MECHANICS AND OPTICS. 215 



If we compare the great axes of ellipse for the different 

 stars, we shall have, theoretically at least, the means 

 determining this absolute velocity. 



This is perhaps less startling than it seems at first. 

 It is not a question, indeed, of the velocity in relation 

 to absolute space, but of the velocity in relation to the 

 ethics, which is regarded, by definition, as being in 

 absolute repose. 



Moreover, this method is purely theoretical. In fact 

 the aberration is very small, and the possible variations 

 of the ellipse of aberration are much smaller still, and, 

 acccordingly, if we regard the aberration as of the first 

 order, the variations must be regarded as of the second 

 order, about a thousandth of a second of arc, and 

 absolutely inappreciable by our instruments. Lastly, 

 we shall see further on why the foregoing theory must 

 be rejected, and why we could not determine this 

 absolute velocity even though our instruments were 

 ten thousand times as accurate. 



Another method may be devised, and, indeed, has 

 been devised. The velocity of light is not the same in 

 the water as in the air : could we not compare the two 

 apparent positions of a star seen through a telescope 

 filled first with air and then with water? The results 

 have been negative ; the apparent laws of reflection 

 and of retraction are not altered by the Earth's motion. 

 This phenomenon admits of two explanations. 



I, We may suppose that the ether is not in repose, 

 but that it is displaced by bodies in motion. It would 

 not then be astonishing that the phenomenon of re- 

 fraction should not be altered by the Earth's motion, 

 since everything — lenses, telescopes, and ether — would 

 be carried along together by the same motion. As for 



