SOLAR-RADIATION RESEARCHES BY JULES CESAR 
JANSSEN.¢ 
[With 1 plate.] 
By A. DE LA BAUME PLUVINEL. 
The scientific career of Janssen was passed far more in temporary 
observatories built in remote parts of the globe and with equipments 
easily transportable than in fixed establishments provided with 
great instruments. Janssen was thus notably a missionary of science 
always ready to devote himself to new efforts in the organization 
and leading to success of some new expedition for science. This 
spirit of enterprise made him love those voyages where he was sus- 
tained with the knowledge that he was devoting himself completely 
to science rather than working quietly in his laboratory. 
The principal missions undertaken by Janssen had for their objects 
the observation of phenomena observable only from some limited 
portion of the earth or where he must find a sky favorable to some 
delicate experiment. But in either case, Janssen carried on researches 
which had the same goal, so his work presents a remarkable unity. 
We might indeed say that all his studies were made upon the selec- 
tive absorption for radiation by gases. His devotion to this class 
of researches was determined by Kirchoff and Bunsen’s discovery 
of absorption spectra; indeed the first spectroscopic experiments of 
Janssen date from 1862, very shortly after the German physicists had 
published their results. 
Janssen studied the absorption for solar radiation, on the one hand, 
by the surrounding envelopes of the sun itself, on the other, by our 
own terrestrial atmosphere. It was while observing these gaseous 
envelopes of the sun, which may be seen only during the few short 
moments of a total solar eclipse, that he accomplished the first part 
of this programme, and in undertaking his classic researches on the 
telluric lines, the second. We will follow Janssen through his 
studies in these two groups of problems. 
@Translated by permission from Astrophysical Journal, September, 1908. 
243 
