times the major axis. This condition was first derived by Sommerfeld from his general theory for the determination of the stationary states of a central motion. It is easily shown that this fixation of the value of the minor axis is equivalent to the statement that the parameter
of the elliptical orbit is given by an expression of exactly the same form as that which gives the major axis
in the unperturbed atom. The only difference from the expression for
in (17) is that
is replaced by