448 Proceedings of Royal Society of Edinburgh. [sess. 
The Theory of Colour Vision. By Dr W. Peddie. 
(Read June 1, 1903.) 
(Abstract.) 
The theories which give the best account of the facts of colour 
vision and colour blindness are the Young-Helmholtz theory and 
Hering’s theory. Both are trichromatic theories, and, apart from 
physiological or anatomical questions, both can, by proper choice 
of fundamentals, be made to give a good account of the main facts. 
The facts of one-eyed colour blindness show that, on the Young- 
Helmholtz theory, colour blindness must be regarded as due to 
fusion of at least two fundamental sensations, but the curves of one 
sensation determined by observations on different eyes differ con- 
siderably among themselves, and this indicates that a broader 
basis for the theory may be desirable. This may be sought for in 
a tetrachromatic theory, but any such theory must explain the 
possibility of trichromatic representation of all colours. A theory 
proposed assumes two pairs of complementary stimulations, say 
RjGx and G 2 ,V. Tn this respect it has a resemblance to Hering’s 
theory ; but whereas in Hering’s theory stimulation of one member 
of a complementary pair means no stimulation of the other member, 
in the proposed theory equal stimulations of two such members 
give white. It is shown that four sets of equivalent trichromatic 
fundamentals must exist. Assuming v. Helmholtz’s fundamentals 
as such a set, the four mathematically possible sets of tetra- 
chromatic equivalents of which no more than one can exist 
physically are deduced, and it is found that one of these does suit 
the known facts of colour vision and colour blindness. Choosing 
this set, the other three trichromatic equivalent sets (Helmholtz’s 
being the fourth) are deduced. The perceptibility curve (ordinates 
being differences of wave-length just appreciable to the eye in the 
spectrum) is given for one of these sets and compared with that 
given by v. Helmholtz’s set. The comparison is found to be very 
satisfactory. The nature of the tetrachromatic set shows that 
