

xxiv.] BRIGHT LINES. 353 



"The line at wave-length 4923 which occurs so often in the 

 chromosphere, according to Young and Tacchini, and is assumed to 

 be due to iron, is so near to lines which come out in our crucibles on 

 the introduction of other metals, that one cannot help feeling some 

 doubt as to its absolute identification with the iron line." l 



But even this does not remove the difficulty, indeed it rather 

 increases it, for if the line in question is not an iron line, then 

 over a large reach of spectrum the iron vapour in the hottest 

 regions of the sun open to our inquiries, instead of giving us 

 two lines only gives us one. 



6. Bright Lines on the Disc and in Sun-spots. 



I may conclude this chapter by referring to another class of ob- 

 servations in which the differentiation of the iron lines is equally 

 complete. How complete it is with regard to calcium I have 

 elsewhere stated in referring to photographs of sun-spot spectra. 

 In these the H and K lines are almost always photographed 

 as bright lines over the spectrum of the umbra, while the blue 

 line of calcium, which is the line of calcium at the temperature 

 of the arc, is absolutely unaffected. Similar phenomena are 

 seen when the iron lines are observed bright on the disc; of 

 adjacent lines of equal intensity one is taken and the other 

 left; and further, contortions may be seen in one differing 

 entirely from those seen in the other. 



On this point I shall content myself by referring the reader 

 to Professor Young's observations recorded in his admirable 

 book on the sun. 2 



1 Proc. Roy. Sac. xxxiii. 432. 



2 The Sun, by Professor Young. International Scientific Series, page 210, 

 Fig. 64. 



A A 



