OF ARTS AND SCIENCES. 383 



died in 1853) he appealed to two young French physicists to under- 

 take the experiment. On July 23, 1849, Fizeau, by a method wholly 

 his own, made a successful experiment. A disk, cut at its circumfer- 

 ence into 720 teeth and intervals, and made by Breguet, was rapidly 

 rotated by a train of wheels and weights. A concentrated beam of 

 light was sent out through one of the intervals between two teeth of 

 the disk, which was mounted in a house in Suresne, near Paris, and 

 was sent back by a mirror placed on Montmartre at a distance of 

 about five miles. The light, on its return, was cut off from the eye or 

 entered it, according as it encountered a tooth or an interval of the 

 disk. If the disk turned 12.6 times in a second, the light encountered 

 the tooth adjacent to the interval through which the light went out. 

 With twice as many rotations in the disk, the light could enter the eye 

 through the adjacent interval. With three times the original velocity, 

 it was cut off by the next tooth but one, and so on. From the num- 

 ber of teeth and the number of rotations in a second, the time taken 

 by the 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 1856 the Institute of France awarded to Fizeau the Imperial Prize 

 of 30,000 francs in recognition of this capital experiment. 



In 1862, Foucault succeeded in measuring the velocity of light by a 

 wholly different method, all parts of the apparatus for it being em- 

 braced within the limits of his laboratory. The light emanated from 

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

 then fell upon a plane mirror revolving 400 times a second, by which 

 it was reflected successively to five other mirrors, the last of which 

 was plane, and returned it back, by the same path, to the revolving 

 mirror and reticule. The total distance travelled was only about 66 

 feet. As the revolving mirror had moved while the light was making 

 this short journey, the image of the reticule was displaced in reference 

 to the reticule itself ; and this displacement was the subject of measure- 

 ment. Although the time involved was only about one 15,000,000th 

 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 

 astronomical methods by only 1,243 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 l'ennemi du bien." His modest claim was that 

 he had suggested to Foucault the problem, and indicated certain means 



