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

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clined about 26° to the plane of the earth's equator, and a tangent to the earth's motion in its orbit makes an angle of 23 1/2° with the plane of the equator. The resultant would be within 25° from the equator. The nearer the components are in magnitude, the more nearly would the resultant coincide with the equator. If the apparatus is placed so that the arms point north and east at noon, the eastern arm would coincide with the resultant motion of the earth, and the northern arm would be at a right angle to it. The displacement produced by revolving the whole through 90° should amount to one-twenty fifth of the interval between two fringes. If the proper motion of the solar system is small compared with the velocity of the earth in its orbit, the displacement would be less. Mr. Michelson drew from these experiments the conclusion that there was not a sufficient displacement of the fringes to support the theory of aberration, which supposes the ӕther to move with a certain fraction of the earth’s velocity. The displacement however was so small that it easily might have been masked by errors of experiment Mr. A. Graham Bell supplied Mr. Michelson with the money required for this investigation,

In 1886, Mr. Michelson and Mr. Morley published a paper on the influence of the motion of the medium traversed by the light on its velocity. Fizeau had made similar experiments. In both cases the interfering rays were changed in velocity in opposite ways by flowing air or water through which they were transmitted. With air having a velocity of about. 82 feet (25 meters) a second, the effect was so small that it might easily be covered up by errors of experiment; but with water it was measurable, and the result corresponded with the assumption of Fresnel, that the ӕther in a moving body is stationary, except the portions which are condensed around its particles. In this sense, it may be said that the ӕther is not affected by the motion of the medium which it permeates. For this investigation, which was made possible by a grant from the Bache Fund of the National Academy, Mr. Michelson and Mr. Morley devised a new instrument, called the refractometer. Cornu writes of Michelson's experiments on moving media: “Leur travail conçu dans l’esprit le plus élevé éxecuté avec ces puissant moyens d'action que les savants des États-Unis aimant a déployer dans les grandes questions scientifiques fait le plus grand honueur à leurs auteurs.”

In 1887, Professor Michelson published another investigation of the question whether the motion of the earth in its orbit carried its ӕther with it. In his previous experiment his apparatus was sensitive to the smallest jars, and it was difficult to revolve it without producing distortion of the fringes, and an effect amounting to only one-twentieth of the distance between the fringes might easily be hidden by accidental errors of experiment. In the new experiment the apparatus was placed on a massive rock, which rested on a wooden base, which floated upon mercury. The stone was 1.5 meters square and 0.3 of a meter thick.

At each corner four mirrors were placed, by reflection from which the length of path traversed by the light was increased to ten times its former value. The width of the fringes of interference, which were the subject of observation, measured from forty to sixty divisions of the observing micrometer. The light came from an Argand burner sent through a lens. To prevent jars from stopping and starting, the float was kept constantly in slow circulation, revolving once in six minutes. Sixteen equidistant marks were made on the stationary frame-work within which the float moved. Observations were taken on the fringes, whenever any one of these marks came in the range of the micrometer. The observations were made near noon and at 6 p. m. The noon and evening observations were plotted on separate curves. One division of the micrometer measured one-fiftieth of a wave-length. Mr. Michelson was confident that there was no displacement of the fringes exceeding one-hundredth of a wave length. It should have been from twenty to forty times greater than this. Mr. Michelson concludes that this result is in opposition to Fresnel's theory of aberration.

As late as 1872, Le Verrier thought that a new measurement of the velocity of light by Fizeau very important in the interest of astronomy; and in 1871, Cornu wrote that the parallax of the sun, and hence the size of the earth’s orbit, were not yet known with the desirable precision. In 1875, Villarceau made a communication to the Paris Academy on the theory of aberration. He says that the parallax of the sun by astronomical measurement is 8″.86. Foucault’s velocity of light combined with Struve’s aberration makes the sun’s parallax 8″.86. Cornu’s velocity of light gives the same result only when it is combined with Bradley’s aberration, which differs from that of Struve by 0″.20. Villarcean thinks that there is an uncertainty about the value of aberration on account of the motion of the solar system. In 1883, M. O. Struve discussed seven series of observations made by his father, Nyrén, and others, with various instruments and by different methods, at the Observatory of Pulkowa. He was certain that the mean result for the value of abeiration was 20″.492, with a probable error of less than 1/100 of a second. This aberration, combined with the velocity of light as deduced from the experiments of Cornu and Michelson, made the parallax of the sun 8″.784; differing from the most exact results of the geometric method by only a few hundredths of a second. Villarceau proposed to get the solar motion by aberration; selecting two places on the earth in latitude 35° 16' north and south, and after the example of Struve, observing the zenith distances of stars near the zenith. The

tangents of these latitudes are ± 1/sqrt(2) so that they contain the best sta

tions for obtaining the constant of aberration, and the three components of the motion of translation of the solar system. In 1887, Ubaghs, a Belgian astronomer, published his results on the determination of the