ABSTRACT
IT has long been matter of common knowledge among psychologists that the colour-sensations which persist, in the ordinary cases of partial colour-blindness, are blue and yellow. 1 This was a requisite consequence of Hering’s theory and was predicted by him; it was proved by the first case of monocular colour-blindness which was observed—that of v. Hippel in 1880—and this proof has been abundantly confirmed by the cases which have been discovered since. But the theory of Young and Helmholtz apparently required that, when two colour-sensations only persisted, if one was blue the other must be either red or green. Now, the physicists (and most physiologists as well) too hastily took the Young-Helmholtz view as expressing fact and not theory, and they continued to infer (although Helmholtz himself had recognized the true state of the case) from the circumstance that the partially colour-blind had two sensations only, that these sensations were, in the ordinary cases, blue and red, or blue and green; and in accordance with this deduction they classified most cases of colour-blindness as red-blindness or green-blindness (without expressly stating that, in their view, in both cases, blindness to yellow was involved as well). There was absolutely no reason except the theory for affirming that the warm colour of the defective person was either red or green; all that was known was that it occupied that portion of the spectrum which, for the normal person, is occupied by red, yellow, and green. Nevertheless, it is stated in twenty textbooks that the sensations of the colour-blind furnish exceedingly strong, if not convincing, evidence of the truth of the Young-Helmholtz theory. Moreover, the belief that the warm colour is either red or green has become so ingrained that the cases by which it has been shown beyond question that it is in fact yellow have often failed to produce any effect. Abney and Festing, in the Philosophical Transactions, say that the examination of colour-blind persons is of prime importance for testing any theory of colour vision, and, nevertheless, they are content, like so many others, to infer the sensations of the colour-blind from a theory which they have already adopted.
