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
produced by the motion of the luminary or of the observer. This prize, consisting of a gold medal or 3,000 francs, was awarded in 1874 to Mascart. He maintained that in Arago’s experiment the change in refraction produced by the fraction of the earth’s motion was compensated by the displacement of the observing telescope. Mascart repeated Babinet’s experiment with gratings, where the effects of the motion of the telescope and of the grating would be additive, and found the sum small compared with Babinet’s calculation. He thinks that the change in the length of the wave caused by the motion is compensated by the displacement of the measuring apparatus. He concludes that reflection, diffraction, double refraction, and circular polarization are powerless to show the motion of the earth, either with solar light or that from a terrestrial source.
In 1871, Airy used a vertical telescope, and measured the meridional zenith distance of γ Draconis, the star by which Bradley discovered aberration. It is about 100″ north of the zenith. The tube of the telescope, which was 35.3 inches long, was filled with water. The days of observation included the seasons of the equinoxes, when the star is most affected in opposite directions by aberration. The observations were repeated in the spring and autumn of 1872. No increase was produced in the aberration by the water in the telescope.
In 1873, Ketteler, in the preface to the “Laws of the Aberration of Light,” enumerates thirty-nine persons who have investigated the effect of motion on the phenomena of sound and light. From his own analysis he concludes: (1) that a motion of the prism and telescope perpendicular to the direction of a star produces no effect on the refraction; (2) that when the motion is in the direction of the star, the velocity of the light is changed according to Fresnel's fraction of that motion; and (3) that for any intermediate direction it is changed to the extent of that fractional part of the motion multiplied by the cosine of the angle between the direction of the motion and the direction of the star.
In 1859, Fizeau proposed an experiment for ascertaining if the azimuth of the plane of polarization of a refracted ray is influenced by the motion of the refracting medium. When a ray of polarized light passes through an inclined plate of glass, the plane of polarization is changed, according to certain laws investigated by Malus, Biot, and Brewster. The degree of change depends upon the inclination of the ray, the azimuth of the plane of primitive polarization, and the index of refraction of the glass. The incidence and azimuth being constant, this rotation of the plane of polarization increases with the index of refraction. This index being inversely as the velocity of light, the rotation is smaller the greater this velocity. Fizeau used two bundles of glass, four plates in each, and slightly prismatic, inclined to one another. One bundle was made of common glass; the other of flint glass. The angle of incidence for the ray was 58° 49′. When the azimuth of the primitive plane of polarization was 20°, the rotation of the plane of