1899.] Centenary Commemoration, 1799-1899. 205 



and speak of Dr. Thomas Young, one of the earliest professors of the 

 Institution. Young occupied a very high place in the estimation of 

 men of science — higher, indeed, now than at the time when he did 

 his work. His " Lectures on Natural Philosophy," containing the 

 substance of courses delivered in the Institution, was a very remark- 

 able book, which was not known as widely as it ought to be. Its ex- 

 positions in some branches were unexcelled even now, and it contained 

 several things which, so far as he knew, were not to be found elsewhere. 

 The earlier lectures dealt with elementary mechanics, and the reader 

 would find as sound an exposition of that science as could be imagined. 

 It was to Young that they owed the term energy, now in everybody's 

 mouth. Elastic resilience was better dealt with there than in any 

 other treatise he knew of, for Young discussed the subject with re- 

 markable ingenuity, showing that the phenomena exhibited by two 

 bodies coming into collision were comprehended under it. If the 

 velocity was moderate, all their motion might be taken up in them in 

 the form of jwtential energy ; but if it exceeded a certain limit their 

 integrity could not be preserved. In the case of a grain of sand pro- 

 jected against a sheet of glass, another element, that of time, had to 

 be considered, for it became a question of the pi-opagation of the wave 

 set up by the impact, and if the region traversed by the wave during 

 collision, and alone available as the seat of potential energy, were too 

 small the glass was bound to break. Young again discussed the 

 problem of a ball supported on a column of air or water, and correctly 

 explained that it preserved its stability and did not fall out of the 

 stream owing to centrifugal force. In the province of sound Young 

 was the originator of many of the most important principles on which 

 the docrine was now expounded, but it was with optics that his name 

 was most closely associated, for he and Fresnel were the builders 

 of the great structure of the undulatory theory. This was a matter 

 that was tolerably familiar. Lord Eayleigh thought he could best 

 utilise the time at his disposal by mentioning some of the points in 

 which Young's good work had been overlooked. In his time a ques- 

 tion of discussion was the change of the focus of the eye for vary- 

 ing distances. One suggested explanation, that accommodation was 

 effected by an alteration in the external convexity of the eye, Young 

 proved to be wrong by drowning his eye in water. This virtually 

 eliminated the convexity, yet the power of accommodation remained ; 

 and he therefore concluded it was due to a muscular alteration in the 

 internal lens of the eye. He also described the phenomenon of astig- 

 matism aud showed his deep knowledge of optical theory by suggest- 

 ing that its effects could be counteracted by the use of a slightly 

 sloping lens. In the study of compound colours, or chromatics as it 

 was then termed, Young's views were correct, though not universally 

 accepted even yet. Lord Eayleigh showed a modification of the ex- 

 periment by which he proved that the combination of green and red 

 gave yellow, and illustrated the fact by a further experiment, not 

 Young's, but following his suggestions, which demonstrated to the 



