Young suggested that colour vision is given by neural mixture from three retinal channels, tuned to different but overlapping spectral bands. The notion was developed by Helmholtz into the Young–Helmholtz trichromatic theory of colour vision, which is now generally accepted as an account of the first stage of colour analysis. Young also was the first to measure (on his own eyes) astigmatism, which he (wrongly) accounted for by supposing the eye's lens to be tilted. He showed by experiments (correctly) that accommodation to different distances is given by changing curvature of the anterior surface of the lens, and not by changes of curvature of the cornea.
He studied medicine at London, Edinburgh, Göttingen, and Cambridge, and he practised in London, but his main work was in research at the newly founded Royal Institution, where he became professor of natural philosophy in 1801. His optical experiments set the wave theory of light and phenomena of interference on a sound experimental and theoretical basis.
Young's classic paper 'On the theory of light and colours' was delivered to the Royal Society, as the Bakerian lecture, on 12 November 1801, and printed in 1802 (Philosophical Transactions, 92). His argument for only a few (three) primary colours is based on the consideration that visual acuity is almost normal in coloured light:
Now, as it is almost impossible to conceive each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation, it becomes necessary to suppose the number limited, for instance, to the three principal colours, red, yellow, and blue, of which the undulations are related in magnitude nearly as the numbers 8, 7, and 6; and that each of the particles is capable of being put in motion less or more forcibly, by undulations differing less or more from a perfect unison; for instance, the undulations of green light, being nearly in the ratio of 6½, will affect equally the particles in unison with yellow and blue, and produce the same effect as a light composed of those two species: and each sensitive filament of the nerve may consist of three portions, one for each principal colour.
In later papers, Young changed his 'principal' colours to red, green, and violet. The concept of 'principal' or 'primary' colours is a treacherous one, and he never fully grasped its multiple ambiguities. He was not, however, ensnared by one common error, that of supposing that the retinal receptors must have their greatest sensitivities at wavelengths which subjectively appear to us as 'pure' or 'unmixed' hues; but he did fall into a second common error, that of supposing that the three spectral regions that are conveniently used as primaries in colour-mixing experiments are those that produce maximal activity in the three kinds of receptor. We now know that the cone receptors have their peak absorbances in the violet, the green, and the yellow–green parts of the spectrum (see colour vision). A light that looks red is not a light that optimally excites the last of these three types of cone; rather it is a light that produces the maximum ratio of absorptions in the last two of these types, and such a light lies at a wavelength much longer than the wavelength of peak sensitivity of either of them. This realization did not come until the second half of the 19th century, and to this day the term 'primary colour' continues to obstruct the proper understanding of colour among non-specialists.
Young's A Course of Lectures on Natural Philosophy and the Mechanical Arts (1807; new edition edited by Kelland in two volumes in 1845) is a useful source. His complete papers were edited in three volumes in 1854 by Peacock and Leitch: Miscellaneous Works of Dr. Thomas Young.
(Published 1987)
— Richard L. Gregory
- Bibliography
- Morse, E. W. (1976). Entry in Gillespie, C. C., Dictionary of Scientific Biography, 14.
- Wood, A. (1954). Thomas Young: Natural Philosopher. (A Life, with a full bibliography of Young's books and papers.)
