THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



DECEMBER 1887. 



LVIII. On the Relative Motion of the Earth and the Lumini- 

 ferous JEiher. By Albeet A. Michelson and Edwaed 

 W. Moeley*. 



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 aberra- 

 tion 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. 



t It may he noticed that most writers admit the sufficiency of the ex- 

 planation 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 he greater in the water 

 telescope, and therefore the angle of aberration should he 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 



