Ixxxiii 



Jean Bernard Leon Foucault, Foreign Member of the Royal Society, 

 was born in Paris on the 18th of September 1819. He began the study 

 of medicine, but soon gave the preference to physics and the sciences 

 of observation. At the age of twenty he employed himself in improving 

 the processes of photography. For three years he assisted M. Donne 

 in preparing the illustrations of his lectures on microscopic anatomy, 

 and was associated with M. Fizeau in conducting a variety of original 

 researches. They investigated the comparative intensities of the light 

 of the sun, of the voltaic arc between carbon poles, and of lime heated 

 before the oxyhydrogen blowpipe. They read memoirs on the interference 

 of calorific rays, on the interference of two rays of light in the case of a 

 large difference in the lengths of their routes, and on the chromatic pola- 

 rization of light. In December of 1849 Foucault described an electromag- 

 netic regulator of the electric light. Conjointly with Regnauit he was the 

 author of a paper on binocular vision. He contributed besides several 

 memoirs on colour, on voltaic and frictional electricity, ^and on the employ- 

 ment of the conical pendulum as a time-keeper. 



M. Arago had suggested the employment of "Wheatstone's revolving 

 mirror, in a manner resembling its use in measuring the propagation of the 

 electric current in a wire, to decide whether the velocity of light within a 

 refractive medium is greater or less than its velocity in air. The former 

 result implies the truth of the emission theory, the latter that of the 

 undulatory theory. The experiment, as devised by M. Arago, was nearly 

 (perhaps quite) impracticable, inasmuch as it depended upon the observa- 

 tion of an image of momentary duration formed in an unknown part of the 

 field of view. By the happy introduction of a concave mirror having its 

 centre in the axis of the revolving mirror, a fixed image was obtained ; and 

 the experiment thus rendered possible proved that the velocity of light 

 is greater in air than in water. This experiment was made in 1850, not 

 long after M. Fizeau had approximately determined the velocity of light 

 in air by measuring the time it occupied in travelling from the place of the 

 observer to a station 8633 metres distant, and back again. Foucault also 

 suggested the means of measuring the velocity of propagation of radiant 

 heat. 



In February 1851 he communicated to the Academy the results of his 

 observations on the rotation of the plane of oscillation of a freely suspended 

 pendulum in the direction east-south-west, and thus supplied an ocular 

 demonstration of the diurnal motion of the earth. By the construction of 

 the gyroscope, in September 1852, he gave a second demonstration of the 

 same phenomenon. For these discoveries the Copley Medal for the year 

 1855 was awarded to him. About this time he was appointed Physical 

 Assistant to the Imperial Observatory. In September of the same year he 

 exhibited a new instance of the conversion of work into heat. A copper 



