will be zero and the perturbing forces will be given by the higher terms in (25).
Let us now assume that for a certain series of stationary states
is different from zero. According to our considerations, the major-axis of the orbit of the outer electron is approximately equal to that in the stationary states of the hydrogen atom. The major-axis will therefore be approximately proportional to
. Accordingly the quantity corresponding to
in the equation (19) and due to the second term in (25) will vary approximately as
. The difference in the energy of the two stationary states corresponding to (19) may therefore be expected to vary approximately as