172 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1924 



of the difficulties were discussed in the presidential address before 

 this association by Sir Oliver Lodge at Birmingham in 1913. It 

 suffices to say that this theory has proved of great value in several 

 branches of science, and is supported by a large mass of direct ex- 

 perimental evidence. 



In applying the quantum theory to the structure of the hydrogen 

 atom Bohr supposed that the single electron could move in a number 

 of stable orbits, controlled by the attractive force of the nucleus, 

 without losing energy by radiation. The position and character of 

 these orbits were defined by certain quantum relations depending 

 on one or more whole numbers. It was assumed that radiation was 

 only emitted when the electron for some reason was transferred 

 from one stable orbit to another of lower energ}^ In such a case it 

 was supposed that a liomogeneous radiation was emitted of fre- 

 quency V determined by the quantum relation E=:Av where E was 

 the difference of the energy of the electron in the two orbits. Some 

 of these possible orbits are circular, others elliptical, with the nucleus 

 as a focus, while if the change of mass of the electron with velocity 

 is taken into account the orbits, as Sommerfeld showed, depend on 

 two quantum numbers, and are not closed, but consist of a nearly 

 elliptical orbit slowly rotating round the nucleus. In this way it is 

 possible not only to account for the series relations between the 

 bright lines of the hydrogen spectrum, but also to explain the fine 

 structure of the lines and the very complicated changes observed 

 when the radiating atoms are exposed in a strong magnetic or elec- 

 tric field. Under ordinary conditions the electron in the hydrogen 

 atom rotates in a circular orbit close to the nucleus, but if the atoms 

 are excited by an electric discharge or other suitable method, the 

 electron may be displaced and occupy any one of the stable positions 

 specified by the theory. In a radiating gas giving the complete 

 hydrogen spectrum there will be present many different kinds of 

 hydrogen atoms, in each of which the electron describes one of the 

 possible orbits specified by the theory. On this view it is seen that 

 the variety of modes of vibration of the hydrogen atom is ascribed, not 

 to complexity of the structure of the atom, but to the variety of stable 

 orbits which an electron may occupy relative to the nucleus. This 

 novel theory of the origin of spectra has been developed so as to 

 apply not only to hydrogen but to all the elements, and has been 

 instrumental in throwing a flood of light on the relations and origin 

 of their spectra, both X ray and optical. The information thus 

 gained has been applied by Bohr to determine the distribution of 

 the electrons round the nucleus of any atom. The problem is ob- 

 viously much less complicated for hydrogen than for a heavy atom, 

 where each of the large number of electrons present acts on the 



