The nature of normal trichromatic and dichromatic vision

Abstract It is shown that if the dichromatic isocolour lines, plotted on any colour diagram of the usual type, are extended, they will either meet at a point which will represent a missing funda­mental sensation, or they will be parallel to a line joining the points representing the two fundamental sensations which are fused. Using these properties it is further shown that protanopia is caused by the absence of the R' sensation, that tritanopia is caused by the absence of the B' sensation, and that deuteranopia is caused by the R' and G' sensations being identical. The B' and R' points determined by these measurements and the G' point as determined by Walters but slightly modified to suit the particular requirements set by the line R'G', have been used to calculate the fundamental sensation curves. Luminosity, colour-mixture, and hue-discrimination data for both dichromatic and trichromatic observers and to a lesser extent trichromatic hue-discrimination and saturation data are satisfactorily explained in terms of these response curves; such an explanation leads to the conclusion that the Young-Helmholtz theory of colour vision is fundamentally correct. The essential difference between the B' sensation and the G' and R' sensations is stressed.