26 SECTIONAL ADDRESSES. 
been made by Wilson, Sommerfeld, Ehrenfest, Kramers, Lande, and 
others. 
Bohr in his theory supposes that each electron in an atom describes 
a central or quasi-central orbit under the attraction of the nucleus 
in combination with the fields of the other extra-nuclear electrons 
present in the atom. He imposes, moreover, upon these motions of 
the electrons in atoms something in the nature of a quantum censorship. 
As a generalised postulate it is laid down that from the continuous 
manifold of all conceivable states of motion that may be ascribed to 
an atomic system there exists a definable number of stationary states 
that possess a peculiar stability, and that are of such a kind that every 
permanent change of motion within the system must involve a complete 
transition from one stationary state to another. 
It is postulated further that while no radiation is emitted by the 
atomic system when it is in one of its stationary states, the process of 
transition from one stationary state to another is accompanied by the 
emission of monochromatic radiation with a frequency given by the 
relation 
vh=H,— &,, 
where h is Planck’s constant and E, and H, are the values of the energy 
of the atom in the initial and final stationary states between which the 
transition takes place. Conversely, it is to be understood that the absorp- 
tion by the atomic system of radiation with the frequency v given above 
results in a transition back from the final stationary state to the initial 
one. These postulates, it will be seen, form the basis of an interpreta- 
tion of the laws of series spectra, for the most general of these—the 
combination principle of Ritz—asserts that the frequency v of each of 
the lines in the spectrum of a selected element can be represented by 
the formula 
v=T,-T), 
where T, and T, are two spectral terms taken from a number that are 
characteristic of the element in question. 
On Bohr’s theory’ the interpretation of the law of Ritz would be 
that the spectrum of the element referred to must originate in transi- 
tions between stationary states for which the atomic energy values are 
obtained simply by multiplying by Planck’s constant the values of 
those spectral terms of which T, and T, are types. 
This, it is evident, indicates the feasibility of establishing a connec- 
tion between the series spectrum of an element and the constitution and 
structure of its atoms. From the spectrum of the element the series 
spectral terms can be selected and evaluated, and these values when 
multiplied by Planck’s constant will give the various energy levels 
within and associated with the atom of the element. As the number 
of electrons within the said atom is given by the atomic number of the 
element, the problem becomes one of assigning to these constituent 
electrons orbits of a size and form that will provide the values of the 
energy levels determined by the spectral series terms. 
2 Bohr, Nature Supplement, July 7, 1923. 
