YOUNG, THOMAS (1773-1829). A British scientist and arclueologist. He was born at Alilverton, in Somersetshire. of Quaker parents, and studied at a school at Compton. in Dorset shire, and under a tutor. Toward the end of 1792 lie began to study medicine in London, and afterwards at the University of Edinburgh. From Edinburgh he went to Giittingen, where lie took the degree of doctor of medicine, returning to England in February, 1797. In order to qualify himself for membership in the College of Physicians, then restricted to graduates of Oxford and Cambridge, he entered at Emmanuel College, Cambridge, as a fellow-commoner, where lie received his degree in 1799. In the year 1799 he took up his residence in London, and began to practice as a physician.
In 1801 be was appointed professor of natural philosophy in the Royal Institution, then newly established. Elected a fellow of the Royal So ciety in 1794, in 1802 he became its foreign secretary, a post which he retained till the end of his life. Ile resigned his professorship in 1803, fearing that his filling a chair of science interfere with his success as a physician.
Young was elected one of the physicians of Saint George's in 1811. He afterwards published several medical works. which, though they were little more than compilations, and are now forgotten, show that he was thoroughly versed in the history of diseases and of medical opinion. Of these the most important were Medical Litrrnture (1813) and Praclieu, and Historical rssau nn Consnmptier Diseases (1815). Ile retired from practice in 1811 on his appointment as inspector of calculations in the Palladium Life Insurance Company. He served in 1814 also on a commission appointed to investigate the dangers involved in the intro duction of illuminating gas in London, and in 1816 he was appointed a member of a commis sion which was to determine the length of the seconds pendulum. and which also determined the 'Imperial gallon' of ten pounds of water. In ISIS Young became secretary of the Board of Longitude and superintendent of the Nu Almanac, in which capacity he served until the dissolution of tlie board in 1828, when he became the sole conductor of 1110 A/MU/UW. During the last years of his life he was a member of a council appointed to advise the Admiralty in scientific matters. Young was the first to de scribe and measure astigmatism, as well as to study the optical constants of the eye, and ex plained color sensation by the theory that the eye contained three sets 14 nerves or receptive systems located at the retina and corresponding to red, green. and violet, and that color-blindness was the result of a deficiency of one or the other of these systems. Young is often called the founder of physiological optics, and his theory was adopted and amplified by Helmholtz with the result that at present it is generally accepted.
His most famous work in optics was his demon stration of the wave theory, which since the time of Huygens (1690) had made hut slight prog ress. This was accomplished by his discovery of the 'interference of light.' which served to ex plain the color of thin and thick plates and various other phenomena. Young stated that "radiant light consists of undulations of fhe InIniniferous; ether," and maintained his position in spite of the numerous attacks to which his theories and papers were subjected. Many of the phenomena of polarized light were also dis cussed by Young, and to bin are due many dis coveries which were later developed by other physicists. To Young is due the use of the terms 'energy' and 'labor expended,' the former mean ing the product of the mass of a body and the square of its velocity, whereas the latter, which is equivalent to the 'work done,' denotes the products of the force exerted and the distance through which the body is moved, both quantities being proportional to each other. The expression known as Young's modulus represents the use of absolute measurements in elasticity and denotes the weight which would stretch a rod or wire of unit cross-section to double its length. His explanation of capillary phenomena was the first correct one.
Young also conducted researches in Egyptology (q.v.), and published an Account of Sonic Recent Discoveries in Hieroglyphical Literature and Egyptian Witianitics (1823). He was at work on an dictionary when he died. Young's work in physics was never adequately appre ciated by his contemporaries and his countrymen. In fact, his great discovery of interference and its hearing on the undulatory theory of light was established largely through the activities of Fresnel and Arago. and to them he is indebted for the general recognition which his theory has since received. Young has been described by Helmholtz as "one of the most clear-sighted men who have ever lived, but lie had the Misfortune to be too greatly superior in sagacity to his con temporaries. They gazed at him with astonish ment. but could not always follow the bold flights of his intellect, and thus a multitude of his most important ideas lay buried and forgotten in the great tomes of the Royal Society of London, till a later generation in tardy advance remade his discoveries :nil convineed itself of the accuracy and force of his inferences." Consult: Peacock, A Life of lonny ; also .11isr•elIaneoUs Works of Young, edited by Peacock and Leitch (Londori, 1857). his lectures before the lloyal Institution have been published in two volumes.