654 BELL SYSTEM TECHNICAL JOURNAL 



of theorists are these: atoms exist in Stationary States — they emit or 

 absorb radiation in passing from one of these States to another — the 

 frequency of the radiation is proportional to the energy-difference 

 between the two States from one to the other of which the atom 

 passes. Moreover, for certain kinds of atoms and molecules there 

 are empirical formulae which express known interrelations among the 

 energy-values of the A^arious Stationary States. These in brief are 

 the major facts to be explained. 



Bohr proved that the energy-values of the Stationary States of the 

 hydrogen atom could be reproduced by affirming, first, that the atom 

 consists of an electron and a nucleus of known masses and equal and 

 opposite known charges; second, that these revolve around their 

 common centre of mass according to the classical laws of mechanics 

 and without radiating energy; third, that among all the conceivable 

 orbits which such particles might describe there are certain ellipses, 

 distinguished by certain especial and peculiar features, which alone 

 the particles are permitted to choose — to each "permitted" ellipse 

 there corresponds a Stationary State, and each Stationary State may 

 be visualized as a permitted ellipse. 



The first of these assumptions has never since departed from the 

 physicists' world-pictures. In wave-mechanics it is still implicit, 

 though easily overlooked. The second and third have not so firm a 

 foothold. As I have elsewhere remarked, they are and always will 

 be as good as they ever really were. If we make the first two of Bohr's 

 assumptions, then it follows as a matter of course that whichever 

 Stationary State of the hydrogen atom we may wish to consider or may 

 hereafter discover, we shall always be able to find an elliptic orbit with 

 the proper energy- value to serve as its picture. Yet this alone is not an 

 important fact; the serious question is, whether the family of all per- 

 mitted elliptic orbits is set apart from the vast multitude of forbidden 

 ones by some simple and striking distinction which they all share and 

 none of the rest possesses, whether they rejoice in some intrinsic patent 

 of aristocracy. At first it seemed so ; now, however, it turns out that 

 the distinctive feature which originally was supposed to ennoble just the 

 orbits required to account for the Stationary States, and no others, 

 is not perfectly suited to every case. This weakened the prestige of 

 the elliptic orbits; and though the introduction of the Spinning 

 Electron has done much to save the situation, it has not done enough 

 to preserve them from the crescent disparagement of those who never 

 really liked them. 



With other atoms and with molecules, the situation is much the 

 same. Bohr and his successors visualized atoms as groups of electrons 



