178 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 



COMPOSITION OF SUN'S ATMOSPHERE 



The identification of solar lines with those of the known terrestrial 

 elements, begun by Kirchoif, was taken up by Rowland, whose ex- 

 tensive findings were incorporated in his Preliminary Table of Solar 

 Spectrum "Wave Lengths, published in 189G. For a third of a cen- 

 tury this has been the world's standard, and the vade-mecum of 

 astronomers, chemists, and physicists. In the Revision'^ recently 

 completed at Mount Wilson Observatory, the original identifications 

 have been checked and corrected by the use of all available data con- 

 jointly with modern astrophysical theory and the results of spectrum 

 analysis. Nearly all, viz. 35, of the elements recognized or suggested 

 by Rowland as present in the sun, have survived the critical examina- 

 tion and retain their place as components of the sun's atmosphere. 

 To these, 2G have been added, so that 61 of the 90 known elements 

 are represented with varying degrees of probability. Of particular 

 interest and importance among the results of the Revision are the 

 identification of the nonmetallic elements — carbon, nitrogen, oxygen, 

 and sulphur in the atomic state; the large proportion of the metals 

 occurring in both the neutral and ionized states; the complete ioniza- 

 tion of the rare-earth metals; and the importance of the role of 

 excitation potential. 



The identification rests upon reasonable correspondence between 

 the positions of solar and laboratory lines, so-called coincidence. For 

 iron this is shown in Plate 2, Figure 1, at A, where it is seen that the 

 bright arc lines of iron are well represented in the sun's spectrum. 

 This is a reproduction of a spectrogram made by simultaneous ex- 

 posures to the center of the sun and to the luminous vapor between 

 the two poles of an iron arc. 



Identifications are furnished also by flash spectra. When the sun's 

 disk is covered by the moon at a total eclipse only the glowing gases 

 of the sun's atmosphere are exposed to view beyond the moon's edge. 

 These then give bright lines which, except in the case of helium, are 

 the reversals of the dark Fraunhofer lines. This may be seen in 

 Plate 3, for the strong iron triplet AA4,045, 4,063, 4,071. Normally 

 helium gives bright lines which appear only in flash and prominence 

 spectra. The helium line A4,026 is conspicuous in the reproduction. 



The symbols of the 90 known chemical elements are given in 

 Table I. 



•Revision of Rowland's Preliminary Table of Solar Spectrum Wave Lengths, with Ex- 

 tension to the I'reseut Limits of the lufra-Ked, Carnegie Institution of Washington, PubL 

 No. 390 ; Pullers of Mount Wilson Observatory, Vol. Ill, 1928. 



