600 Resonance Spectra of Sodium Vapour. 
IVJiite Light and C at J i ode-ray Excitation. 
At the close of the present year an attempt will be made 
to photograph the white-light fluorescence spectrum with 
the 21-foot grating in the second-order spectrum. This will 
give us a record on the same scale as the absorption-spectrum 
map. I have already found that the structure is much more 
complicated than was at first supposed, the darker regions 
between the strong lines being- filled with a multitude of fine 
lines. The structure of the spectrum which I attempted to 
show with a drawing in one of the earlier papers is quite 
incorrect, the peculiar appearance being due to a peculiar 
distribution of the light in the spectrum, combined with a 
rather wide slit. The white-light fluorescence spectrum, 
which begins at about -4600, ends quite abruptly at the point 
where the yellow-green doublet of the first subordinate series 
appears in the arc spectrum of the metal. 
This may be an accident of course, but it may also be due 
to some relation between the parts of the vibrating mechanism, 
with which we are at present unfamiliar. The general 
appearance of the upper half of this spectrum is shown on 
PI. XVIII. fig. 2 (upper spectrum), with an iron comparison 
spectrum. The upper end of this spectrum is also shown on 
PI. XIX. fig. 3 (upper spectrum), together with the D lines 
which come out bright, and a portion of the red and orange 
fluorescence spectrum. The red-orange fluorescence extends 
a little below the D lines, but not quite down to the terminus 
of the yellow-green fluorescence, the region between the two 
spectra being destitute of light. The bands which border 
the edge of this black region give it the appearance of a deep 
chasm. I have already obtained some excellent photographs 
of this region, and they will be published with the final paper. 
Exciting the vapour with cathode rays causes it to emit a 
very remarkable spectrum. The lines of the principal and 
subordinate series come out with great intensity, as well as 
the lines which appear in the fluorescence excited by white 
light. In addition there is an entirely new spectrum, which 
in a way appears to be symmetrical about the D lines, at 
least in their immediate vicinity. This spectrum is shown on 
PI. XIX. fig. 3 (lower spectrum). It is on the same scale as, 
and placed in coincidence with, the white-light fluorescence 
spectrum. The lines of the subordinate series I have marked 
with black dots. It is a very remarkable fact that the bright 
bands to the right and left of the D lines coincide with broad 
dark bands in the fluorescence spectrum. This scarcely 
shows on the print which I am reproducing, and it will in all 
probability disappear completely in the process of repro- 
