Closing Years of the Nineteenth Century. 445 



on Newton's laws of motion, does not supply any criterion by 

 which rest may be distinguished from uniform motion ; for if 

 the laws of motion are applicable when the position of bodies 

 is referred to any particular set of axes, they will be equally 

 applicable when position is referred to any other set of axes 

 which have a uniform motion of translation relative to these. 



The older theories of electrostatics, magnetism, and electro- 

 dynamics, which are based on the conception of action at a 

 distance, are concerned only with relative configurations and 

 motions, and are therefore useless in the search for a basis of 

 absolute reckoning. 



But the existence of an aether, which is postulated in the 

 undulatory theory of light, seems at first sight to involve the 

 conceptions of rest and motion relative to it, and thus to afford 

 a means of specifying absolute position. Suppose, for instance, 

 that a disturbance is generated at any point in free aether; 

 this disturbance will spread outwards in the form of a sphere ; 

 and the centre of this sphere will for all subsequent time 

 occupy an unchanged position relative to the aether. In this 

 way, or in many other ways, we might hope to determine, by 

 electrical or optical experiments, the velocity of the earth 

 relative to the aether. 



The failure of such experiments as had been tried led 

 Fitz Gerald* to suggest that the dimensions of material bodies 

 undergo contraction when the bodies are in motion relative 

 to the aether. By the transformation of Lorentz and Larmor, 

 as we have seen, this hypothesis came to be expressed in a new 

 form ; namely that the equation of the figure of the body, 

 referred to a frame of reference moving with it, is always the 

 same, but that frames of reference which are in motion relative to 

 each other are based on different standards of length and time. 

 This way of regarding the matter brings into prominence the 

 fundamental questions involved. Before speaking of lengths 

 and velocities, it is necessary to examine the nature of systems 

 of measurement of space and time. 



* Cf . p. 432. 



