THE ELECTRON— COMPTON 219 



had appeared, one devised by chemist Lewis and extended by Lang- 

 muir to explain the directional symmetries of atoms as indicated by 

 their molecular combining forms, and the other devised by physicist 

 Bohr to account for spectra. Gradually the Bohr theory has been 

 developed to include the symmetries of the Lewis-Langmuir theory, so 

 that both may be said to be merged, with many major additions too 

 numerous to mention. 



It was Bohr's bold genius to cast off some of the fetters of classical 

 mechanics, which had been fairly well proved inadequate to meet the 

 situation, and to devise a new mechanics franldy to meet the simplest 

 known facts of atomic structure and spectroscopy — the hydrogen 

 atom and the atomic hydrogen spectrum. In doing so, he at one 

 stroke brought into the same picture the quantum theory of radia- 

 tion, the electronic structure of the atom and the facts of spectroscopy. 

 He had his electron moving in a circular orbit around the nucleus 

 under the regular laws of electrostatic attraction and centrifugal 

 force. But he stipulated that only such orbits were possible in which 

 the angular momentum of the electron was an integral multiple of 

 Planck's constant h divided by 27r. He also stipulated that the 

 electrons should not radiate energy wliile revolving in their orbits, 

 but only when they jumped from one orbit to another. In this case 

 the frequency of light radiated was equal to the change of energy of 

 the electron between the two orbits, divided by Planck's constant h. 

 With these assumptions, the spectra of hydrogen and of ionized helium 

 were quantitatively explained in their main features, but not in their 

 finer details. 



Then came the Great War, and we heard little of atomic structure 

 in the United States. But in Germany, Sommerfeld was extending 

 Bohr's ideas in most interesting ways. He showed that, by considering 

 elliptic as well as circular orbits, and taldng account of the variation 

 of the electron's mass with speed, the fine details as well as the main 

 features in the spectra of hydrogen and ionized hehum were accurately 

 explained. He also showed how the theory could be extended to deal 

 with atoms where there were many electrons moving in orbits. He 

 showed that these additional concepts were in the right direction to 

 explain the more complicated spectra both in the visible and in the 

 X-ray regions. 



SPECTRAL LINES 



When this new work first was known in America, it started the most 

 feverish and earnest scientific activity that the country has ever 

 known, which is still in progress with undiminished zeal and with 

 increasing productive effectiveness, I well remember when the first 

 copy of Sommerfeld's Atombau und Spectrallinen came to America 

 in the possession of Prof. P. W. Bridgman. Until later copies arrived, 



