with 

 any three-sensation theory. Supporters of such an elemental 

 theory have in many cases contented themselves with 

 describing a case of colour-blindness in the terms of the 

 theory without showing that the ascertained facts are con- 

 sistent with the theory. No one, for instance, has shown 

 how on an elemental theory 50 per cent, of dangerously 

 colour-blind can get through the now obsolete wool test. 

 On the non-elemental theory the explanation is easy enough: 

 the man has defective colour discrimination, but not sufficient 

 to prevent him matching wools in favourable circumstances, 

 particularly when colour names are not used. On an 

 elemental theory, why should the trichromic mark out about 

 half the number of monochromatic divisions in the spectrum, 

 designate yellow as red-green, and have an increased simul- 

 taneous colour contrast? When there are three definite 

 colour sensations, how can colour-blindness be explained ? 

 The recent paper by Houstoun * should be read on this 

 point. 



The Anomalous White Equation without Colour-Blindness. 



Just as a man may make an anomalous Rayleigh equation 

 without any evidence of colour-blindness, so may a man 

 make an anomalous white equation without being colour- 

 blind f- As an example of this, a man was examined who 

 presented no sign of colour weakness. He passed my card 

 test, lantern test, and spectrometer with the ease and 

 accuracy ot an absolutely normal-sighted person. His 

 luminosity curye was taken by the flicker method, and 

 corresponded with the normal. The wave-length of the 

 apex of the luminosity curve was at 585/a/x, which is the 

 normal point. When, however, his white equation was 

 taken, he put only 8 scale divisions of green, instead of 13^ 

 or 14, which is normal, and the mixed light appeared red to 

 the normal-sighted. An important fact was noted — namely, 

 that after fatigue with red of the region of \670yit/x, the 



* Proc. Koy. Soc. Ediii. Vol. xlii. pt. i. no. 7, p. 75 (1922). 

 t Proc. Roy. Soc, B. vol. lxxxvi. p. 164 (1913). 



