MATTER, SPACE, AND TIME 251 



what progress can be made on the general lines 

 of Bohr's theory is all consistent with observed 

 facts. 



Here, then, in the quantum theory and Bohr's 

 application of it to atomic structure, we have quite 

 a new departure in science. No explanation can 

 be given at present on the principles of classical 

 dynamics of the existence of an indivisible 

 quantum of action, or of its consequence the 

 restriction of atomic planetary electrons to a few 

 definite orbits. The quantum seems a brute fact, 

 which we must accept, but cannot, yet at any rate, 

 explain. But this, as Eddington holds, may be a 

 sign of its real importance. It is certain that we 

 ourselves have not read it into the story of Nature. 



We must now pass to the consideration of 

 the theory of relativity, which has shared with 

 the theory of quanta and the allied problem of 

 atomic structure the chief attention of mathe- 

 matical physicists during recent years. 



The idea of relativity arose from the results 

 of various measurements of the velocity of light, 

 and the discordant indications they gave of the 

 relation between the earth and the hypothetical 

 aether of space. 



Astronomical observations suggest that the 

 earth moves through a quiescent aether, the aether 

 streaming through the moving atoms of matter 

 as wind through a grove of trees. Moreover, 

 Lodge found that the velocity of light between 

 two parallel steel discs, whirled round their axis 

 at great speed, was the same whether the light 

 passed in the same or the opposite direction to 

 the movement of the plates. In this case, the 



