GASEOUS SPECTRA UNDER HIGH DISPERSION. 441 



cially with time and use; and show at last nothing but the Hydrogen tube- 

 spectrum with brilliancy and certainty at any pressures between O'l and 0*5 

 inch. 



One particular impurity, however, has to be guarded against; for its lines 

 though few, are strong, viz., Mercury, — whose vapour always has a chance of 

 entering, in connection with the Sprengel air-pumps now so generally used in 

 the exhausting operations. To this end therefore some tubes specially contain- 

 ing Mercury have been made for me in Edinburgh by Mr W. H. Shaep (of 

 Kemp & Co.), and have furnished the Mercury low-temperature spectrum 

 which appears as No. 13 in the second division of my Index Map; and will 

 enable any one with great ease to eliminate Mercury, from the H, lines ; espe- 

 cially if they try it at various lamp temperatures. 



But not a great deal has yet been written on the tube, or "low electric 

 temperature," H spectrum, though every one knows about the 4 grand lines in 

 its high-temperature spectrum, and which reappear in the tubes, together with 

 all their low-temperature lines. MM. Angstrom and Thalen, for instance, 

 are silent on the subject in 1875; and still more remarkably Hydrogen does 

 not figure in the otherwise very comprehensive list of gaseous spectra treated 

 of by the British Association's Committee's Report of 1880. 



Fortunately these low-temperature Hydrogen lines attracted the attention 

 several years ago of the savants of the Imperial Central Observatory of Russia 

 at Pulkowa ; and Dr Hasselberg, their chief spectroscopist (a former pupil 

 too of M. Angstrom), entered into the subject with intense enthusiasm. He 

 had begun with other gases, but soon alighted, as I had done in my paper of 

 1880, on the existence of low-temperature lines, as an addition to the well-known 

 high temperature spectrum of 4 lines only for Hydrogen. He found them 

 too so invariably in any and every mode of preparing Hydrogen gas, that he 

 concluded they must belong to it alone, and under such impression prepared 

 a map of them about 18 inches long, with three of the four great lines intro- 

 duced amongst them. 



But that size of map he afterwards considered did by no means do 

 justice to the richness of this H spectrum, wherefore he laboured again with 

 new prisms; and in the course of last year, published with the Imperial 

 Academy of St Petersburg, a new map of Hydrogen " tube " lines by simple 

 electric spark, or the many low-temperature + the few high-temperature lines; 

 the map having a length altogether of nearly 90 inches from the Red to near 

 the Violet line. 



This last map of Dr Hasselberg's is a very grand work as compared to 

 anything yet published, either on the H, or indeed any other, gaseous 

 spectrum; and it will doubtless be long regarded as a high authority for the 

 absolute spectrum place of the lines which it contains. 



