1878.] Analysis in connexion with the Spectrum of the Swi. 171 



Further, in the eclipse observed in Siam in 1875, the H and K lines 

 left the strongest record in the spectrum of the chromosphere, while 

 the line near G in a photographic region of nmch greater intensity was 

 not recorded at all. In the American eclipse of the present year the 

 H and K lines of calcium were distinctly visible at the base of the 

 corona, in which, for the first time, the observers could scarcely trace 

 the existence of any hydrogen. 



To sum up, then, the facts regarding calcium, we have first of all the 

 H line differentiated from the others by its almost solitary existence in 

 Sirius. We have the K line differentiated from the rest by its birth, 

 so to speak, in a Aquilse, and the thickness of its line in the sun, as 

 compared to that in the arc. We have the blue line differentiated 

 from H and K by its thinness in the solar spectrum while they are 

 thick, and by its thickness in the arc while they are thin. We have 

 it again differentiated from them by its absence in solar storms in 



K H 



Fig. 4. 

 i 



BLUE 

 LINE 



RED 

 LINE 



1 1 



SIRIUS 



1 



III 1 



SUN 









1 ARC 







H 



FLAME 



which they are almost universally seen, and, finally, by its absence 

 during eclipses, while the H and K lines have been the brightest seen 

 or photographed. Last stage of all, we have calcium, distinguished 

 from its salts by the fact that the blue line is only visible when a high 

 temperature is employed, each salt having a definite spectrum of its 

 own, in which none of the lines to which I have drawn attention 

 appear, so long as the temperature is kept below a certain point. 



Iron. 



With regard to the iron spectrum, I shall limit my remarks to that 

 portion of it visible on my photographic plates, between H and G. It 

 may be described as a very complicated spectrum, so far as the 

 number of lines is concerned, in comparison with such bodies as 

 sodium and potassium, lead, thallium, and the like ; but unlike them, 

 again, it contains no one line which is clearly and unmistakably re- 

 versed on all occasions. Compared, however, with the spectrum of 

 such bodies as cerium and uranium, the spectrum is simplicity itself. 



Now, among these lines are two triplets, two sets of three lines 

 .each, giving us beautiful examples of those repetitions of structure in 



