MODERN THEORIES OF THE SUN.! 
By Jean Boster, 
Astronomer at the Meudon Observatory. 
[With 2 plates.] 
It is the sun alone among all the stars that we can ever hope to 
see in detail. It alone can aid us in understanding all the others 
and throw light on their evolution. Furthermore, whatever our ideas 
may once have been, they had no solid experimental basis until the 
invention of the spectroscope toward the middle of the last century. 
This remarkable discovery taught us that there was something 
further to observe in the sun than the spots, the facule, and the 
prominences visible at the eclipses; the appearance of these phe- 
nomena to the eye has therefore lost something of the exclusive 
interest it formerly usurped. The Janssen-Lockyer method of 
utilizing the monochromatic hydrogen light from the prominences 
had already enabled us to see them at any time on the limb of the 
solar disk. The spectro-heliograph, based upon a bold and ingenious 
generalization of analogous principles, now reveals to us in the 
flocculi (pl. 2, fig. 1), the filaments and alignments (pl. 2, fig. 2), 
new phenomena formerly invisible which, perhaps, equal or even 
surpass in importance the spots and facule. It is on these new 
appearances that the interest of the present-day astronomer is 
especially centered. 
Our knowledge of the constitution of the sun is naturally increased 
by all this progress. The fact that the solar spectrum is made up of 
black lines upon a bright continuous background shows, according 
to Kirchhoff’s law, the existence of a very hot source of light sur- 
rounded by a cooler absorbing layer of gas. The latter produces in 
the spectrum the lines of a great number of terrestrial substances— 
iron, hydrogen, calcium, magnesium, sodium, etc. Its inner portion, 
situated close to the brilliant photosphere, has been called the revers- 
ing layer. It contains the heavier elements, while the outer layer or 
chromosphere contains principally hydrogen and calcium. Farther 
1 Summary of two lectures delivered Apr. 11, 1913, and Jan. 9, 1914, at the observatory of the Société 
Astronomique de France. Translated by permission from L’ Astronomie, 28th year, February, 1914, Paris. 
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