252 Mr. J. R. Capron on the Comparison of some Tube 



Dr. Vogel's spectrum does not comprise the line near G ; but 

 I have added this (in an approximate place only) in order to 

 complete the set of lines. For drawing of Dr. VogePs spectrum, 

 with its scales attached, see Plate III. 



Hydrogen- Tube. 



This tube was one of Geissler's and of rather small calibre. On 

 illumination the wide ends were easily lighted with a silver-grey 

 glow, having a considerable amount of stratification. The ca- 

 pillary part glowed brilliantly with silver-white, bright green, 

 and crimson light according to the intensity of the current ; with 

 the commutator slowly working, white running into green and 

 bright green were the main features of the thread of light ; on 

 the current passing more rapidly the capillary thread became of 

 an intense crimson, at the same time apparently increasing in 

 diameter, an effect probably due to irradiation. 



The spectrum was very brilliant, consisting of the three bright 

 lines usually distinguished as Ha, H/3, and H7, and a number 

 of shaded bands and fainter lines between these, with a bright 

 continuous spectrum as a background to the whole. 



The lines Ha, H/3, and H7 were found to vary in intensity 

 with the current, and in accordance with the colour of the light 

 as seen by the eye — a fact, as I think, not without bearing on 

 the question of the aurora, the varying tints of which are so well 

 known. The fainter lines or bands were mostly stripes of pretty 

 equal intensify throughout, and all about the width of the H/3 

 line. I did not trace any marked degrading on either side of the 

 lines, though the edges were not uniformly so sharp as Ha and 

 H/3 ; some of the lines are coincident in position with lines of 

 the air-spectrum. 



One line only actually coincides with the aurora-spectrum, 

 this being that to which Dr. Vogel assigns an identical wave- 

 length, viz. 5189. Other lines, however, fall somewhat near the 

 aurora-lines 5569, 5390, 5233, and 5004, two faint lines also 

 falling within the band 4694 to 4629. 



The lines (adopting Dr. Vogel's wave-lengths for the H lines) 

 are, when compared, as under : — 



r 4694 

 Aurora . 5569 5390 5233 5189 5004 \ to 



I 4629 



Hydrogen 5555 5422 . . 5189 5008 * 4632 



I remarked that a line 5596, described by Dr. Vogel as "very 

 bright" in his H spectrum, does not appear in my tube, though 

 in most other respects our H spectra agree. 



I thought this tube afforded a good opportunity for testing the 



