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

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combined. The difference in the values of the rotation according as the light came from the west or east was consistent with a change in the index of refraction corresponding to Fresnel's hypothesis. Fizeau indicated his intention of renewing the research with improved apparatus, but no further publication on the subject by him can be found.

Faye has criticised this investigation of Fizeau, on the ground that he has taken no account of the motion of the solar system towards the constellation Hercules. This motion, recognized by astronomers on substantial evidence, amounts to 25,889 feet per second (7,894 meters) at its maximum. Its influence is almost zero at noon of the solstices. But it increases after noonday. Faye examines Fizeau's observations at 4 P. M., and finds discrepancies of 12′ or 15′ between the results of theory and observation. By neglecting the term which corresponds to the motion of the solar system, Fizeau's observations accord better at all hours of the day. Must the inference be, Faye asks, that the solar system does not move? Tessan, in reply to Faye, says that the sun, from which Fizeau derived the light used in his experiments, moves with the rest of the solar system; and that therefore Fizeau was justified in neglecting the term which expresses this motion, as of no effect on his calculations. Fizeau's theory depends only on the relative velocity between the source of light and the body which receives it; that is, the velocity of revolution and rotation of the earth.

In 1881, Professor Michelson published the results of his investigation on this delicate problem. He first calculates the probable difference of time taken by the light in going and returning over a given distance, according as that distance lies in the direction of the earth’s motion or at right angles to it. If the distance were 1,200 millimeters, the difference of time translated into space would be equal to one-twenty-fifth of a wave-length of yellow light. The apparatus was ingeniously devised so as to bring about fringes of interference between the two rays which have travelled on rectangular paths. The whole apparatus was then turned round bodily through 90°, so as to exchange the conditions of the two interfering rays. Special apparatus was made for this experiment by Schmidt and Haensch of Berlin, and was mounted on a stone pier at the Physical Institute of Berlin. It was so sensitive to accidental vibrations that it could not be used in the day-time, nor indeed earlier than midnight. To secure greater stability the apparatus was moved to the Astrophysikalisches Observatorium in Potsdam, in charge of Professor Vogel. But even here the stone piers did not give sufficient protection against vibration. The apparatus was then placed in the cellar, the walls of which formed the foundation for an equatorial. But stamping with the feet, though at a distance of 100 meters, made the fringes disappear.

The experiments were made in April, 1881. At this time of the year, the earth’s motion in its orbit coincides roughly with the motion of the solar system, viz, towards constellation Hercules. This direction is in-

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.