The proceedings of scientific societies teem with such facts and "working hypotheses," and the best of them have been collected by Miss Clerke in her _History of Astronomy during the Nineteenth Century_. As to established facts, we learn from the spectroscopic researches (1) that the continuous spectrum is derived from the _photosphere_ or solar gaseous material compressed almost to liquid consistency; (2) that the _reversing layer_ surrounds it and gives rise to black lines in the spectrum; that the _chromosphere_ surrounds this, is composed mainly of hydrogen, and is the cause of the red prominences in eclipses; and that the gaseous _corona_ surrounds all of these, and extends to vast distances outside the sun's visible surface.
[1] _Rosa Ursina_, by C. Scheiner, _fol_.; Bracciani, 1630.
[4] _Observations on the Spots on the Sun, etc.,_ 4°; London and Edinburgh, 1863.
[5] _Periodicität der Sonnenflecken. Astron. Nach. XXI._, 1844, P. 234.
[6] _R.S. Phil. Trans._ (ser. A), 1906, p. 69-100.
[7] "Researches on Solar Physics," by De la Rue, Stewart and Loewy; _R. S. Phil. Trans_., 1869, 1870.
[8] "The Sun as Photographed on the K line"; _Knowledge_, London, 1903, p. 229.
[10] _Acad. des Sc._, Paris; _C. R._, lxvii., 1868, p. 121.
_The Moon_.--Telescopic discoveries about the moon commence with Galileo's discovery that her surface has mountains and valleys, like the earth. He also found that, while she always turns the same face to us, there is periodically a slight twist to let us see a little round the eastern or western edge. This was called _libration_, and the explanation was clear when it was understood that in showing always the same face to us she makes one revolution a month on her axis _uniformly_, and that her revolution round the earth is not uniform.
Galileo said that the mountains on the moon showed greater differences of level than those on the earth. Shröter supported this opinion. W. Herschel opposed it. But Beer and Mädler measured the heights of lunar mountains by their shadows, and found four of them over 20,000 feet above the surrounding plains.
Langrenus [1] was the first to do serious work on selenography, and named the lunar features after eminent men. Riccioli also made lunar charts. In 1692 Cassini made a chart of the full moon. Since then we have the charts of Schröter, Beer and Mädler (1837), and of Schmidt, of Athens (1878); and, above all, the photographic atlas by Loewy and Puiseux.