﻿412 M. A. Wiillner on the Spectra of 



liant under a pressure of 30 inillims., 21 millims., 16 millims. ; 

 and even under a pressure of 8 millims. it was doubtful whether 

 the continuous part was really fainter, or whether it only ap- 

 peared so in comparison with the dazzling lustre of the line H /3. 



Several observations which we made on a phosphorus-tube and 

 a sulphur-tube, prepared by Dr.Geissler, favour, I think, the view 

 that the nature of the electrodes exerts an influence on the pro- 

 duction of the continuous spectrum. These tubes also contained 

 hydrogen. If the induction-current was allowed to pass without 

 heating the tubes to the melting-point of phosphorus or of sul- 

 phur, they exhibited a beautiful continuous spectrum exactly as 

 has been previously described ; the spectrum of phosphorus or 

 sulphur only appeared on stronger heating. In these tubes the 

 electrodes are always more or less covered with phosphorus or 

 sulphur, by which, since these substances do not conduct, the 

 resistance to be overcome is greater, and therefore the intensity 

 of the current must be less. 



5. It has already been mentioned, in § 3, that when the pres- 

 sure of the gas in the tube only amounted to fractions of a milli- 

 metre the continuous spectrum of hydrogen was again formed, 

 particularly in the green. If the tube was then still further 

 exhausted by means of the Sprenger's pump, the light in the 

 tube first became feebler and its colour paler ; and in the spec- 

 trum, while all the rest was weakened, the green part stood out 

 still more beautifully. It appears in the form of six beautifully 

 shaded bright bands, which are connected with each other by less 

 bright intermediate spaces. In the brightest parts of the blue 

 in the continuous spectrum bright fields also appear, of which 

 that in the middle between H /3 and H y is seen as columnar 

 grouped lines. 



On further pumping, the light in the tube suddenly becomes 

 of a splendid green like the light of a thallium-flame, and the 

 spectrum is quite changed ; the red line H u can scarcely be 

 seen ; the reddish-yellow part of the spectrum has completely 

 disappeared ; and in the green six splendid groups of lines appear 

 on an almost black ground. Repeated measurements gave for 

 the least deflection of these groups the following values* : — 



(1) Middle bright line of the first group, consist-"] fi ° - » 

 ing of three bright lines, this being the brightest j 



(2) Middle line of the second group, consisting") pt > in K 

 of three J b6 i0 15 



* The flint-glass prism used for these measurements has a refractive 

 angle of 60° 2' 00"; its refractive indices are, for Roc 1 743355, for 

 H/3 = 1772210, for H 7=1*790564 : the measurements were made with a 

 Meyerstein's spectrometer with a circle divided to 10". 



