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Dr. William Hug gins 



[Feb. 6, 



study the spectrum better by a reference to this diagram (Fig. 4), in 

 which the lines are put down according to their wave lengths. The 

 two lines H and K, as is well known, coincide with two lines of cal- 

 cium, and we attribute them usually to the vapour of that substance. 



Fig. 5. 



The remarkable behaviour of these lines in the stars was pointed 

 nut by me at the end of 1876. A few months previously, Mr. 

 Lockyer suggested that photographs of the brighter stars might show 

 a modification of this character, and that if such were the case, it 

 would support his view as to the dissociation of the vapour of calcium 

 in the hottest stars. In a subsequent paper to the Koyal Society 

 Mr. Lockyer explained in more detail his views of the dissociation of 

 tlie terrestrial elements, and also the bearing of his views in con- 

 nection with the difierent kinds of visible star spectra. I wish also here 

 to acknowledge the kindness of Professor Dewar, who permitted me to 

 witness the experiments conducted by himself and Professor Liveing. 

 I saw the lines of calcium corresponding to H and K in the emission 

 spectnnn of that substance vary in relative brightness until for a 

 moment the line corresponding to 11 alone remained. 



Are this thick line of the star spectrum and the thin line at K really 

 due to calcium ? 



