Resonance Spectra of Sodium Vapour. 599 
lines and still have enough light to work with is now the 
chief difficulty. I have recently found a chlorate of potash 
film which in transmitted light shows a very black interference 
band not much over 15 Angstrom units in width. The 
position of this band varies with the angle of incidence of 
the light, and it can be set so as to cut off either one of the 
copper lines at will. It cannot, however, do this for a 
divergent beam of light, owing to the variation of the 
incidence angle in this case ; and as the crystal is less than a 
square centimetre in area, little or nothing is gained by its 
use over the monochromatic illuminator. If we could prepare 
crystal flakes similar to this, measuring several cms. in width, 
they could be used with light made parallel by a lens, and 
the problem of separating spectrum lines would be solved. 
In the resonance spectrum excited by the three green 
copper lines I have already measured over one hundred and 
twenty-five lines, but until more satisfactory spectrograms 
have been obtained with the separated radiations I prefer 
not to publish this table. The spectrum is very complicated, 
and I do not feel at all sure of the lines which belong 
together, although there are a number of very well marked 
series. My surmise of a year ago that the shortest of the 
three lines excited a series of doublets has been verified. 
The Magnetic Rotation Spectrum. 
The present work has shown that there are not many coinci- 
dences between the bright lines of the magnetic rotation spec- 
trum and the lines of the resonance spectra. The belief that 
these coincidences existed, which was expressed in the earlier 
paper, was due to the low dispersion used. There appear to be 
six or more series in the magnetic spectrum, but they do not 
coincide with any of the series excited by monochromatic 
radiations. The lines which show the magnetic rotation are 
the lines which come out with especial prominence in the 
spectrum of the fluorescence excited by white light. At the 
present time I am engaged in a study of the magnetic spec- 
trum by the method described in my recent paper on the 
possible existence of positive electrons in the sodium atom. 
I have photographed the spectrum over again under more 
favourable conditions, and have obtained a much larger 
number of lines, nearly 200 in all. The table of their wave- 
lengths will not be published until the final paper is ready 
for publication, as I have not yet determined whether all of 
the lines rotate the plane of polarization in the same direction. 
2S2 
