It was in May 1801 that I discovered, by reflecting on the beautiful experiments of Newton, a law which appears to me to account for a greater variety of interesting phenomena, than any other optical principle that has yet been made known. I shall endeavour to explain this law by a comparison:--Suppose a number of equal waves of water to move upon the surface of a stagnant lake, with a certain constant velocity, and to enter a narrow channel leading out of the lake; suppose, then, another similar cause to have existed, another equal series of waves will arrive at the same channel with the same velocity, and at the same time with the first. Neither series of waves will destroy the other, but their effects will be combined. If they enter the channel in such a manner that the elevations of one series coincide with those of the other, they must together produce a series of greater joint elevations; but if the elevations of one series are so situated as to correspond to the depressions of the other, they must exactly fill up those depressions, and the surface of the water must remain smooth; at least I can discover no alternative either from theory or from experiment. Now I maintain that similar effects take place whenever two portions of light are thus mixed, and this I call the general law of the interference of light. Within three months he became Professor at the Royal Institution. In 1801, on the 6th of July, Count Rumford reported to the managers that, ‘at the recommendation of Sir Joseph Banks, he had had a conversation with Dr. Young respecting his engaging as Professor of Natural Philosophy at the Royal Institution and Editor of the Journals, together with a general superintendency of the house, and it appearing from the report of Count Rumford that Dr. Young is a man of abilities equal to these undertakings, it was resolved that Count Rumford be authorised to engage Dr. Young in the aforesaid capacities at a salary of 300_l._ per annum.’[23] On August 3, at the managers’ meeting (Count Rumford in the chair; present, Henry Cavendish, R. J. Sullivan; secretary J. P. Auriol), Count Rumford reported that, agreeably to the authority granted him by the managers, he had engaged Dr. Thomas Young. A copy of his letter to Dr. Young, expressing the conditions of his engagement, was at the same time laid before the committee. The first number of the Journal had been published by Rumford on April 5, 1800. Dr. Young alone edited the fourth number in the autumn of 1801, the fifth number in December 1801, and, after editing two more numbers alone, he joined with Davy in the editorship till the Journal stopped in 1803. On January 19 Dr. Young printed, at the press of the Royal Institution, a syllabus of his first course of lectures.