MECHANICS AND OPTICS. 217 



XL 



The Principle of Relativity. 



But if the ether is not displaced by the Earth's 

 motion, is it possible by means of optical phenomena 

 to demonstrate the' absolute velocity of the Earth, or 

 rather its velocity in relation to the motionless ether ? 

 Experience has given a negative reply, and yet the 

 experimental processes have been varied in every 

 possible way. Whatever be the method employed, 

 we shall never succeed in disclosing any but relative 

 velocities ; I mean the velocities of certain material 

 bodies in relation to other material bodies. Indeed, 

 when the source of the light and the apparatus for 

 observation are both on the Earth and participate in 

 its motion, the experimental results have always been 

 the same, whatever be the direction of the apparatus 

 in relation to the direction of the Earth's orbital motion. 

 That astronomical aberration takes place is due to the 

 fact that the source, which is a star, is in motion in 

 relation to the observer. 



The hypotheses formed up to now account perfectly 

 for this general result, if we neglect very small quanti- 

 ties on the order of the square of aberration. The 

 explanation relies on the notion oi local time introduced 

 by Lorentz, which I will try to make clear. Imagine 

 two observers placed, one at a point A and the other 

 at a point B, wishing to set their watches by means of 

 optical signals. They agree that B shall send a signal 

 to A at a given hour by his watch, and A sets his 

 watch to that hour as .soon as he sees the signal. If 

 ihc operation were performed in this way onl)-, there 



