L. Page — A Century's Progress in Physics. 351 



tive nucleus of a hydrogen atom, must, according to elec- 

 trodynamic laws, radiate energy. This radiation will 

 act as a resistance to its motion, causing its orbit to 

 become smaller and its frequency to increase. Hence 

 luminous hydrogen would be expected to give off a con- 

 tinuous spectrum. The very fine lines actually found 

 seem inexplicable on the classical dynamical and electro- 

 dynamical theories. These lines, and those of many 

 other spectra, may even be grouped into series, and the 

 relations between them expressed in mathematical form. 

 Formulas have been proposed by Balmer, Bydberg, Ritz 

 and others, all of which contain a universal constant N 

 as well as certain parameters which must be varied by 

 unity in passing from one line of a series to the next. 



In 1913 Bohr 18 proposed anatomic theory which brings 

 to light a remarkable numerical relationship between 

 this quantity N and Planck's constant h. He postulated 

 that the electron in the hydrogen atom, for instance, can- 

 not revolve in a circle of any arbitrary radius, but is con- 

 fined to those orbits for which its kinetic energy is an 

 integral multiple of |- h n, n being its orbital frequency. 

 Now at times this electron is supposed to jump from an 

 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. 



18 N. Bohr, Phil. Mag., 26, 1, 1913 et seq. 



