SOLAR RESEARCHES BY JANSSEN—PLUVINEL. 249 
snatch the secrets of the formation of the clouds; and finally, the 
physiologist, in this elevated laboratory, could study the conditions 
of life where the atmospheric pressure had become one-half that at 
the ordinary level. 
When this observatory at Mount Blanc had been decided upon it 
required no common strength of purpose in Janssen to carry out 
the project. By his convincing words he succeeded in gathering 
together the necessary money and then braving his critics he fear- 
lessly built his structure upon the snow on the very top of Mount 
Blanc, overlooking from that culminating point all the surrounding 
Alps. Perhaps jt is in this project of the creation of the observatory 
of Mount Blanc that we get the best measure of the energy, the 
tenacity of purpose, and the audacity of which Janssen was capable. 
And it was indeed concerning the realization of this project that he 
said, “I have always thought that there are very few difficulties 
which can not be surmounted by a will strong enough or by study 
sufficiently profound.” During the last years of his life Janssen had 
for this observatory on Mount Blanc the solicitude of a father for 
the child. Each year he took delight in giving advice to those ob- 
servers who proposed to ascend this giant of the Alps for the purpose 
of some new research; he organized the expeditions even to the small- 
est details, aided in this task by both Mme. and Mlle. Janssen. 
Janssen was an enthusiast of the mountains and never ceased to 
praise their benefits. He loved to repeat to the mountain climbers 
that phrase of our great physicist, Foucault, “The mountain makes 
the man, the city destroys him.” 
We have seen the train of thought which led Janssen to observe 
eclipses of the sun, to carry out his laboratory researches, to analyze 
at high altitudes the light of the sun, to study the absorption pro- 
duced by the gases of the sun and by our own atmosphere. But 
beyond the limits of these studies Janssen took interest in other 
questions which brought with them the opportunity of satisfying his 
taste for long voyages. In 1874 and in 1882 he was in charge of the 
parties sent by France to observe the transits of Venus over the sun’s 
disk. For the study of this phenomenon the idea occurred to him 
of using a revolving photographic plate. With this instrument could 
be obtained a series of photographs separated by short intervals of 
time, so that the precise moment of the contact of Venus with the 
solar disk could be told. This revolving photograph was the pre- 
cursor of the apparatus of M. Marey for the study of the movements 
of animals, and it is upon its principle that the motion pictures of 
to-day are made. 
The study of volcanoes, and especially the spectrum analysis of 
the gases which escape from the craters, attracted the attention of 
