xxiv.] IRON IN SPOTS AND PROMINENCES. 343 



To make the matter quite clear, the sun-spot frequencies are 

 reproduced. In the next horizon are entered the frequencies 

 with which iron lines were seen in observations of a hundred 

 flames by Tacchini. 



I am anxious to give these diagrams because they bring out 

 the perfectly natural fact for it is the natural fact that over 

 this region of the spectrum, at all events, no iron lines affected 

 in the spots are visible in the prominences as a rule. If 

 we were unacquainted with the spectrum of iron, we should be 

 justified in saying that the " iron " spectrum of the prominences 

 was due to one substance, and that of the spots to another. 



If we assume, and we know this assumption is legitimate, 

 that the region occupied by prominences is hotter than that 

 occupied by spots, that hotter region ought to do its work, 

 and it ought to be a work of simplification. Therefore 

 I say it is a perfectly natural result, and not one to be 

 wondered at, that in the spectra of the flames there is no line 

 coincident with any of the lines seen frequently widened in 

 the spots. 



We have finally three solar spectra which we can compare 

 one with the other. First of all we have the iron spectrum 

 of the sun taken as a whole. Next we have the spectrum of 

 spots, which we know to be hotter that the sun's atmosphere, 

 taken as a whole. Then we have the spectrum of flames, which 

 we know to be hotter than the spots. It will be seen that the 

 story, as it runs from the top of the diagram downwards, is a 

 story of greater simplicity, as it ought to be ; the simplicity 

 brought about by the reduction of lines actually seen as to 

 number is accompanied by the appearance of new lines (pro- 

 duced by the transcendental temperatures) in these regions. 



It will be observed that the line at 501 7'5 given among the 

 flame observations in the map is not recorded as an iron line by 

 either Angstrom or Thalen. The way, however, in which it 

 sympathised with the line at 4923 in the flames induced me ^0 



