DR. A. SCHUSTER ON THE SPECTRA OF METALLOIDS. 
51 
7. The Compound Line — Spectrum of Oxygen. 
In one of his early papers (Pogg. Ann., 107), Plucker gives the spectrum of oxygen 
as consisting of four lines, with the following wave-lengths. 
6150 
5328 
5185 
4367 
In his later drawing of the spectrum of oxygen (Phil. Trans., 1865) he gives a great 
many lines of which these four form part. Wullner says that these four lines are 
always the first to appear in oxygen tubes. Angstrom and Thalen do not give 
these lines ; Huggins does not give them ; Salet does not give them. Plucker and 
Wullner are the only observers who experimented under the circumstances under 
which the lines come out, and they seem to assume that they form part of the 
ordinary line spectrum of oxygen ; yet when the ordinary line spectrum is brightest, 
these lines are quite invisible, and when these lines are brightest, no trace of the 
ordinary line spectrum can be detected. I must repeat that these lines come out 
equally well, whatever way the oxygen is prepared. Wullner prepared his gas by 
electrolysis and saw the lines; I prepared my gas, as already mentioned, in three 
different ways. When I began work I was under the impression that these lines 
must be due to impurities ; but after having spent a great deal of time in trying to 
trace their origin, I feel certain that they are due to pure oxygen. 1 have seen the 
lines almost daily for many months, and in at least 30 different tubes. 
The appearance of an oxygen tube when a spark passes while it undergoes gradual 
exhaustion is as follows :— 
Experiment 8.—At first the spark has a yellow colour, and the spectrum is per¬ 
fectly continuous. Almost immediately, however, four lines are seen in the capillary 
part above the continuous spectrum. One of these lines is in the red, two are in the 
green, and one is in the blue. The discharge still passes as a narrow spark throughout 
the length of the tube. In the wide part the spectrum remains continuous, and 
it extends more towards the red and blue than in the capillary part. It seems as if 
the four lines had taken away part of the energy of the continuous spectrum. 
As the pressure diminishes, these lines increase considerably in strength ; the spark 
spreads out in the wide part of the tube, and the intensity of the continuous spectrum 
is therefore considerably diminished, while it still forms a prominent part in the 
spectrum of the capillary part. When the pressure is small the continuous spec¬ 
trum decreases in intensity. At the same time the negative glow with its own 
characteristic spectrum, gradually extends through the negative half of the tube into 
the capillary part. The continuous spectrum has now entirely disappeared ; the bands 
of the negative pole and the four lines stand out on a perfectly black background. It 
is under these conditions that the change from the compound line spectrum to the 
h 2 
