‘Experiments on Sound and Light’ (January 1800, Phil. Trans.), and next year (Nicholson's Journal, August 1801) extended the conclusions he had drawn in that paper ‘on the coalescence of musical sounds’ to the ‘interference’ of light. A more detailed account of his doctrine of ‘interference’ and its applications appeared in his brilliant memoir ‘On the Theory of Light and Colours’ (Bakerian Lecture, November 1801, Phil. Trans.), which marks an epoch in the history of the subject. In it he showed that the colours of thin and of thick plates, of striated surfaces, and those seen at the edge of the shadow of an obstacle, could all be explained by the interference of light undulations which had traversed different paths, and concluded with the proposition ‘Radiant light consists of undulations of the luminiferous ether.’ Other phenomena were explained in two subsequent papers (July 1802, Phil. Trans.; Bakerian Lecture, November 1803, Phil. Trans.) The vital importance of Young's work was, however, not understood, and the three memoirs met with severe and unjust criticism at the hands of Henry Brougham [q. v.] in the ‘Edinburgh Review’ (Nos. ii. and ix. 1803). The critic could find in them ‘nothing which deserves the name either of experiment or discovery,’ considered them ‘destitute of every species of merit,’ and admonished the Royal Society for printing such ‘paltry and unsubstantial papers.’ Young's masterly reply was published in the form of a pamphlet (London, 1804), which, remaining almost unknown, did nothing to counteract the effect produced by these unfortunate assertions; and the principle of interference remained unnoticed till fourteen years later it was rediscovered by Fresnel.
A further advance was made by Young in 1809, when he showed (Quarterly Review, ii. 344) that the variation of the index of refraction of a uniaxal crystal, which the emission theory had been unable to explain satisfactorily, was on the wave theory a simple consequence of the elasticity of the crystal being different in different directions. The idea thus introduced was developed by Fresnel into a complete theory of double refraction (1821).
Dispersion in transparent media was explained by Young (Theory of Light and Colours, Prop. vii.) as due to the oscillations of the material particles set in motion by the ether vibrations, affecting the latter to an amount depending on their frequency. This explanation has been extended by Sellmeyer, Helmholtz, and others into complete theories of dispersion for absorbent media.
The phenomena exhibited by polarised light had proved too difficult of explanation by either the emission or the wave theory, although Young had suggested (Quarterly Rev. April 1814) that the colours produced by the passage of polarised light through crystalline plates were due to interference of the two polarised rays into which the crystal divided the incident light. When in 1816 Arago and Fresnel showed that two rays polarised at right angles to each other would not interfere, Young pointed out immediately that this implied that the vibrations of light were transverse to the ray. Next year he showed (‘Chromatics,’ Encycl. Brit. 6th edit.; Works, i. 335) that the law of Malus for the intensities of the two rays into which a crystal divides polarised light incident on it, was a consequence of the transverse nature of the vibrations, and in a few years, principally by the work of Fresnel and Arago, most of the phenomena of polarisation had been explained on the wave theory.
In his ‘Essay on the Cohesion of Fluids’ (December 1804, Phil. Trans.) Young gave in non-mathematical language the theory of capillary action soon after and independently (1805) brought forward by Laplace, and now known by his name. In this essay Young for the first time accounted on physical grounds for the constancy of the angle of contact of a solid and a liquid.
He was the first to use the term ‘energy’ for the product of the mass of a body into the square of its velocity, and the expression ‘labour expended’ (work done) for the product of the force exerted on a body into the distance through which it is moved, and to state that these two products were proportional to each other (Lectures, i. 78–9). He introduced absolute measurements in elasticity by defining the ‘modulus’ (Young's modulus) as the weight which would double the length of a rod of unit cross-section to which it was hung (Lectures, i. 137). He agreed with Rumford [see Thompson, Benjamin], Pictet, and Sir Humphry Davy [q. v.] as to the impossibility of any ‘material’ theory of heat (November 1801, Phil. Trans.), and held that it consisted of vibrations of the particles of bodies, ‘larger and stronger than those of light’ (Lectures, i. 654).
Young's ‘Theory of the Tides,’ given first in his ‘Lectures’ (p. 576), then in ‘Nicholson's Journal’ (1813), and more completely in the ‘Encyclopædia Britannica’ 6th edit. (1823) (Works, ii. 291), explained more tidal phenomena than any other theory till (Sir) George B. Airy's article on ‘Tides and Waves’ appeared in the ‘Encyclopædia Metropolitana,’ vol. v. (1844).
Young contributed to the supplement to
