EXPERIMENTS ON ETHER-DRIFT 23 



could be given on the assumption that the whole of the ether 

 is carried with the earth, and freely passes the star under 

 observation ; but such a conception is obviously absurd. In 

 truth, it probably drifts past both earth and star; and it is 

 important to remember, in this connection, as has been shown 

 by Lodge, 1 that all that the aberration angle measures is the 

 relative motion of the source and the receiver, and not the-total 

 movement of the latter with reference to the medium. A 

 uniform drift of the ether past both star and observer would 

 produce no variation in the magnitude of the observed aberra- 

 tion. Hence, although this observation may be taken as a 

 guide as to the size and direction of the earth's motion, the 

 total motion may be considerably different ; and a successful 

 experiment of the type to be described would measure the 

 whole drift of the ether. 



Considerations of this kind are sufficiently weighty to justify 

 completely the attempts which have been made to determine 

 this motion ; but, as has been already pointed out, none of them 

 has succeeded in throwing light on the main question. They 

 have only afforded ground for speculation as to their non- 

 success — interesting and important speculation, it is true, but, 

 nevertheless, only speculation. Let us now proceed to consider 

 in detail the experiments themselves, and also some suggestions 

 which have an important bearing on the subject. 



In the first place, it will be well to consider what is known 

 from an astronomical point of view regarding the movement of 

 the earth. Besides rotating on its axis, it is moving in a nearly 

 circular orbit round the sun as centre, with a velocity of about 

 19 miles per second. In comparison with this, the rotary 

 motion of the earth is negligibly small, only amounting, at the 

 equator, to about -^ of the above value. If, then, the ether be 

 supposed stationary relative to the sun, the velocity with which 

 it is to be expected to drift past the earth amounts to 19 miles 

 per second, or very nearly one ten-thousandth part of the 

 velocity of light. Now, although the rotation of the earth does 

 not considerably alter the magnitude of this velocity relative to 

 an observer at a particular point on the earth's surface, it does 

 produce marked diurnal variation of its direction. This direction 

 will obviously at any time be in the plane of the ecliptic — i.e. 

 the plane of the earth's orbit — and perpendicular to the line 



1 O. J. Lodge, "Aberration Problems," Phil. Trans, vol. clxxxiv. 1893. 



