Michelson's recent researches on light. By Joseph Lovering, President (April 10, 1889).

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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

polarization was 18° 40′ and 24° 58′ for the two bundles. By Fresnel's hypothesis the change in the velocity of light from the motion of the

medium is ±((μ–1)/μ2)v. The greatest available velocity for the medium

is that of the earth in its orbit, viz, 101,708 feet per second (31,000 meters). At the time of the solstices this motion is horizontal, and from east to west at noon. If the incident light comes from the west, the velocity of light is diminished by Fresnel's fraction of the velocity of the earth. If the light comes from the east, its velocity is increased by

the same amount. The change in the index of refraction (or δμ/μ) is

equal to (v′/v) (μ2–1); this for an index of 1.513 amounts to 1/11740. Measurements show that in glass, the index increasing by a certain fraction, the rotation increases by a fraction four and one-half times greater, and the consequent change in the plane of polarization would be 1/2500. The

total change on reversing the direction from which the light came would be 1/1250. If the incidence is 70°, and allowance is made for the change

of direction inside of the glass, the fraction becomes 1/1500. When a ray of light falls on a single plate of glass at an angle of 70°, if its plane of primitive polarization makes an angle of 20° with the plane of refraction, this plane is changed by 6° 40′. This multiplied by gives sixteen seconds for the probable effect of the earth’s motion. With forty such plates the effect would be increased to ten and two-third minutes. Two mirrors were used, one to the east and the other to the west, and light could be sent by a heliostat upon either one. The apparatus was easily turned through 180° so as to receive successively the light which travelled with or against the earth’s motion.

With a single pile of plates highly inclined and a second pile less inclined, of more highly tempered glass and in the opposite azimuth, a rotation of 50° could be produced, while the tendencies to elliptical polarization were exactly balanced. The motion of the earth could modify this result to the extent of only two minutes; which is too small for accurate observation. Fizeau then resorted to a device already indicated by Botzenhart for amplifying this effect. A small variation in the primitive plane of polarization produces a greater effect the smaller the azimuth of this plane. If the original azimuth is only 5°, a small change in the azimuth trebles the value of the rotation. A large rotation is first produced on a ray whose azimuth is large, and then this rotation is largely changed by another pile so placed that the ray enters it under a small azimuth. More than two thousand measurements were made under various conditions. For noon observations at the time of solstice the rotation was always greater when the light came from the west, and was less at other times of day. The excess in the value of the rotation when the light came from the west varied between 30′ and 155′, according to the different ways in which the piles of plates were