A. Michelson and E. Morley. On the Relative Motion of the Earth and the Luminiferous Æther. // Phil. Mag. S. 5. Vol. 24. No. 151. Dec. 1887.

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LVIII. On the Relative Motion of the Earth and the Luminiferous Æther. By Albert A. Michelson and Edward W. Morley*.

THE discovery of the aberration of light was soon followed by an explanation according to the emission theory. The effect was attributed to a simple composition of the velocity of light with the velocity of the earth in its orbit. The difficulties in this apparently sufficient explanation were overlooked until after an explanation on the undulatory theory of light was proposed. This new explanation was at first almost as simple as the former. But it failed to account for the fact proved by experiment that the aberration was unchanged when observations were made with a telescope filled with water. For if the tangent of the angle of aberration is the ratio of the velocity of the earth to the velocity of light, then, since the latter velocity in water is three-fourths its velocity in a vacuum, the aberration observed with a water telescope should be four-thirds of its true value †.

On the undulatory theory, according to Fresnel, first, the

* Communicated by the Authors.

This research was carried out with the aid of the Bache Fund,

† It may be noticed that most writers admit the sufficiency of the explanation according to the emission theory of light; while in fact the difficulty is even greater than according to the undulatory theory. For on the emission theory the velocity of light must be greater in the water telescope, and therefore the angle of aberration should be less; hence, in order to reduce it to its true value, we must make the absurd hypothesis that the motion of the water in the telescope carries the ray of light in the opposite direction!

Phil. Mag. S. 5. Vol. 24. No. 151. Dec. 1887. 2 H

æther is supposed to bo at rest, except in the interior of transparent media, in which, secondly, it is supposed to move with a velocity less than the velocity of the medium in the

ratiowhere n is the index of refraction. These two

hypotheses give a complete and satisfactory explanation of aberration. The second hypothesis, notwithstanding its seeming improbability, must be considered as fully proved, first, by the celebrated experiment of Fizeau*, and secondly, by the ample confirmation of our own work†. The experimental trial of the first hypothesis forms the subject of the present paper.

If the earth were a transparent body, it might perhaps be conceded, in view of the experiments just cited, that the inter-molecular æther was at rest in space, notwithstanding the motion of the earth in its orbit; but we have no right to extend the conclusion from these experiments to opaque bodies. But there can hardly be any question that the æther can and does pass through metals. Lorentz cites the illustration of a metallic barometer tube. When the tube is inclined, the æther in the space above the mercury is certainly forced out, for it is incompressible‡. But again we have no right to assume that it makes its escape with perfect freedom, and if there be any resistance, however slight, we certainly could not assume an opaque body such as the whole earth to offer free passage through its entire mass. But as Lorentz aptly remarks: “Quoi qu'il en soit, on fera bien, à mon avis, de ne pas se laisser guider, dans une question aussi importante, par des considérations sur le degré de probabilité ou de simplicité de l'une ou de l’autre hypothèse, mais de s'addresser a l’expérience pour appendre à connaître l’état, de repos ou de mouvement, dans lequel se trouve l'éther à la surface terrestre. ” §

In April, 1881, a method was proposed and carried out for testing the question experimentally ||.

In deducing the formula for the quantity to be measured,

* Comptes Rendus, xxxiii. p. 349 (1851); Pogg. Ann. Ergänzungsband, iii. p. 457 (1853); Ann. Chim. Phys. [3]. lvii. p. 385 (1859).

† "Influence of Motion of the Medium on the Velocity of Light. ” Am. J. Sci. [3], xxxi. p. 377 (1886).

‡ It may be objected that it may escape by the space between the mercury and the walls; but this could be prevented oy amalgamating the latter.

§ Archives Néerlandaises, xxi. 2me livr. Phil. Mag. [5], xiii. p. 23(5.

|| “ The Relative Motion of the Earth and the Luminiferous Æther/ by Albert A. Michelson. Am. J. Sci. [3], xxii. p. 120.

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