Alfred Marie Cornu. 185 



to speak, of the scientific memoirs which from time to time he 

 published, all bespeak a man of no ordinary capabilities a master in 

 his profession. Clear in his exposition of scientific matters, exquisitely 

 clear alike in his experimental demonstrations and in the language in 

 which he expounded their theory, he was as great in teaching as in 

 research. His first publication was the thesis written for his doctorate 

 upon the reflexion of light in crystalline media, in which he sought 

 to perfect the theory of Fresnel by modifying the conditions at the 

 limiting surface between the two media. This research exercised a 

 notable influence upon his scientific life. Optics was his first love, and 

 though he laboured successfully in other branches of experimental 

 physics, it was to optics that he returned, and in the field of optics 

 were achieved his greatest successes in physical investigation. The 

 pages of the Comptes Rendus and of the Journal de Physique bear 

 eloquent testimony to the activity and penetration of his mind. 

 Already, from 1863 to 1865, while at the Ecole des Mines, he had 

 begun to contribute, to the Academie des Sciences, notes on the 

 refraction and reflexion of light. Following up the work of Jamin, he 

 later pursued the subjects of vitreous and metallic reflexion, and 

 studied the connection between them. He showed that they were but 

 parts of one and the same phenomenon, though affecting different 

 regions of the spectrum, there being, as he demonstrated, a true 

 continuity between them. 



Soon after entering upon the duties of his chair, Cornu began, with 

 laborious and patient preparation, those experiments upon the velocity 

 of propagation of light which have become classical. Fizeau on the 

 one hand, Foucault on the other, had already made determinations, 

 each on his own lines. Foucault's value, then supposed to be the best, 

 was 2-98 x 10 10 in C.G.S. units. Cornu's results raised this figure to 

 3-004 x 10 10 in vacuo, or 3'0033 x 10 10 in air. His method, which 

 was fundamentally the same as that of Fizeau, was applied to the 

 transit of light over a total distance of 46 kilometres, or between two 

 stations 23 kilometres apart, the one at the Observatoire in Paris, the 

 other at Montlhe'ry. The instrumental perfection of his rotatory 

 apparatus enabled him to observe up to the twenty-first extinction of 

 the beam, thus securing a precision far in advance of that attained by 

 Fizeau. For his determination of the velocity of light he was 

 awarded the prix Lacaze in 1878, the same year in which his merits 

 were recognised by his admission into the Academie des Sciences. He 

 cherished the intention in later years of making a still more perfect 

 determination by sending the light between Corsica and Mont 

 Mourner, which is an annexe of the Observatory of Nice. In 1872 he 

 wrote papers " On Electrostatic Measurement," dealing with the 

 potential theories of Gauss and Green, then little known in France. 



