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PROFESSOR A. FOWLER, F.R.S., OX 



is now generally agreed that this " flash " spectrum observed 

 during total eclipses corresponds essentially with the dark line 

 spectrum ordinarily observed. There are, it is true, certain 

 divergences between the intensities of the dark and bright lines 

 arising from the apparently undue brightness of enhanced lines 

 in the flash spectrum, but it would take too long to discuss 

 them and to indicate how the differences may be explained. 



The reversing layer, as ordinarily understood, is about 500 

 miles in depth and is situated close to the sun's surface, at the 

 base of the chromosphere, which extends to a height of about 

 5000 miles. The upper part of the chromosphere may be 

 observed any time that the sun shines. The bright lines thus 

 observed indicate that the chief gases in this region are 

 hydrogen and helium, but during solar eruptions the lines of 

 various elements, projected into it from the reversing layer 

 below, are also observed. The bright helium lines of the 

 chromosphere do not ordinarily occur among the Fraunhofer 

 lines, but they are occasionally observed as absorption lines in 

 the neighbourhood of sun-spots, where, it may be supposed, 

 there are special accumulations of this gas. 



It should be mentioned that the solar corona, which is the 

 most striking feature of a total eclipse of the sun, has apparently 

 nothing to do with the production of Fraunhofer lines. The 

 greater part of the corona gives a feeble continuous spectrum, 

 and it is only the inner corona which gives distinctive bright lines. 

 The chief line is usually one in the middle green, and has been 

 attributed to a hypothetical element, " coronium," which is not 

 yet known in terrestrial chemistry. Strangely enough, this line 

 was not observed in the corona of last August, but was replaced 

 by a bright line in the red, which is also at present of unknown 

 origin. There are no Fraunhofer lines corresponding to these 

 unknown emissions of the corona. 



From the point of view of the chemical composition of the 

 sun, very little advance has been made on the work of Kirchhoff, 

 Lockyer, and Kowland. Very substantial progress has been 

 made, however, in the identification of additional Fraunhofer 

 lines with lines of elements previously known to exist in the 

 sun. The unidentified lines are mostly of low intensities, and it 

 is too early to conclude that they may represent forms of matter 

 which are special to the sun. They may, in part, correspond to 

 still uncharted faint lines of the various metals ; or, with equal 

 probability, they may represent the constituent lines of a com- 

 plex band spectrum, the origin of which has not yet been traced. 

 It has sometimes been supposed that these unidentified lines 



