THE 
LONDON, EDINBURGH, and DUBLIN 
PHILOSOPHICAL MAGAZINE 
AND 
JOURNAL OF SCIENCE. 
[SIXTH SERIES.] 
MA Y 1908. 
LV. The Resonance Spectra of Sodium Vapour. By R. W. 
Wood, Professor of Experimental Physics, Johns Hopkins 
University *. 
[Plates XVIII. & XIX.] 
TI^HE vapour o£ sodium, obtained by heating the metal in 
I a highly exhausted steel tube to a temperature of about 
400 degrees, yields an absorption spectrum of great com- 
plexity. In addition to the D lines and the other lines (ultra- 
violet) of the principal series, which come out reversed, we 
find the entire visible spectrum, with the exception of a very 
narrow region in the yellow, filled with fine and sharp 
absorption-lines. This we shall call the channelled absorption 
spectrum, and we find it divided into two distinct regions, one 
extending from wave-length 4500 to 5700, and the other 
extending from about 5800 to the extreme limit of the red. It 
is probable that it extends out as far into the infra-red as 10 
or 12 fi, for recent investigations by the writer, in collaboration 
with Professor A. Trowbridge of Princeton, have shown that 
the vapour exhibits heavy absorption in this region, though 
the bolometer strip was not narrow enough to resolve the 
lines. This remarkable absorption spectrum was investigated 
by the writer in collaboration with J. H. Moore a number of 
years ago ; but the precision which is now being obtained in 
the work upon the fluorescence of the vapour has made a 
* Communicated by the Author. 
Phil. Mag. S. 6. Vol. 15. No. 89. May 1908. 2 R 
