Behaviour of H and K Lines of Spectrum of Calcium. 433 



loses none of its activity. An experiment has been made with the 

 object of reflecting the zinc action from glass. This did not succeed ; 

 whether this arose from the glass not being capable of effecting such 

 a reflection, or whether a fortnight was not sufficient time to produce 

 in this way a visible effect, is not known, bat the experiment is being' 

 repeated. A photographic plate, suspended film upwards over a 

 copal plate, was acted on round the edges in the way one would 

 imagine a vapour to act. A similar experiment is being made over a 

 zinc plate. The action of glass proves that there is at least a marked 

 difference between the action exerted by metallic uranium and that 

 by zinc and other metals. 



It should be stated that it is only the most sensitive photographic 

 plates which, without extremely long exposures, give the results 

 described. The Mawson plate has generally been used in the fore- 

 going experiments, but the Ilford special rapid plate acts equally 

 well, and Edwards' isochromatic snap-shot plates are particularly 

 sensitive to the action of the uranium salts. Lumiere's extra rapid 

 are not so sensitive as the Mawson and Ilford plates, and still less 

 sensitive are the same firm's plates for yellow and green, and for red 

 and yellow. Other sensitive plates have not been experimented 

 with. 



4< On the Relative Behaviour of the H and K lines of the 

 Spectrum of Calcium." By William HuGGlNS, D.C.L., 

 LLD., F.R.S., and Mrs. HuGGlNS. Received May 27 — 

 Read June 17, 1897. 



[Plate 4.] 



The remarkable relative behaviour of the lines in the spectra of 

 certain substances as they appear at and near the sun's limb, and in 

 the atmospheres of stars of different classes, has long been before 

 our minds as a problem of great interest, which there is reason to 

 believe is capable of solution by the methods of the laboratory, and 

 on which we have worked from time to time for many years. 

 Without waiting for the results of other researches which are in 

 progress, we think that it is desirable to put on record some definite 

 results on the behaviour of the lines of calcium, which appear to us 

 to be conclusive, and of great importance in forming a correct inter- 

 pretation of many solar and stellar phenomena. 



As early as 1872, Professor Young from a few weeks' work at 

 Sherman on the spectra of the chromosphere and of the prominences, 

 was able to point out that " tbe selection of lines seems most capri- 

 cious; one is taken and another is left, though belonging to the same 

 element, of equal intensity, and close beside the first." Especially 

 he noticed that while the H and K lines of calcium are almost always 



