CLASSICAL AND NEW MECHANICS 165 



decided only after an exhaustive trial. But there is no 

 doubt as to the startling effect it has produced on scien- 

 tific theory nor of the far-reaching importance of its 

 conclusions. For example, Professor Planck of Berlin 

 is said to have declared that this new idea of time and 

 space surpasses in boldness anything that has appeared 

 up to the present time in speculative science. 



The name, Principle of Relativity, is derived from 

 Professor Einstein's first postulate, which is as fol- 

 lows: the idea of absolute rest or absolute motion is 

 an impossibility to the human mind, and not only in 

 mechanics, but also in electro-dynamics corresponds 

 to no properties of phenomena. 



In addition he announced as a second postulate that 

 the velocity of light, V, in empty space is an absolute 

 constant of nature. Hence this velocity, contrary to 

 all others, is independent of the motion of the body 

 emitting and of the one receiving the light. The 

 startling nature of this postulate is readily under- 

 stood when we remember that the closest analogue to 

 light is sound, the velocity of which is known by ex- 

 perience to be dependent on these quantities. 



He has recorded for us, that he became convinced of 

 the necessity for these postulates, because the theory 

 of electro-dynamics developed by Maxwell leads to 

 an asymmetry, when applied to moving bodies, which 

 is not true experimentally. For instance, when a mag- 

 net and an electric conductor are moved with respect 



