648 DELL SYSTEM TECIIXICAL JOURNAL 



that when tlic electron is re\olving in one or another of these, the 

 energy- of the atom shall have one or another of the values prescribed 

 by the Stationary States. If we arbitrarily say that the electron can 

 revolve only in one or another of these orbits, then we have an atom- 

 model competent to interpret the Stationary States of the hydrogen 

 atom. But is there anything distinctive about these selected orbits, 

 anything peculiar, anything which marks them out and sets them apart 

 from the other, from the discarded orbits? Have they any feature in 

 common, apart from being necessary to give the observed energj-- 

 values of the Stationary States? 



It is hardly possible to lay too strong an emphasis upon this require- 

 ment; the value of the contemporary atom-model depends upon satis- 

 fying it. Let me put the matter another way. From the moment 

 that we imagine that the electrons within the atom are cruising around 

 the nucleus in orbits without radiating energ\' and without dropping 

 into the nucleus, we are sacrificing the unity and the coherence of the 

 classical theory of electricity. So grav'e an action is not to be under- 

 taken lightly nor with indifference; it were foolish to make such a 

 sacrifice without recompense; and there is no recompense to be found 

 in merely proving that especial orbits can be so selected as to copy the 

 energy-values of the Stationary States. If one is going to deviate 

 from the rules of the classical theory of electricity, one must deviate 

 by rule. If one is going to disrupt the system which prevails in one 

 great department of theoretical physics, one must systematize another 

 department in exchange. If one proposes to violate some of the prin- 

 ciples of modern physics, by asserting that electrons can travel in 

 certain orbits without radiating, he must reconcile the congregation 

 of physicists to his sacrilege by proving that the selected motions are 

 themselves go\-erned by a principle, as imposing as those he lacerated. 

 If the innovator cannot show that his innovations are systematic, he 

 is not likely to prosper; but if his innovations are derived from a 

 principle, it may supersede those which he contradicted. 



To discover such a principle is the ambition of, jirohabl)', halt ot 

 the theoretical physicists who are active today. 



There are other general statements which miglu hv made at this 

 point; but they will be more intelligible, and so will the foregoing para- 

 graphs be, after I ha\e given an illustration. For this purpose I will 

 describe two models of the hydrogen atom, each of them consisting 

 of a nucleus and a single electron, each capable of being so constrained 

 that its energy-values will copy those of the Stationary States of 

 hydrogen. With one of these, however, the description can be carried 

 no farther. With the other, I shall show — following Bohr that the 



