In December 1797 Young's uncle, Dr. Brocklesby, died, bequeathing to his nephew the sum of 10,000_l._, besides his house, furniture, and a choice collection of pictures. Dr. Young was now entirely at liberty to form his own scheme of life, and he determined to commence practice as a physician, for which purpose, after having completed his terms of residence at Cambridge, he took a house in Welbeck Street (No. 48), which he continued to occupy for five-and-twenty years. His practice as a physician, although respectable, was never large. He wanted that confidence or assurance which is so necessary to the successful exercise of the profession. He was perhaps too deeply informed, and therefore too sensible of the difficulty of arriving at true knowledge in the science of medicine ever to form a hasty judgment; while his great love of, and adherence to truth, made him often hesitate where others would have felt no difficulty in expressing an opinion. It was perhaps a happy circumstance for the fame of Dr. Young that this should be the case, as he was thereby enabled to devote a considerable portion of his time to those literary and scientific studies in which so few could compete with him. In 1799 he published his memoir entitled 'Outlines and Experiments respecting Sound and Light,' which was read before the Royal Society and printed in their 'Transactions.' Other papers, 'On the Theory of Light and Colours,' followed, which the council of the Royal Society selected for the Bakerian lectures. In the year 1801 Dr. Young accepted the office of Professor of Natural Philosophy at the Royal Institution, which had been established the year previously. The conducting of the journal of the Institution was also entrusted to his care, in conjunction with his colleague Sir Humphry Davy, at that time Professor of Chemistry. Dr. Young remained at the Royal Institution two years, during which period he gave a course of lectures on 'Natural and Experimental Philosophy,' a syllabus of which he published in 1802, announcing for the first time his great discovery of the general law of the interference of the undulations of light. His lectures were not, however, popular; they embodied too much knowledge to be intelligible to any considerable portion of his hearers; and the matter was so abundant and the style so condensed, that students tolerably versed in science might have found it extremely difficult to follow him in his masterly discussions. Dr. Young had been elected a Fellow of the Royal Society as early as the year 1794, when he had just completed his twenty-first year; he was now appointed (1802) Foreign Secretary to the same Society, an office which he held during the remainder of his life, and for which he was well qualified by his knowledge of the principal languages of Europe.