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LXVII. The Series Spectrum of Mercury. By S. R. MiLNER, 

 D.Sc, Lecturer in Physics, University of Sheffield *. 



AS we pass up from group to group of the elements in 

 the periodic table, the lines in their spectra connected 

 by series relations become in general less and less marked. 

 Thus, while the spectra of the alkalies show on the average 

 some seven or eight of the lines in each series, those of the 

 alkaline earths show only the first three or four of the lines. 

 In mercury, Kayser and Runge have observed only three 

 complete members of the triplets of which the sharp and the 

 diffuse series of this element are composed, and the principal 

 series was entirely unknown until last year, when Paschen 

 discovered the first three members of it in the ultra-red. 



In taking some photographs of the spectrum of the 

 mercury arc in vacuo recently, I was struck by the almost 

 complete absence of a visible background to the spectrum 

 which it showed. In the spectrum of the arc in air a limit 

 is set to the faintness of lines which can be observed by the 

 brightness of the continuous spectrum which is always 

 present, but trial showed that with the arc in vacuo it was 

 possible to give exposures over 50 times the normal without 

 any background making itself evident. Photographs taken 

 with these long exposures showed a great many lines which 

 have not been previously observed, but the chief interest 

 about them was that the lines which form the continuation 

 of the various series of mercury were very strikingly 

 developed. 



The mercury arc used was a very simple home-made 

 apparatus similar to that described by Pfund f ; the arc was 

 about 5 cm. long, and was worked at 4 amp., 15 volts. 

 With a Hilger single-prism quartz spectrograph, with which 

 the normal exposure was about 30 seconds, an exposure of 

 half an hour showed the lines of the diffuse series up to 

 m = 16, and those of the sharp to m=14. Traces of still 

 higher lines could be seen, but they did not come out any 

 better with a longer exposure of two hours, as a continuous 

 background then appeared sufficiently strong to mask the 

 continuation of the series. With a suitable instrument of 

 higher dispersion (which would diminish the continuous 

 spectrum) it would doubtless be possible to extend the 

 series further, but attempts with a two-prism calcite spectro- 

 graph were not very successful through the much greater 



* Communicated by the Author. 



t Astrophys. Journ. xxvii. p. 299 (1908). 



