334 Michelson and Morley — Relative Motion of the 



On the undu.lato.ry 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 vel ociy 



w 2 — 1 

 less than the velocity of the medium in the ratio — , where 



n is the index of refraction. These two hypotheses give a com- 

 plete 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 mo- 

 tion of the earth in its orbit ; but we have no right to ex- 

 tend 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 resist- 

 ance, 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, a mon avis, de ne pas se laisser guider, dans 

 une question aussi importante, par des considerations sur le 

 degre* de probability ou de simplicite* de l'une ou de 1'autre 

 hypothese, mais de s'addresser a l'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 the 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, xxxiii, 349, 1851 ; Pogg. Ann. Erganzungsband, hi, 457, 

 1853; Ann. Chim. Phys., Ill, lvii, 385, 1859. 



f Influence of Motion of the Medium on the Velocity of Light. This Journal, 

 III, xxxi, 377, 1886. 



X It may be objected that it may escape by the space betweeu the mercury and 

 the walls ; but this could be prevented by amalgamating the walls. 



§ Archives Neerlandaises, xxi, 2 me livr. 



j[The relative motion of the earth and the luminiferous ether, by Albert A. 

 Michelson, this Jour., Ill, xxii, 120. 



^[ 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. 



