A.—MATHEMATICS AND PHYSICS. 55 
polarisation of the light passing through the vapour with magnetic fields 
as low as a few tenths otf a gauss. 
This idea of space quantisation may perhaps throw some light 
on the interesting and suggestive experiments of R. W. Wood and 
A. Ellett ** on the polarisation of the resonance light emitted by mercury 
and sodium vapours. In their experiments, it will be recalled, strong 
polarisation of the resonance light from mercury or sodium vapours 
could be produced by weak magnetic fields properly orientated. More- 
over, they found that the polarisation of the resonance light emitted by 
these vapours in the presence of the earth’s magnetic field could be 
destroyed by applying a magnetic field of less than one gauss provided 
it was suitably orientated. It is highly desirable that the experiments 
of Wood and Ellett should be followed up in order that sufficient 
information may be gained to enable us to elucidate the principles 
- underlying the modifications in the polarisation of the resonance light 
observed by them. 
It seems clear that atoms of sodium, for example, when excited 
by the absorption of resonance radiation would tend during the period 
of excitation to take up definite and characteristic orientations even in 
weak magnetic fields that would result in the polarisation of the re- 
sonance radiation emitted being different from that of the radiation 
emitted from atoms of the vapour situated in space in which absolutely 
no magnetic field existed. It should be remembered, too, that in the 
normal atom of sodium the orbit in which the valency electron is bound 
has the value 1 for its characteristic azimuthal quantum numter k. 
When the atom is excited by the absorption of resonance radiation the 
azimuthal quantum number of the orbit, in which the valency electron . 
becomes bound for a time, takes on the value 2. It seems clear then 
that the electronic orbit of the valency electron may be subject to 
different orientations relative to the rest of the atom when the atom 
is in the excited state from what it would be with the atom in its 
normal state. These relative orientations, moreover, would again be 
different in the presence of even a weak external magnetic field from 
what they would be in the complete absence of such a field. It is, 
therefore, quite conceivable that changes in orientation of electron 
orbits may be able to account for the phenomena observed by Wood 
and Ellett, but at present the whole matter appears to be rather involved 
and rather difficult to clear up with the information as yet available. 
Quantum Theory and the Zeeman Effect. 
Among the most fruitful of the prinéiples utilised by Bohr in the 
development of his theory of radiation is the Adiabatic Hypothesis 
enunciated by Ehrenfest.‘7 To this hypothesis Bohr has given the 
name the Principle of Mechanical Transformability. Numerous examples 
of the application of this principle might be cited, but the one that 
concerns us most here is that which deals with the effect of the 
establishment of a magnetic field on the electronic orbits in atoms. It 
46 Wood and Ellett, Proc. Roy. Soc., A, June 1923, p. 396. 
47 Ehrenfest, Die Naturwissenschaften, vol. 11, Heft 27, July 6, 1923, 
p. 543. 
