596 Prof. R. W. Wood on the 
The Hues which form the series are marked with a star. 
5336-5 
5309-5 
5299 
5272 
5263 
5254 
5238 
5233 Differences. 
5201* 
5195 on 
5188 6y 
5162* 
5158 
5154 39 
5127 
5123 * 
51195 39 
5084* 
5081 39 
5045* 
5042 39 
5006* 
5002 ss 
4968* 
g9» 38 
49o6 
4930* 
4928 
4925 
The line in the lead spectrum which stimulates this 
resonance spectrum has a wave-length of 5006 : there is in 
addition a weaker line at 5002, and I am not sure whether 
the light of this line is responsible for any of the spectrum. 
It comes within one Angstrom unit of fitting into the series, 
but it is much fainter than the 5006 line. 
Bismuth Excitation. 
The bismuth arc is by far the best exciter of the vapour 
which we have, and it furnishes the most typical resonance 
spectrum. It has but a single line which is operative, and 
this line is located in the remote blue. The resonance 
spectrum consists of a series in the blue, which with long 
exposures can be traced well up into the green, and a host 
of lines at the upper end of the spectrum in the yellow-green 
region. 
The exciting line in the resonance spectrum is accompanied 
by two faint companions, one on each side, the first line above 
it is a close doublet, the second a wider one, and the first 
line below is a suspected doublet, judging by its width. In 
all probability the second and third lines below the exciting 
