MECHANICS AND OPTICS. 221 



observations of astronomical aberration could make us 

 acquainted with the absolute velocity of the Earth, if 

 our instruments were a thousand times as accurate, 

 but this conclusion must be modified. It is true that 

 the angles observed would be modified by the effect of 

 this absolute velocity, but the graduated circles we use 

 for measuring the angles would be deformed by the 

 motion ; they would become ellipses, the result would 

 be an error in the angle measured, and this second 

 error would exactly compensate the former. 



This hypothesis of Lorentz and Fitz-Gerald will 

 appear most extraordinary at first sight. All that can 

 be said in its favour for the moment is that it is merely 

 the immediate interpretation of Michelson's experi- 

 mental result, if we define distances by the time taken 

 by light to traverse them. 



However that be, it is impossible to escape the 

 impression that the Principle of Relativity is a general 

 law of Nature, and that we shall never succeed, by any 

 imaginable method, in demonstrating any but relative 

 velocities ; and by this I mean not merely the velocities 

 of bodies in relation to the ether, but the velocities ot 

 bodies in relation to each other. So many different 

 experiments have given similar results that we cannot 

 but feel tempted to attribute to this Principle of 

 Relativity a value comparable, for instance, to that of 

 the Principle of Equivalence. It is well in any case to 

 see what are the consequences to which this point of 

 view would lead, and then to submit these consequences 

 to the test of experiment. 



