AMERICAN JOURNAL OF SCIENCE.
[THIRD SERIES.]
Art. SXXYI.— On the Relative Motion of the Earth and the Luminiferous Ether; 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.f
* This research was carried out with the aid of the Bache Fund, f It may be noticed that most writers admit the sufficiency ot the explanation according to the emissiou 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, m 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 I
Ah. Jour. Sci.—Third Series, Yoii. XXXIY, No. 203.—Nov., 1887.
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On the undulatory theory, according to Fresnel, first, the ether is supposed to be at rest except in the interior of transparent media, in which secondly, it is supposed to move with a v elociy
less than the velocity of the medium in the ratio ~ \ where
n
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.f 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 ether was at rest in space, notwithstanding the motion of the earth in iis orbit; but we have no right to extend the conclusion from these experiments to opaque bodies. But there can hardly be question that the ether can and does pass through metals. Lorentz cites the illustration of a metallic barometer tube. When the tube is inclined the ether in the space above the mercury is certainly forced out, for it is im-compressible4 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 qui’l en soit, on fera bien, & mon avis, de ne pas se laisser guider, dans une question aussi importante, par des considerations sur le degr£ de probability ou de simplicity de l’une ou de 1’autre hypothese, mais de s’addresser a ^experience pour apprendre a connaitre l’etat, de repos ou de mouvement, dans lequel se trouve Tether a la surface terrestre.”§
In April, 1881, a method was proposed and carried out for testing the question experimentally.!
In deducing the formula for th^ quantity to be measured, the effect of the motion of the earth through the ether on the path of the ray at right angles to this motion was overlooked.^"
* Comptes Rendus, xzxiii, 349, 1851; Pogg. Ann. Erganzungsband, iii, 457, 1853; Ann. Chim. Phys., Ill, lvii, 385, 1859.
f Influence of Motion of the Medium on the Yelocity of Light. This Journal, III, ixxi, 377. 1886.
% It may be objected that it may escape by the space between the mercury and the walls; but this could be prevented by amalgamatitf^ the walls.
§ Archives Neerlandaises, xxi, 2me livr.
(The relative motion of the earth and the luminiferous ether, by Albert A. Michelson, this Jour., Ill, xxii, 120. 1
It may be mentioned here that the error was pointed out to the author of the >former paper by M. A. Potier, of Paris, in the winter of 1881.
