102 BELL SYSTEM TECHNICAL JOURNAL 



or can be used as stront; sui)i)()rt to that theory. If those two spectra 

 of hydrogen and ionized hehum were unknown, it is unHkely that 

 the atom-model would e\er ha\e been suggested; it is more than 

 unhkely that the alom-model could ever have been regarded as 

 satisfactory. To this day the prestige of the atom-model results 

 almost entireh- from its achievement with those two spectra. 



Why then trouble with apphing it to the interpretation of other 

 spectra? Several good reasons can be given. For instance, it may 

 be that a system of several electrons about a nucleus acts in some 

 respects as a unit — that its motion can be considered in some ways 

 as the motion of a rigid body, that principles of quantization can be 

 found for the system as a whole, similar to the principles used for 

 quantizing the smaller and yet perhaps not more consolidated system 

 which a single electron is. Here and there in the discussion we shall 

 find indications that this way of thinking is suitable. 



Again, one is justified in arguing that if in simple cases a certam 

 law is proved, and if in complex cases neither that nor any other law 

 can be proved nor disproved, than we should assume that the law 

 proved for the simple cases extends over the complex ones. F"ew 

 events in this world take place under such conditions that conserva- 

 tion of energy can be proved to prevail during them; yet, from the 

 fact that conser\'ation of energy has been verified in whatever events 

 it has been tested, we do not hesitate to infer that it prevails in all. 

 Bohr's model having been so strongly fortified by the data for the 

 only two atoms for which it can be completely tested, why not assume 

 it for the others? 



And finally, there is the point that many of the data of experiment 

 are almost universally expressed in terms of the model, so that the 

 physical literature of today is almost incomprehensible without 

 some knowledge of it. Unfortunate as this is, it shows that the model 

 is a valuable aid for visualizing the facts. This justifies any model; 

 but must not be construed as evidence for it. 



It will be expedient to divide the subject substantially under these 

 following headings. 



(a) The Helium Atom. This, as the case of an atom composed 

 presumably of a nucleus and two electrons, comes nearest to being 

 amenable to calculation. C'ertain mechanically possible orbits of 

 the two electrons, possessing the peculiar features of the "permitted" 

 orbits of a single electron rexohing around a nucleus, have been 

 traced and their energy-values calculated. Not one of them has 

 given the observed energy-value of a Stationary State of the helium 

 atom. It is the consensus of opinion that whatever the features 



