Rods and Cones, and Thomas Young'S Theory of Colour Vision

Abstract

MR. E. N. WILLMER'S recent communications^{1, 2} on the physiology of colour vision once more direct attention to the essential principles involved in Thomas Young's theory. As is well known, it is on purely logical grounds that Young in 1801 came to the conclusion that the existence of three kinds of receptors having different spectral sensitivities— as we would say to-day—should account for the facts of normal colour vision³. At the basis of the theory lies⁴ the principle of the 'specific energy of nerves', formulated by J. Müller⁵ in 1840, according to which each nerve fibre can send only one kind of sensory message to the brain. This would seem to lead to the conclusion that there must be as many different kinds of receptors as there are colours; but Young's well-known hypothesis allows this number to be reduced to three. For the abnormal colour vision of such persons as his contemporary Dalton, Young himself suggested as an explanation "the absence or paralysis of those fibres of the retina which are calculated to perceive red"⁶; that is, dichromats would possess only two colour receptors instead of three. To-day the strength of this theoretical argument referring to the whole retina remains undiminished^{7, 8, 9, 10}.