174 THE CHEMISTRY OF THE SUN. [CHAP. 



in the solar spectrum, but the breadth and degree of shade of ttie two 

 sets of lines were seen to agree in the most perfect manner, the brightest 

 iron lines corresponding to the darkest solar lines." 



This statement (the italics are mine) was made to prove the 

 absolutely identical nature of the iron vapour in the sun's atmo- 

 sphere and in the electric spark. 



Minute study however revealed the fact that the intensities of 

 the various lines really vary very greatly from vapour to sun. 



I wrote in 187i : l 



" It is obvious that greater attention will have to be given to the pre- 

 cise character as well as to the position of each of the Fraunhof er lines, 

 in the thickness of which I have already observed several anomalies. 

 I may refer more particularly at present to the two H lines 3933 

 and 3968 belonging to calcium, which are much thicker in all photo- 

 graphs of the solar spectrum " (I might have added that they were by 

 far the thickest lines in the solar spectrum) " than the longest calcium 

 line of this region (4226'3), this latter being invariably thicker than 

 the H lines in all photographs of the calcium spectrum, and remaining, 

 moreover, visible in the spectrum of substances containing calcium 

 in such small quantities as not to show any traces of the H lines. 



" How far this and similar variations between photographic records 

 and the solar spectrum are due to causes incident to the photo- 

 graphic record itself, or to variations of the intensities of the various 

 molecular vibrations under solar and terrestrial conditions, are 

 questions which up to the present time I have been unable to 

 discuss." 



The progress of the work showed that the differences here in- 

 dicated are not exceptions, but are truly typical when the minute 

 anatomy of the solar spectrum is studied. 



5. Divergences between the Spectra of Vapours and the Lines 

 visible in Spots and Prominences. 



We have dealt as yet with the spectroscopic difficulties so far 

 as the ordinary spectrum of the sun is concerned, but in 1874 a 



1 Phil. Trans, vol. clxiv. part ii. p. 807. 



