298 TRANSACTIONS OF SECTION A. 
tudes. In the second case it is made up by relatively large amounts from a 
proportion only of the systems, and an increase would be due to a larger 
proportion of atoms changing from one system to another. In the first case, 
although the constancy of period follows as a matter of course, it is difficult 
to see how the conditions of Planck’s quanta can be met, and that the ideas 
lying at the base of his theory are well founded there can be little doubt. 
In the second case the energy for each line is transferred in the same amount, 
and the constancy of the frequency follows at once from Planck’s theory. 
These general considerations seem to point to the conclusion that the cause of 
spectral emission is change from one configuration to another of less internal 
energy. But there is experimental evidence pointing in the same direction. 
Stark has shown that the series lines in a spectrum are due to molecules which 
have lost one or more electrons. For instance, doublet series are due to 
molecules which have lost one electron, triplets two, &c., and we should there- 
fore expect the energy emitted to be given out by their recombination to the 
neutral state. Since in general the larger proportion of spectral lines—both 
are and spark—are either series or. seem to be closely related to series lines, it 
would appear that change of state is one of the chief causes of radiation. 
A further consideration pointing in the same direction is afforded by the 
fact that the formule to which series lines conform give the frequency itself, 
and not the square of the frequency, which latter is always the case when the 
forces of displacement are proportional to the displacements themselves. As 
Rayleigh has pointed out, the former case requires forces proportional to the 
velocities, and hence suggests motion in magnetic fields. Now we know these 
fields exist, and as a fact the only theory which reproduces Rydberg’s formulz 
is that of Ritz, depending only on magnetic fields. Unfortunately electrostatic 
fields exist and must be taken account of. If we could conceive of shells of 
constant magnetic force produced by electric charges moving in such a way 
that the electric forces between the moving charges themselves are annulled, we 
should have made a first step towards forming a basis of a satisfactory theory. 
That such motions are possible is rendered probable from consideration of 
Maxwell’s classical case of two uniformly charged parallel plates moving 
parallel to one another with the velocity of light. The great desideratum in 
the present state of the question is, not attempts at forming a complete theory, 
but mathematical discussions of as many simple cases as possible, in order to. 
obtain a clearer comprehension of what such systems may be expected to 
explain. From this point of view the recent most suggestive paper of Conway 
on ‘ An Electromagnetic Hypothesis as to the Origin of Series Spectra’? is of the 
greatest value. We want more of a similar nature. 
Whilst, however, in all probability the greater portion of a spectrum is due 
to changes of configuration, it does not necessarily follow that lines related to 
the series are the only ones emitted. In fact, the high-frequency vibrations 
discovered by Barkla and measured quite recently by Moseley are clearly a case 
in point. Nicholson has determined recently the frequencies of small oscilla- 
tion of electrons revolving round positively charged nuclei on the basis of the 
Rutherford theory. More especially he finds that the sets of lines observed in 
the corona and in nebule fit in very exactly for elements in which the nuclear 
charges are respectively 4 and 5, and the lines are due to neutral atoms 
and also to atoms which have lost or gained one or two or more electrons. The 
agreements are so close and so numerous as to leave little doubt of the general 
correctness of the theory. But the lines are certainly not connected in any way 
with the series type of line. Their appearance is probably due to the vast 
number of atoms in the corona and nebule in the line of sight all emitting 
vibrations, whilst the absence of the series type may be due to the rarefaction 
of the gas causing comparatively few changes from one configuration to another. 
Nicholson’s theory stands alone as a first satisfactory theory of one type of 
spectra. Unfortunately this type contains so few examples that if they exist in 
other elements they have not been noticed. It affords considerable evidence 
that Rutherford’s theory approximates to the actual case when the nuclear 
charge is a small multiple of the fundamental charge. Several attempts have 
* Phil. Mag. xxvi. p. 1010, December 1913, 
