246 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
birth to these lines, for its existence in the envelopes of the sun could 
as well play in the production in the phenomena. With respect to 
the theory of the sun, it is of the greatest importance to know 
whether oxygen coexists with hydrogen in the solar envelopes. 
Janssen, indeed, attributed to the question of this existence of oxygen 
in the sun a capital importance, and so he searched by all the methods 
available to find out whether the bands, A, a, and B, had their origin 
in both solar and terrestrial absorption or were produced solely by 
our atmosphere. In order to solve this problem he produced these 
absorption bands in his own laboratory in such a manner as to see 
whether a column of oxygen equivalent to the oxygen contained in 
the air would produce bands of the same intensity as those we ob- 
serve in the solar spectrum. The same train of thought caused him 
to observe the spectrum of a luminous source so far distant that the 
air intervening between it and his apparatus would produce an ab- 
sorption equivalent to that of all the air between us and the sun 
- and at different heights of the sun above the horizon. Then we find 
him making, in a sense, the reverse experiment—diminishing sufh- 
ciently the action of the interposed air until the bands under ex- 
amination are no longer visible. 
The study of gaseous spectra was carried on very extensively by 
Janssen. He examined at his spectroscope the light from a source 
producing a continuous spectrum after this hght had traversed tubes 
containing gases under various pressures and temperatures. His 
laboratory, situated in the old stables of the chateau at Meudon, 
allowed him to use tubes reaching a length of 60 meters. His re- 
searches related chiefly to oxygen. By varying the pressure of the 
gas in the tube he could at will make the absorption lines of oxygen 
disappear, especially the line B. His experiments showed that cer- 
tain absorption lines—B, for instance—always appeared when the 
product of the length of the tube by the pressure of the gas reached 
a certain value. 
The experiments upon the absorption lines of oxygen led Janssen 
to a remarkable phenomenon which required repetition with the per- 
fected means at the disposal of modern physics. He discovered 
that, besides the telluric rays, the absorption spectrum of oxygen 
showed, under certain conditions, a system of bands difficult to re- 
solve into lines, and whose production was ruled by an entirely 
different law than that stated above for the telluric lines. These bands 
appear when the product of the length of the tube by the square of - 
the pressure reaches a certain value. This law had a remarkable 
confirmation when Olszewski studied the spectrum of liquid oxygen. 
It was found that the bands of Janssen appeared when the layer of 
liquid oxygen reached a thickness which the law of the square of the 
pressure would require. Janssen confirmed his law in yet another 
