THEORY OF THE ATOM 191 



sent out from or taken into the atom, the amount and 

 period must correspond exactly to h. But as regards 

 its internal arrangements the atom has no objection to 

 2/*, 3/*, 4/i, etc.; it only insists that fractions shall be 

 excluded. That is why there are many alternative orbits 

 for the electron corresponding to different integral mul- 

 tipliers of h. We call these multipliers quantum num- 

 bers, and speak of 1 -quantum orbits, 2-quantum orbits, 

 etc. I will not enter here into the exact definition of 

 what it is that has to be an exact multiple of h; but it 

 is something which, viewed in the four-dimensional 

 world, is at once seen to be action though this may not 

 be so apparent when we view it in the ordinary way in 

 three-dimensional sections. Also several features of the 

 atom are regulated independently by this rule, and 

 accordingly there are several quantum numbers — one for 

 each feature; but to avoid technical complication I shall 

 refer only to the quantum numbers belonging to one 

 leading feature. 



According to this picture of the atom, which is due 

 to Niels Bohr, the only possible change of state is the 

 transfer of an electron from one quantum orbit to 

 another. Such a jump must occur whenever light is 

 absorbed or emitted. Suppose then that an electron which 

 has been travelling in one of the higher orbits jumps 

 down into an orbit of less energy. The atom will then 

 have a certain amount of surplus energy that must be got 

 rid of. The lump of energy is fixed, and it remains to 

 settle the period of vibration that it shall have when it 

 changes into aether-waves. It seems incredible that the 

 atom should get hold of the aether and shake it in any 

 other period than one of those in which it is itself 

 vibrating. Yet it is the experimental fact that, when the 

 atom by radiating sets the aether in vibration, the 



