42 SECTIONAL ADDRESSES. 
separations of H, and H,, and in some cases of H, and H,, to look 
for evidence that would lead to a confirmation of Sommerfeld’s theory. 
Up to the present the results obtained could not be considered as satis- 
factory. There was a lack of agreement in the values obtained for the 
separations by different investigators, and on the whole the values 
obtained were less than that demanded by the theory. The matter was 
reinvestigated recently, at my suggestion, by one of the research workers 
in the Physical Laboratory of the University of Toronto, Mr. G. M. 
Shrum, and in his experiments he succeeded in eliminating practically 
the whole of the secondary spectrum, and as a result was able to include 
in his measurements of the doublet separations that of H, as well as 
those of H., Hs, H,, and Ha. 
The results are the following : — 
Separation of the Components 
Line Wave-length Probable Error 
dr dy 
H, 6562-79 A 0-143 A 0:33 cm.—! 0-02 em.—! 
H 4861-33 ,, 0-085 ,, 0-36 ,, 0-01 ,, 
H, 4340-46 ,, 0-070 ,, | 0-37 ,, 0-02 ,, 
H, 4101-73 ,, 0-061 ,, 0°36 ,, 0-02 ,, 
H 3970-07 ,, 0-055 ,, 0°35 ,, | 0-02 ,, 
It will be seen that as far as the doublet separations are concerned they 
afford a striking confirmation of Sommerfeld’s theory. 
Model of the Atom of Helium. 
Considerable interest attaches to the atom of Helium. From the 
chemical point of view it has been considered to be inert, and conse- 
quently not likely to enter into chemical combination. Of all atoms 
it is the most stable, for it has the highest ionisation potential, namely 
24.5 volts. A study of the X-radiation emitted by the elements gener- 
ally makes it appear that the configuration we assign to the electronic 
orbits in helium atoms is maintained intact throughout the whole of 
the remaining heavier elements. These orbits, as Table I. shows, 
constitute for all atoms the K X-ray group the innermost and most stable 
system. For these reasons it is highly desirable that a model of the 
atom of helium be realised possessing high stability endowed with the 
capacity to emit radiation exhibiting the characteristic features of the 
helium spectrum, and having energy values for its normal and tem- 
porary stationary states that fit in with the experimentally determined 
values of its ionisation, resonance, and other critical excitation 
potentials. 
The earlier models of the atom of helium put forward failed entirely 
to meet these requirements. Models recently conceived by Lande * and 
by Bohr® are at the present time receiving considerable attention. In 
these the two electrons in the normal atom are taken to move in equiva- 
lent 1, orbits. As a first approximation these may be described as 
8 Lande, Phys. Zeit., No. 20, p. 228, 1919. 
* Bohr, Zeit. fiir Phys., No. 2, p. 464, 1920. 
