1878.] 



President's Address. 



67 



and nomenclature of the unrivalled collection of fishes in the British 

 Museum, and of which he prepared a systematic catalogue in eight 

 volumes, which has been published by order of the Trustees. This is a 

 work of prodigious labour; it required for its satisfactory execution 

 an intimate knowledge of the fish of all parts of the world, and an intui- 

 tive perception of the natural character upon which a sound classifica- 

 tion should be based. From possessing these attributes it has been 

 accepted as the standard authority on the order by all zoologists. 

 Under this head too I must specially allude to his excellent work on the 

 Ceratodus. The Reptilian collections of the Museum have been no 

 less successfully dealt with by Dr. Giinther, and have afforded the 

 material for some of his most important works, amongst which his 

 "Reptiles of British India," "Memoir on Hatteria," and "Mono- 

 graph of the Gigantic Land Tortoises of certain islands in the Pacific 

 and Indian Oceans," are examples conspicuous for their completeness 

 and accuracy. 



The Rumford Medal has been awarded to Mr. Alfred Cornu for his 

 various Optical Researches, and especially for his recent redetermina- 

 tion of the Velocity of Propagation of Light. 



Mr. Alfred Cornu is the author of papers on optical and other 

 subjects published in the " Comptes Rendus " and other scientific 

 periodicals. He has been engaged, for example, with the difficult 

 subject of crystalline reflection, and kindred researches. 



It was in 1849 that Fizeau astonished the scientific world by an 

 actual experimental determination of the velocity of light, a velocity 

 so enormous that hitherto its finiteness has been proved, and its value 

 approximately determined, only by two astronomical phenomena. 

 Foucault almost simultaneously brought out an experimental deter- 

 mination by a totally different method. 



The method of Fizeau gave at once a near approximation to the 

 Aalue got from those two astronomical phenomena, combined with the 

 jmrallax of the sun, assumed known. But the difficulties of obtaining 

 a sufficiently accurate result were such that the velocity obtained by 

 Fizeau's method was not considered to rival in exactness the velocity 

 determined astronomically. Indeed, Foucault's method seemed to be 

 preferred, though Fizeau's is the simpler in principle, and is free from 

 certain doubts which may be raised as regards the other. But the 

 difficulties alluded to, which turned mainly on the determination of 

 the velocity of the revolving wheel, were such that almost twenty 

 years have elapsed without the method having been brought to a 

 sufficient degree of perfection to make it astronomically available. 



Such was the state of the problem when it was taken up by 

 M. Cornu. By methods of his own devising he succeeded in getting 

 over the difficulties with which Fizeau's further progress had been 



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