Resonance Spectra of Sodium Vapour. 583 
interesting, for the light transmitted appears to be as 
homogeneous as the light in the spectrum of the iron 
arc. 
To attempt to unravel this spectrum, or find any regular- 
ities in it by the usual means, is quite out of the question, 
for the lines are so numerous and so close together that we 
could pick out series that would conform to any law that 
we might choose to invent. 
As I have shown in a previous paper, however, we possess 
a very beautiful experimental method of analysing the 
spectrum, and of determining just which lines belong 
together ; a method, moreover, which may in time yield 
results which will enable the theoretical physicists to tell us 
the exact nature of the piece of machinery which we call the 
sodium molecule. 
If we illuminate the vapour with a powerful beam of 
white light, it becomes strongly fluorescent, emitting a 
spectrum which I now believe to be the exact counterpart 
of' the absorption spectrum. During the earlier part of the 
work, before the methods and apparatus had been perfected 
to the high degree which they have now reached, it was 
believed that comparatively few of the absorption-lines gave 
rise to fluorescence, as the fluorescence spectrum could not 
be photographed with the high dispersion which is now 
employed. This fluorescent spectrum is obviously of little 
help to us, for it is quite as complicated as the absorption 
spectrum. If, however, we throw monochromatic light into 
the vapour, instead of white light, we observe a very remark- 
able phenomenon. We now have series of bright lines 
spaced at very nearly equal intervals along a normal spec- 
trum, and separated by a distance equal to about 37 Angstrom 
units. Various series of lines with varying distribution of 
intensity can be brought out by changing the wave-length 
of the exciting light. In every case, light of the same wave- 
length as that of the exciting light is emitted by the vapour, 
and in addition a large number of other frequencies, which 
bear a definite relation to each other. In the absence of the 
exciting light the vapour is non-luminous, that is the 
electrons, which we may perhaps regard as revolving in 
concentric rings, do not radiate any energy. The passage of 
an intense beam of monochromatic radiation through the 
medium disturbs this non-radiating system in some way, and 
causes it to emit a series of bright spectrum-lines. The 
phenomenon can of course be classed under fluorescence if 
we choose, but as we appear to be dealing with a much more 
tangible phenomenon than is usuallv the case, and as these 
2R2 
