while the number of physical sciences has received numerous additions during the last half-century, new affiliations and a more intimate correlation have been manifested. In this mutual helpfulness light has played an important part. The optical method of studying sound, and the many varieties of flame apparatus, have made acoustics as intelligible through the eye as through the ear.
Velocity being expressed by space divided by time, it is evident that in measuring an immense velocity we must have at our command an enormous distance, such as we find only in astronomy, or else possess the means of measuring fractions of time as small as one-millionth of a second. The first successful attempt to measure such a velocity was made by Wheatstone in 1834. Discharges from a Leyden jar were sent through a wire, having two breaks in it one-fourth of a mile apart. The wire was in the form of a loop, so as to bring the breaks into the same vertical line. The sparks seen at these breaks were reflected by a mirror at the distance of 10 feet, and revolving eight hundred times per second. The images of the two sparks were relatively displaced in a horizontal direction. As the displacement did not exceed one-half of an inch, the time taken by electricity to go from one break to the other was less than a millionth of a second. Since the distance was one-quarter of a mile, the electricity travelled in that case at the rate of 288,000 miles a second. If this experiment is interpreted to mean that electricity would go over 288,000 miles of similar wire in one second, as it probably often was at that time, the conclusion is fallacious. The velocity of electricity, unlike that of sound or light, diminishes when the length of wire increases.
In 1838, Wheatstone suggested a method for measuring the velocity of light, which he thought was adequate for giving not only the absolute velocity but the difference of velocity in different media.
In that year Arago communicated to the French Academy the details of an experiment which he thought would give the velocity of light in air or a vacuum. As his own health was broken down (he died in 1853) he appealed to two young French physicists to undertake the experiment. On July 23,1849, Fizeau, by a method wholly his own, made a successful experiment. A disk cut at its circumference 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 5 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 number 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 embraced 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 four hundred 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 traveled 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 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 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 be 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 of resolving it. Babinet thought that the experiment admitted of ten times greater accuracy. With three times only it might correct Struve’s value of aberration.
In 1873, Cornu, another French physicist, repeated the experiments of Fizeau with a toothed wheel, the work extending over three years. The observer was stationed at the Ecole Polytechnique. The reflecting mirror and collimating telescope were placed on Mont Valerian, at a distance of about 33,816 feet. Three different wheels were tried, having 104,116, and 140 teeth respectively, and rotating between seven and eight hundred times a second, the velocity being registered by electricity. 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 with three pens recorded automatically seconds, the rotations 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 probable error was 1 per cent. In 1874, Cornu gave the result of a new set of experiments made by him in conjunction with Fizeau over a distance of more than 14 miles between the Observatory