386 A CENTURY OF SCIENCE 



outer to an inner orbit, when the excess energy of the first 

 orbit over the second is radiated away. But the energy 

 emitted is also taken to be equal to hv, where v is the fre- 

 quency of the radiation. Hence v can be determined, and 

 the expression obtained for it is exactly that given long 

 before by Balmer as an empirical law. The most 

 remarkable thing about it, however, is that Bohr's result 

 contains a constant involving h and the electronic charge 

 and mass which has precisely the value of the universal 

 constant N of Balmer 's and Rydberg's formulae. In all, 

 the theory accounts for three series of hydrogen, and 

 yields satisfactory results for helium atoms which have 

 lost an electron, or lithium atoms which have a double 

 positive charge. But for atoms which retain more than 

 a single electron it seems no longer to hold. 



The three mentioned are only the most clearly defined 

 of a growing group of phenomena in which the quantum 

 manifests itself. Its significance and the alteration in 

 our fundamental conceptions to which it seems to be 

 leading is for the future to make clear. That it presents 

 the most important and interesting problem as yet 

 unsolved few physicists would deny. 



American Physicists. In attempting to cover the 

 progress of physics during the last hundred years in the 

 space of a few pages, many important developments of 

 the subject have of necessity remained untouched, and 

 the treatment of many others has been entirely inade- 

 quate. Among those appearing in the Journal of which 

 no mention has been made are LeConte's (25, 62, 1858) 

 discovery of the sensitive flame and Rood's (46, 173, 

 1893) invention of the flicker photometer. However, 

 enough has been recounted to indicate the pre-eminent 

 position in the history of physics in America occupied by 

 four men: Joseph Henry, of the Albany Academy, 

 Princeton, and the Smithsonian Institution; Henry 

 Augustus Rowland, of Johns Hopkins University; 

 Josiah Willard Gibbs, of Yale; and Albert Abraham 

 Michelson, of the United States Naval Academy, Case 

 School of Applied Science, Clark University, and the 

 University of Chicago. Of these, the last named has the 

 distinction of being the only American physicist to have 

 received the Nobel prize, though there is little doubt that 



