264 THOMAS YOUNG. 



graduated in 1803, when he was thirty years of age, 

 and five years more had to elapse before he could 

 take the degree of M.D. Meanwhile he had begun the 

 practice of medicine. Dr. Brocklesby died in 1797, 

 on the night of a day when he had entertained his 

 relative and some other friends at dinner. During 

 dinner he seemed perfectly well, but he expired a few 

 minutes after he went to bed. He left Young his 

 house and furniture in Norfolk Street, Park Lane, his 

 library, his prints, a collection of pictures chiefly selected 

 by Sir Joshua Keynolds, and about 10,0001. in money. 



THE WATE THEORY. 



On January 16, 1800, Young communicated to the 

 Royal Society a memoir entitled ' Outlines and Ex- 

 periments respecting Sound and Light-' In this paper 

 he treated of the ' interference ' of sound, and his re- 

 searches on this subject led him on to the discovery of 

 the interference of light ' which has proved,' says 

 Sir John Herschel, c the key to all the more abstruse 

 and puzzling properties of light, and which would alone 

 have sufficed to place its author in the highest rank of 

 scientific immortality, even were his other almost in- 

 numerable claims to such a distinction disregarded. 5 

 Newton considered the sensation of light to be aroused 

 by the impinging of particles, inconceivably minute, 

 against the retina. Huyghens, on the contrary, sup- 

 posed the sensation of light to be aroused by the 

 impact of minute waves. Young favoured the theory 

 of undulation, and by his researches on sound he was 

 specially equipped for its thorough examination. Before 

 he formally attacked the subject he gave, in a paper 

 dealing with other matters, his reasons for espousing 

 the wave theory. The velocity of light, for instance, 

 in the same medium is constant. All refractions are 



