4 fi !

54 ON THE SEBIES SPECTKA OF THE ELEMENTS

combination tones in acoustics. It is due to the fact that the
perturbation of the motion will not only consist in an effect upon
the components originally present, but in addition will give rise to
new components. The frequencies of these new components may be
TO* -f /ca; where K is different from ±1. According to the correspond-
ence principle we must therefore expect that the electric field will
not only influence the lines appearing under ordinary circumstances,
but that it will also render possible new types of transitions which
give rise to the "new" combination lines observed. From an esti-
mate of the amplitudes of the particular components in the initial
and final states it has even been found possible to account for the
varying facility with which the new lines are brought up by the
external field.

The general problem of the effect of an electric field on the spectra
of elements of higher atomic number differs essentially from the
simple Stark effect of the hydrogen lines, since we are here con-
cerned not with the perturbation of a purely periodic system, but
with the effect of the field on a periodic motion already subject to
a perturbation. The problem to a certain extent resembles the
effect of a weak electric force on the fine structure components of
the hydrogen atom. In much the same way the effect of an electric
field upon the series spectra of the elements may be treated directly
by investigating the perturbations of the external electron. A | I
continuation of my paper in the Transactions of the Copenhagen ff j
Academy will soon appear in which I shall show how this method I ;
enables us to understand the interesting observations Stark and ( j
others have made in this field. I !

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' - • " ' - > ' i •

The spectra of helium and lithium. We see that it has been f j j.

possible to obtain a certain general insight into the origin of the f I '.•!

series spectra of a type like that of sodium. The difficulties en- I p

countered in an attempt to give a detailed explanation of the | |

spectrum of a particular element, however, become very serious, f I i

even when we consider the spectrum of helium whose neutral atom « I * f

contains only two electrons. The spectrum of this element has a ft

simple structure in that it consists of single lines or at any rate of | j

double lines whose components are very close together. We find, it

however, that the lines fall into two groups each of which can be ? 1"
' • ' '• ' ' '• :- ' .' '. • ' ' . • • "' '• kf '

 to the correspond-