452 michelson's recent researches on light. 



one, and so on. From the number of teeth and the number of rotations 

 in a second the time taken by tlie light in going and returning was 

 easily calculated. In this way the velocity of light was found to be 

 195,741 miles per second. In 1850, the Institute of France awarded to 

 Fizeau the Imperial prize of 30,000 francs in recognition of this capital 

 experiment. 



In 1802, Foucault succeeded in measuring the velocity of light by a 

 wholly different method, all parts of theapparatusfor it being embraced 

 within the limits of his laboratory. The light emanated from a hue 

 reticule, ruled on glass and strongly illuminated by the sun. It then 

 fell upon a plane mirror revolving four hundred times a second, b^'" 

 which it was rellected successively to live other mirrors, the last of 

 which was plane, and returned it back by the same path to the revolv- 

 ing mirror and reticule. The total distance traveled was only about 

 06 feet. As the revolving mirror had moved while the light was mak- 

 ing this short journey, the image of the reticule was displaced in refer- 

 ence to the reticule itself; and this displacement was the subject of 

 measurement. Although the time involved was only about one fifteen- 

 millionth of a second, this brief interval was translated by the method 

 of the experiment into a measurable space, and gave 185,177 miles per 

 second for the velocity of light, differing from the best results of astro- 

 nomical methods by only 1,213 miles. Foucault was prompted to this 

 experiment by Leverrier, director of the observatory. Arago was the 

 first to propose the experiment. To obtain greater accuracy he placed 

 the moving mirror in a vacuum, but without any advantage. He said, 

 "Le mieux est I'ennemi du bien." His modest claim was that he had 

 suggested to Foucault the problem and indicated certain means of re- 

 solving it. Babinet thought that the experiment admitted oftentimes 

 greater accuracy. With three times only it might correct Struve's 

 value of aberration. 



In 1873, Cornu, another French physicist, repeate<l the (experiments 

 of Fizeau with a toothed wheel, the work extending over three years. 

 The observer was stationed at the f^cole Poly technique. The reflecting 

 mirror and collimating telescope were placed on Mont Valerian, at a 

 distance of about 33,810 feet. Three different wheels were tried, hav- 

 ing 104, 110, and 140 teeth respectively, and rotating between seven and 

 eight huiulred times a second, the velocity being registered by electric- 

 ity. Cornu used at times all the eclipses from the first to the seventh 

 order. Calcium and petroleum light were tried, as well as sunlight. A 

 chronograph Avith three pens recorded automatically seconds, the rota- 

 tions of the toothed wheel, and the time of the eclipse. More than a 

 thousand experiments were made, six hundred of which were reduced. 

 The velocity of light as published by Cornu in 1873, was 185,425.6 miles 

 per second. The proba.l)le error was 1 })er cent. In 1874, Cornu gave 

 the result of a new set of experiments made by him in conjunction with 

 Fizeau over a dist^uQ© of juore thaq J4 miles between the Observatory 



