. According to the Ritz combination principle the entire spectrum may then be obtained by forming every possible combination of two values among all the quantities
.
The fact that the frequency of each line of the spectrum may be written as the difference of two simple expressions depending upon whole numbers suggests at once that the terms on the right-hand side multiplied by
may be placed equal to the energy in the various stationary states of the atom. The existence in the spectra of the other elements of a number of separate functions of
compels us to assume the presence not of one but of a number of series of stationary states, the energy of the