ing through it at all. Hence the refraction is the same whether the prism be at rest or in motion through space.” Maxwell is more guarded in his own statement of the case. He says: “We can not conclude certainly that the ӕther moves with the earth, for Stokes has shown from Fresnel’s hypothesis that the relative velocities of the ӕther in the prism and that outside are inversely as the square of the index of refraction, and the deviation in this case would not be sensibly altered, the velocity of the earth being only one ten-thousandth of the velocity of light.”
In 1879, Maxwell wrote to Prof. D. P. Todd, then at the Nautical Almanac Office in Washington, asking him if he had observed an apparent retardation of the eclipses of Jupiter’s satellites depending on the geocentric position of the planet. Such observations, be thought, would furnish the only method he knew of finding the direction and velocity of the sun’s motion through the surrounding medium. In terrestrial methods of measuring the velocity of light, it returns on its path, and the velocity of the earth in relation to the ӕther would alter the whole time of passage by a quantity depending on the square of the ratio of the velocities of the earth and light, and this is quite too small to be observed.
In 1839, Babinet made a very delicate experiment on the relation of the luminiferous ӕther to the motion of the earth. He found that when two pieces of glass of equal thickness were placed across two beams of light which interfered so as to produce fringes, one of them moving in the direction of the earth’s motion and the other contrary to it, the fringes were not displaced. The experiment was made three-times by Babinet, with new apparatus each time. He concludes that here is a new condition to be fulfilled by all theories in regard to the propagation of light in refracting media. According to all the theories admitted or proposed, the displacement of the fringes should have been equal to many lengths of a fringe — that is, many millimeters — while by observation it was nothing. Stokes has calculated the result according to Fresnel’s theory, or his own modification of it, and found that the retardation expressed in time was the same as if the earth were at rest. Fizeau has pointed out a compensation in the effect of Babinet’s experiment. he says: “When two rays have a certain difference of march, this difference is altered by the reflection from the turning mirror.” By calculating the two effects in Babinet’s experiment, Fizeau finds that they have sensibly equal values, and of opposite sign.
In 1860, Angström communicated to the Royal Society of Upsala a method of determining the motion of the solar system by observations on the bands of interference produced by a glass grating. In 1863, he published the results which he had obtained. After allowing for Babinet’s correction on account of the motion of the grating, Angström finds that a difference in the direction of the observing telescope with reference to the earth’s motion might produce a displacement of the
fringes amounting to 49″.8. Selecting the line D in the fourth spectrum, he thought that the influence of the earth’s annual motion was verified, but that of the motion of the solar system was less decided. The observations were more consistent with the assumption that the solar system moved with a velocity equal to one-third of that in its orbit, than with an equal velocity, or none at all. In 1862-’63, Babinet presented to the Academy of Paris a paper on the influence of the motion of the earth on the phenomena produced by gratings, which depend not on reflection, refraction, or diffraction, but on interference. His principal object was a study of the motion of the solar system. He calculated the effects to be expected, but published no observations. In 1867, Van der Willigen measured the length of waves of light by means of a grating. When a slit was used, no effect was produced by the motion of the earth, the slit partaking of that motion. With a star, a movement of the earth in the direction of the light had an effect. This is the theoretical result, and agrees with Babinet’s experiment, but is not applicable to solar light when reflected by a mirror. That behaves as light from a terrestrial source. In 1873, he rejects the proposition that the refraction of light is modified by the motion of its source or of the prism. In 1874, he seems to doubt the reality of the effect produced on diffraction.
In 1867, Klinkerfues used a transit instrument having a focal length of 18 inches. In the tube was a column of water 8 inches long, and a prism. He observed transits of the sun and of certain stars whose north polar distance was equal to the sun’s, and which passed the meridian at midnight. The difference of right ascension is affected by double the coefficient of aberration. He computed that the column of water and the prism would increase the aberration by 8″. The amount observed was 7″.1. In 1868-’69, Hoek of Amsterdam discussed the influence of the earth’s motion on aberration. Delambre had calculated from the eclipses of Jupiter’s satellites that light must take 493s.2 in coming from the sun. Hence the aberration must be 20″.255. Struve’s observed aberration made the time 497s.8. Hoek decided in favor of Struve; but he thought that it was desirable that a new set of observations should be made on the eclipses. Klinkerfues espoused the side of Delambre. Hoek said that, if the earth’s motion was taken into account, according to Fresnel’s fraction, different results would be harmonized. In 1868, he made experiments on a divided beam of light, the two parts going in opposite directions through tubes filled with water. There was no interference attributable to the effect of the earth’s motion. As to any influence to be expected from the motion of the solar system, he thinks that motion must be insignificant compared with the initial motion of the comets, and with the cometary orbits, which are parabolas with few hyperbolas.
In 1872, and on several previous occasions, one of the grand prizes of the Academy of Paris was offered for an investigation of the effect