and the difference of time for air and water would be only a fraction of that small fraction. Hence the exceeding delicacy of the experiment.
In 1883, Mr. Michelson, at the request of Professor Newcomb, repeated Foucault’s experiments for finding the difference of velocity of light in air and water. Foucault did not aspire to quantitative precision in his results. The experiments of Michelson proved that the ratio of the velocities was inversely as the indices of refraction. The velocity with sunlight was a little greater than with the electric light; which opposes the conclusion of Young and Forbes. When Mr. Michelson covered half of the slit with red glass, the two halves of the image were exactly in line. Experiments were also made on the velocity of light in carbon disulphide, which led to the inference that its index of refraction was 1.77, and that orange-red light traveled from one to two per cent, faster than greenish blue light. Mr. Michelson was enabled to make this investigation by a grant from the trustees of the Bache Fund.
Various other methods of measuring the velocity of light have been proposed. About 1850, Laborde suggested, in a letter to Arago, a mechanical method of measuring the velocity of light. He supposes two disks, with many holes at the outside, connected by a very long axis and rotating rapidly. The light which was sent out through a hole in one wheel would be transmitted or arrested by the second wheel, behind which an observer was stationed. The distance between the wheels, the time of rotation, and the order of the eclipse, would be sufficient for calculating the velocity of light. Laborde imagined an enormous axis more than 200,000 miles long. Moigno recommended the substitution of a mirror for the observer and the second wheel, which would double the distance travelled by the light. A distance of 1,640 feet, a disk 25 feet in radius, with 1,000 holes, and turning 360 times a second, would be more than sufficient to surprise the reflected ray and stop it.
In 1874, Burgue suggested a new way of finding the velocity of light by experiment. If a white disk, with a black radius, is rotated rapidly, and at each turn is illuminated by an instantaneous flash, this radius will appear immovable. If this flash is reflected on the disk from a distant mirror, the black radius will be displaced. No details of the arrangement of apparatus and no experiments were published.
In 1885, Wolf proposed the following arrangements: Two mirrors were placed 5 meters apart and facing each other. The radius of curvature of each mirror was 5 meters. The first mirror was 0.20 of a meter in diameter; the other, 0.05 meter, revolved rapidly (two hundred turns a second). A slit was made in the center of the large mirror through which light was sent to the small mirror, forming an image on the surface of the large mirror; this image became an object for the small mirror, forming another image on the larger mirror, at a distance from the first mirror depending on the velocity of rotation. These images could be sent out laterally by an inclined plate of thin glass, and their distance measured by a micrometer. Wolf expected advantages from
the proximity of the two mirrors which would more than balance those of the long distances used by Foucault and Michelson.
The greatest difficulty which the undulatory theory of light has encountered is found in the attempted reconciliation between the requirements of the refraction of light and the aberration of light. To explain refraction, the density of the luminiferous ӕther must be greater when the index of refraction is greater. If a body moves, it must carry its inclosed ӕther with it, as its refractive power does not change. On the other hand, to explain the aberration of light, it must be supposed that the ӕther in the telescope does not move with the telescope; that the ӕther sifts through the telescope, the ӕther in front taking the place of the ӕther left behind; or, as Young expressed it, that the ӕther flows through the air and solid earth as easily as the wind blows through the trees of a forest.
The difficulty can be eluded by supposing that a refracting body carries along with it as much of the ӕther as it possesses in excess of what would exist in a vacuum of the same bulk. This, added to what is always sifting through it, would maintain its ӕther at a constant density. "What this fraction is which must travel with the body was calculated by Fresnel. But while the refracting power has been protected, how is it with aberration? That would be increased to a small extent. But as the aberration is very small, only about 201/2″ at its maximum, the required change in its value might be masked by ordinary errors of observation. Boscovich suggested to Lalande, in 1766, that a telescope filled with water instead of air would test the theory; but he made no experiment. Wilson, of Glasgow, also proposed a water telescope in 1782. In the course of time it appeared that not only was the effect of the earth’s motion on refraction and aberration under trial, but also the solar parallax, the motion of the solar system, and that of other stars.
The case is clearly stated by Lodge in this way: Sound travels quicker with the wind than against it. Is it the same with light? Does light travel quicker with the wind! Well, that depends altogether on whether the ӕther is blowing along as well as the air. If it is, then its motion must help the light on a little; but if the ӕther is at rest, no motion of the air, or of matter of any kind, can make any difference. According to Fresnel, the free ӕther is at rest, the bound is in motion. Therefore the speed of light will be changed by the motion of the medium; but only by a fraction, depending on its index of refraction, — infinitesimal for air, but sensible for water.
At an early day Arago investigated the effect which a change in the velocity of light would produce on aberration and refraction. He saw that a change of 5 per cent in the velocity of light would alter the aberration by only one second, whereas the refraction in a prism of 45° would be affected to the extent of two minutes. He observed the zenith distances of stars with and without the prism; and also the deviation of stars which passed the meridian at 6 a. m. and 6 p. m. The observa-