LONDON, EDINBURGH and DUBLIN
PHILOSOPHICAL MAGAZINE
AND
JOURNAL OF SCIENCE.
[FIFTH SERIES.]
DECEMBER 1860.
LVIII. On the Relative Motion of the Earth and theLuminiferous Æther.
The discovery of the aberration of light wassoon followed by an explanation according to the emission theory. Theeffect was attributed to a simple composition of the velocity of light withthe velocity of the earth in its orbit. The difficulties in this apparentlysufficient explanation were overlooked until after an explanation on theundulatory theory of light was proposed. This new explanation was at firstalmost as simple as the former. But it failed to account for the factproved by experiment that the aberration was unchanged when observationswere made with a telescope filled with water. For if the tangent of theangle of aberration is the ratio of the velocity of the earth to thevelocity of light, then, since the latter velocity in water isthree-fourths its velocity in a vacuum, the aberration observed with awater telescope should be four-thirds of its true value.[2]
On the undulatory theory,according to Fresnel, first, the æther is supposed to be at rest exceptin the interior of transparent media, in which secondly, it is supposedto move with a velocity less than the velocity of the medium in the ratio
, where 
is the index of refraction. Thesetwo hypotheses give a complete and satisfactory explanation of aberration.The second hypothesis, notwithstanding its seeming improbability, mustbe considered as fully proved, first, by the celebrated experiment ofFizeau,[3]and secondly, by the ample confirmation of our own work.[4].The experimental trial of the first hypothesis forms the subject of thepresent paper.
If the earth were a transparent body, it might perhaps be conceded, inview of the experiments just cited, that the inter-molecular æther wasat rest in space, notwithstanding the motion of the earth in its orbit;but we have no right to extend the conclusion from these experiments toopaque bodies. But there can hardly be question that the æther can anddoes pass through metals. Lorentz cites the illustration of a metallicbarometer tube. When the tube is inclined the æther in the space abovethe mercury is certainly forced out, for it is imcompressible.[5].But again we have no right to assume that it makes its escape with perfectfreedom, and if there be any resistance, however slight, we certainlycould not assume an opaque body such as the whole earth to offer freepassage through its entire mass. But as Lorentz aptly remarks: "Quoiqu'il e