594: Prof. R. W. Wood on the 
Thallium Excitation. 
The great line o£ the thallium arc excites a resonance series 
of lines some of which are accompanied by companion lines 
on the short wave-length side. A photograph of the spectrum, 
taken with a wide slit, is reproduced on PL XIX. fig. 1. The 
wave-lengths are as follows : — 
Differences. 
5206* 
5235 36 
5242 * ™ 
5278* 6l ° 
5304 36 
5314 * 37 
5387* db 
5419 365 
5423-5 * 
5450 36-5 
5460* 
The thallium arc burnt very steadily. The lower carbon, 
which was bored with a hole 2 mms. in diameter, was kept 
full of metallic thallium, and used as the anode. 
The lines of the series are a little closer together in this 
case, as appears from the table of differences. 
Magnesium Excitation . 
A great deal of time was wasted in trying to obtain a 
satisfactory magnesium arc between metallic electrodes in 
hydrogen at low pressure. It was finally found that the 
simple expedient of introducing magnesium powder into a 
hollow carbon answered every purpose, the arc burning 
steadily. Fresh powder was put in every four or five 
minutes. 
The green triplet (b group) excite brilliant resonance 
spectra, which are reproduced on PL XVIII. fig. 4. In this 
spectrum we find the triplet reproduced more or less perfectly 
at regular intervals. If the spacing of the individual lines 
forming the series was the same for each series, it is clear 
that we should have the triplet reproduced over and over 
again along the spectrum. That this is so only to an 
imperfect degree is due to the difference in the spacing of 
the lines of the different series. Xo means has as yet been 
found of exciting the spectra separately, owing to the 
proximity of the three exciting lines. 
