THE PROGRESS OF PHYSICS. 155 



theory by proving that the velocity of light in water 

 is less than that in air. 



Fizeau. The determination of the velocity of 

 light, which thus became of importance in relation to 

 the general theory, had been previously based, e.g., by 

 Rb'rner and Bradley, on astronomical data, derived 

 from aberration-observations, or from timing the 

 eclipses of Jupiter's satellites when at their greatest 

 and least distances from the Earth, but a direct ex- 

 perimental method was devised by Fizeau (1819- 

 1896). In 1849, in the suburbs of Paris, he ar- 

 ranged a rapidly rotating cog-wheel which inter- 

 cepted light at regular intervals, and found what 

 speed must be given to the wheel so that it rotated 

 one tooth's breadth while the light travelled to a 

 distant mirror and was reflected back again. Fou- 

 cault modified this method by observing " the posi- 

 tion ultimately assumed by a ray which travels 

 from a source to a rotating mirror, thence to a dis- 

 tant mirror, and thence back to the original mirror, 

 which by this time has been rotated somewhat." * 

 The determination of the velocity of light thus 

 effected by Fizeau and Foucault was revised by 

 Cornu in Paris, by James Young and George Forbes 

 in Britain, but the most accurate determinations 

 are said to be those made by Michelson, Kewcomb, 

 and Holcomte, in the United States. A mean result 

 is that light travels in vacuo at the rate of 186,772 

 miles per second, and in air at a velocity less than 

 this in the ratio of 10,000 to 10,003. 



As Professor Alfred Cornu points out in his Rede 

 lecture, to which we have already been much in- 

 debted in this section, the emission theory was a 

 natural but primitive one, with its germ in the ex- 



* Article Light, by Dr. Daniell. Chambers' Encyclopaedia, 



