571 



The Copley Medal for the present year has been awarded to 

 M. Foucault, of Paris. 



M. Foucault has been engaged in various remarkable researches 

 during the last ten or twelve years. His earliest labours were de- 

 voted to photography. In the year 1844, he published, along with 

 M. Fizeau, an investigation of the comparative intensity, both che- 

 mical and optical, of three of the most brilliant sources of light of 

 which we can avail ourselves, the sun, the voltaic arc, and incan- 

 descent lime*. The investigations of these philosophers led to 

 numerical results, from which the vast inferiority of the lime-light 

 came out in a very striking manner. While the voltaic arc with coke 

 poles gave a light of which the intrinsic intensity was nearly ^ths of 

 that of the sun, the intensity of the oxy-hydrogen lime-light was 

 found to be only about T Vth of that of the poles of the voltaic arc. 



Shortly afterwards, M. Foucault was engaged, in conjunction with 

 M. Fizeau, in a series of important researches on the interference of 

 light produced by a considerable difference of path in the interfering 

 streamsf. In ordinary cases, all signs of interference cease when the 

 difference of path amounts to a few undulations, though inter- 

 ferences of a high order had been observed with the light of a spirit- 

 lamp with a salted wick, and also in certain peculiar phenomena 

 exhibited in a pure spectrum. But the prism does not seem to have 

 been employed in the investigation of interference, except for the 

 sake of analysing the tints produced by means of polarized light ; 

 and theorists even doubted whether the vibrations of the body emit- 

 ting the light in the first instance were sufficiently regular to render 

 interference possible in the case of great differences of path. But 

 by subjecting a narrow band of interfering light to prismatic analysis, 

 the authors were able to detect perfectly distinct interference pro- 

 duced by great retardations, amounting in one case to no less than 

 7394 undulations, proving that the undulations are regular in a very 

 high degree. A similar method of research enabled the authors to 

 study the modifications of polarized light with great advantage. 



* Annales de Chimie, torn. xi. p. 370. 



t Annales de Chimie, torn. xxvi. (1849) p. 138, having been read to the Aca- 

 demy May 24, 1845 ; and Ann. de Ch. torn. xxx. (1850), p. 146, having been read 

 March 9, 1846. 



