584 Prof. R. W. Wood on the 
spectra are radically different from spectra excited by any 
other means, I propose that they be named Resonance spectra, 
since they are without doubt excited by the resonance of one 
or more of the electrons of the system when monochromatic 
radiation plays upon it. 
During the past year much additional information has 
been gathered regarding these remarkable spectra, and 
though the work is by no means completed at the present 
time, it seems advisable to place on record the results ob- 
tained thus far, in view of the fact that theoretical papers 
(notably those by Gr. A. Schott) are appearing from time to 
time on the nature of the radiation emitted by electron 
systems. 
I have already shown that the D lines are present in the 
emission spectrum when the vapour is excited by light of 
the same wave-length. This fact is of interest in connexion 
with statements recently made by some writers that the 
D lines are never present unless oxidation is going on, and 
in the air-tight steel tubes, highly exhausted and repeatedly 
washed out by the hydrogen evolved by the sodium, it is 
difficult to see how any oxygen can be present. 
The very remarkable fact has now been established that 
the D lines can be caused to appear in the emission spectrum 
by stimulating the vapour with a very intense beam of blue- 
green light, from which all yellow light has been completely 
eliminated. This proves that the mechanism which produces 
the principal series is a part of, or connected in some way 
with, that very complicated piece of machinery which gives 
rise to the channelled absorption spectrum with its thousands 
of lines. 
The second point of interest is the discovery of series of 
equidistant lines in the red fluorescent spectrum, excited 
by monochromatic red light, the photographic recording 
of w T hich has been made possible by the panchromatic 
plates recently placed on the market by "\Vratten and 
Wainwright. 
The spectrum emitted when the vapour is stimulated by 
white light has been photographed under more favourable 
conditions with a large grating, and has been found to 
possess much more structure than was at first supposed, 
especially in the yellow-green region, where very little fine 
detail could be made out in the earlier photographs. 
The magnetic-rotation spectrum has been studied with a 
view of determining whether the rotatory power is positive 
or negative for the different electrons. The results obtained 
