238 THE KOYAL INSTITUTION. [CHAP. IV. 



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 pro- 

 duce 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 

 Koyal Institution. 



In 1801, on the 6th of July, Count Rumford reported 

 to the managers that, c at the recommendation of Sir 

 Joseph Banks, he had had a conversation with Dr. Young 

 respecting his engaging as Professor of Natural Philo- 

 sophy 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. per annum.' l 



1 See Appendix II. 



