450 Messrs. Michelson and Morley on the Relative Motion 



aether is supposed to be 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 



n 2 — 1 

 ratio — a — , where n is the index of refraction. These two 

 n z 



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 workf. The experi- 

 mental 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 sether was at rest in space, notwithstanding the 

 motion 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 any question that the aether can and 

 does pass through metals. Lorentz cites the illustration of a 

 metallic barometer tube. When the tube is inclined, the 

 aether 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, a mon avis, de ne 

 pas se laisser guider, dans une question aussi importante, par 

 des considerations sur le degre de probabilite ou de simplicite 

 de Tune ou de l'autre hypothese, mais de s'addresser a 1' ex- 

 perience pour appendre 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, 



* Comptes Rendus, xxxiii. p. 349 (1851) ; Pogg. Ann. Erganzungsband, 

 iii. p. 457 (1853) ; Ann. Chim. Pkys. [3], lvii. p. 385 (1859). 



f "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 by amalgamating 

 the latter. 



§ Archives Neerlandaises , xxi. 2 me livr. Phil. Mag. [5], xiii. p. 236. 



|| " The Relative Motion of the Earth and the Luminiferous ^Ether/ 

 by Albert A. Michelson. Am. J. Sci. [3], xxii. p. 120. 



