220 ANNUAL IlEPOKT SMITHSONIAN INSTITUTION, 1937 



he knew no peace and enjoyed no privacy, for he was besieged by 

 friends wanting to read the book — which he would not allow to go 

 out of his possession. I recall, too, the sudden popularity of the only 

 two or three men in America who knew what a spectral series was. 

 Heretofore, practically our only interest in spectra had been in the 

 cuhnary variety of spectroscopy used by chemists in identifying 

 chemical elements. No interjHetive quality to speak of had hitherto 

 been attached to the peculiar numerical regularities which had been 

 discovered in the vibration frequencies of groups of spectrum lines. 



I recall, too, the dismay with which we found only a handful of 

 mathematical jihysicists versed in the analytical dynamics underlying 

 the new atomic structure theories. In the summer of 1921, having 

 been taught by one of these few mathematical physicists, I went to the 

 University of Michigan to lecture on Sommerfeld's theory, and found 

 there also F. A. Saunders, invited to impart his knowledge of spectrum 

 series. In the winter of 192G, Born and Jordan having just announced 

 a new development in quantum mechanics, I found more than 20 

 Americans in Gottingcn at this fount of quantum wisdom. A year 

 later they were at Zurich, with Schrodinger. A couple of years later, 

 Heisenberg at Leipzig and then Dirac at Cambridge held the Elijah 

 mantle of quantum theory. In America, contributions are coming 

 rapidly, particularly in the fields of apphcation to chemical inter- 

 pretations, metals and other complex situations. 



From all this has come the situation which permitted Dirac, a few 

 years ago, to write: "The underlying physical laws necessary for the 

 mathematical theory of a largo part of i)hysics and the whole of chem- 

 istiy are thus completely known, and the difficulty is only that the 

 exact application of these laws leads to equations much too complicated 

 to be soluble." But if any ambitious young scientist be discouraged 

 lest there be little left to do, let him consider the unexplored atomic 

 nucleus, or the fact that every attempt to apply these laws, which 

 look so satisfactory to us now, discloses new realms of knowledge still 

 unexplored. 



Time forbids mention of the most interesting work which was done 

 to check and extend the theories of atomic structure, through direct 

 measurement of the energy states of atoms and molecules by carefully 

 controlled bombardment of these molecules by electrons. Begun by 

 Franck and Hertz in Germany, much of this work was done in America 

 by Foote and Mohler at the Bureau of Standards, by my students at 

 Princeton and by Tate's group at Minnesota, all since 1920. 



Before leaving the interpretive triumphs of the electron, however, 

 I cannot refrain from jumping from the atom to the universe, to the 

 interpretation of conditions on the stars. Spectra of stars had long 

 been known, and these were interpreted as indicating that some stars 

 consist principally of hydrogen, others of hehum and others of many 



