250 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
Janssen and was the cause of voyages to Santorin, to the Azores, 
to the Sandwich Islands, and finally to Mount Vesuvius. 
Oftentimes these voyages were utilized for the determination of 
the magnetic elements of the earth. Indeed, his first scientific expe- 
dition had for its object the determination of the magnetic equator 
in Peru; then later he made, in the Azores, magnetic observations 
for geological purposes, and to him also we owe the determination 
of the magnetic equator in India and the neighboring island of 
Malacca. 
But all these voyages were not enough to use up his energy. Be- 
tween whiles he founded, in 1874, the Observatory of Meudon, and 
apart from the observations on the absorption spectra of gases, of 
which we have already spoken, he was busy with applications of 
photography, notably to the solar disk. Janssen was one of the 
first to foresee the services which the photographic plate could render 
in the observatory, and he tersely indicated the role which it would 
‘play in the science of observation when he said that it would be the 
“true retina of the scientist.” ; 
Assisted first by M. Arents, and then by M. Pasteur, he obtained 
at Meudon the remarkable series of solar photographs of which the 
more characteristic ones have been embodied in an atlas which is a 
true monument to the history of the sun. These photographs were 
made especially for the study of the character of the solar surface 
and show most delicate details of the photosphere. Examining 
them closely, Janssen found that the solar surface has a peculiar 
texture, to which he gave the name photospheric “reseau.” The 
existence of this reseau was at first attributed to real displacements 
of the granulated structure of the photosphere, but later it seemed 
more probable that the cause must be looked for in the irregular 
refractions produced perhaps by our own atmosphere but possibly 
by the gases surrounding the sun itself. 
Photography has always occupied a place of honor at the Observa- 
tory of Meudon, and, not content with the qualitative results which 
it had hitherto given, Janssen wished to make use of it for quanti- 
tative observations. And so we owe to him numerous photographic 
photometric researches and, notably, the determination of the rela- 
tive brillancy of the sun and the stars. He was one of the first to 
make use of out-of-focus stellar disks for photometric measures. 
Janssen had a predilection for photography which embraced all the 
application of this science and of which he gave proof by accepting 
the invitations to preside over numerous reunions and congresses of 
photographic societies. 
Janssen was above all an observer and man of action. He did not 
allow himself to be tempted by the desire of giving his name to a 
