1823, and Croonian lecturer to the college in 1822 and 1823. During the winters of 1809 and 1810 he delivered at the Middlesex Hospital two courses of lectures on medical science and the practice of physic, afterwards (1813) embodied in his work on medical literature. Like his lectures at the Royal Institution, they were too condensed to attract many students, and were only sparsely attended. On 24 Jan. 1811 he was elected physician to St. George's Hospital, London, a position he retained till his death. In 1814, at the request of the admiralty, he reported on a proposed change in the method of building ships (Phil. Trans. 1814). During the same year he became a member of a committee of the Royal Society appointed at the request of the secretary of state to investigate the risk attending the proposed general introduction of gas in London. The results of the experiments made by the committee removed all fear of danger. In 1816 he was appointed secretary of a commission for ascertaining the length of the seconds pendulum. This commission subsequently recommended the establishment of the present ‘imperial gallon’ of ten pounds of water.
In November 1818 Young was appointed superintendent of the ‘Nautical Almanac’ with a salary of 100l., and secretary of the reconstituted board of longitude, with a further salary of 300l. per annum. His opinion that the ‘Nautical Almanac’ should, as in the past, supply only information of importance in navigation, brought him into conflict with many astronomers of the day, who wished it to be carried out on the more liberal scale of the ‘Ephemerides’ published abroad. When in 1828 the board of longitude was dissolved, and the admiralty assumed its functions, Young, as superintendent of the ‘Nautical Almanac,’ was appointed an adviser to the admiralty, and the agitation for reform was resumed. His death put an end to the contest, and on the report of a committee of the Astronomical Society appointed to consider the matter, the ‘Nautical Almanac’ took its present form.
In 1814 Young retired from practice as a physician, having been appointed inspector of calculations to the Palladium Insurance Company at a salary of 500l. per annum. Within the next two years he published several papers dealing with life assurance.
During two visits to Paris in 1817 and 1821 he had met most of the distinguished French scientists, and was elected on 6 Aug. 1827 one of the eight foreign members of the French Academy of Science. In 1828 he visited Paris on his way to Geneva, where his strength commenced to show signs of decay. He sank gradually and died on 10 May 1829, at his house in Park Square, to which he had removed in 1826. He was interred at Farnborough, Kent. He left no issue.
He married, on 14 June 1804, Eliza (1785–1859), second daughter of James Primrose Maxwell of 69 Harley Street, London, and Tubbendens, Kent. The marriage was a remarkably happy one, and the relations between Young and his wife's family were always most affectionate. Mrs. Young's brother was Captain George Berkeley Maxwell, R.N. (1791–1854).
Young has been justly called ‘the founder of physiological optics’ (Tscherning). He was the first to prove conclusively that the accommodation of the eye for vision at different distances was due to change of curvature of the crystalline lens (Phil. Trans. 1793). His opinion that the lens itself was muscular has, however, not been confirmed by more recent work. His memoir ‘On the Mechanism of the Eye’ (ib. 1801) contained the first description and measurement of astigmatism, and a table of optical constants of the eye in close agreement with modern determinations. He first explained colour sensation as due to the presence in the retina of structures which respond to the three colours, red, green, and violet respectively (Lectures, i. 139, 440), and colour blindness as due to the inability of one or more of these structures to respond normally to stimulus (ib. ii. 315). Young's theory has been supported and extended by Helmholtz; and although a rival theory due to Hering is regarded with favour by many physiologists (Foster), there are phenomena unfavourable to that theory (Tscherning; Burch).
Of other contributions connected with his profession two of the most noteworthy are the Croonian lecture to the Royal Society ‘On the Functions of the Heart and Arteries’ (November 1808, Phil. Trans.), in which the laws regulating the flow of blood through the body are clearly stated; and its predecessor, ‘Hydraulic Investigations’ (May 1808, Phil. Trans.), on which it depends. His work on ‘Medical Literature’ (1813) was the most complete of its kind for many years, and reached a second edition (1823); while his ‘Practical and Historical Essay on Consumptive Diseases’ (1815) was a condensed account of all that was then known on the subject.
When Young began to write on physical optics, the wave theory of light (Huyghens, Traité de la Lumière, 1690) had made little headway against its rival the emission theory. Young supported the wave theory in his
