248 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
Let us return to Janssen’s researches on oxygen. After his labora- 
tory studies of the conditions relating to the production of these 
absorption bands of oxygen, Janssen wished to obtain them by inter- 
posing between the luminous source and the observer a column of air 
sufficiently long to produce them. In 1889 the Eiffel tower had just 
been constructed and a great electric light placed at the summit 
could be turned toward the observatory at Meudon, and furthermore 
the distance between the tower and the observatory was 7.7 kilometers, 
so that the light, before reaching the observatory, traversed a column 
of air exactly equivalent, so far as its absorption goes, to our own at- 
mosphere. These conditions produced the lines of the absorption 
spectrum of oxygen of exactly the same intensity as in the solar 
spectrum and brought a new confirmation of the exclusively terres- 
trial origin of these bands. 
This experiment of the tower of Eiffel is practically a repetition of 
another ingenious one made by Janssen, about 1864, on the shores of 
~ Lake Geneva. <A large wood fire was kindled at Nyon upon one of 
the banks of the lake; although close-to the spectrum of this fire 
appeared continuous, yet it showed, when observed at Geneva, at a 
distance from the fire of 21 kilometers, the telluric rays, including 
those of water vapor. 
We have already said that Janssen was not content with having 
produced artificially by one method the absorption lines of water 
vapor; he wished to make the reverse experiment and assure him- 
self that as we go higher and higher up in the atmosphere the telluric 
lines tend to disappear; that is to say, in proportion as we decrease 
the layer of air interposed between the sun and the observer. It 
was with this object in mind that Janssen undertook several mountain 
ascents; that of Faulhorn first, in 1864, then the Pic du Midi, and 
more recently of Mount Blanc. In the first ascent of the Grands 
Mulets, in 1888, he demonstrated clearly that the lines of the group 
B were less intense at an altitude of 3,000 meters than at Meudon, and 
in the ascent to the summit in 1893 and 1895 he believed that the last 
doublet of the B group had disappeared completely. His lameness 
made this mountain climbing particularly difficult, as in order to 
reach the summit of Mount Blanc he had to be carried along either 
on a litter or on a sledge. From these expeditions to Mount Blane 
Janssen came back convinced that an observatory built on the very 
summit of this famous peak would render important service to 
science and help to solve many problems of astronomy, meteorology, 
and physiology. The astronomer ascending to this altitude would be 
free from what has been called by some one “ atmospheric mud,” and 
the light from the stars would appear less deviated and diffuse; the 
meteorologist, placed in the very bosom of the atmosphere, could 
