On tlic Spectrum of tJ' more Volatile Gases of Air. 469 



these, a vast number of rays, generally less brilliant, are 

 distributed through the whole length of the visible spectrum. They 

 are obscured in the spectrum of the. capillary part of the tube by the 

 greater strength of the second spectrum of hydrogen, but are easily 

 seen in the spectrum of the negative pole, which does not include the 

 second spectrum of hydrogen, or only faint traces of it. Putting a 

 Leyden jar in the circuit, while it more or less completely obliterates 

 the second spectrum of hydrogen, also has a similar effect on the 

 greater part of these other rays of, as yet, unknown origin. The 

 violet and ultra-violet part of the spectrum seems to rival in strength 

 that of the red and yellow rays, if we may judge of it by the intensity 

 of its impressions on photographic plates. We were surprised to find 

 how vivid these impressions are up to a wave-length 314, notwith- 

 standing the opacity of glass for rays in that part of the spectrum. 

 The photographs were taken with a quartz calcite train, but the rays 

 had to pass through the glass of the tube containing the gases. 



We have made approximate measurements of the wave-lengths of all 

 the rays which are sufficiently strong to be seen easily or photographed 

 with an exposure of thirty minutes, and .give a list of them below. 

 These wave-lengths are computed to Rowland's scale, and were deduced 

 from the deviations produced by two prisms of white flint glass for the 

 visible, and of calcite for the invisible, rays. The wave-lengths assigned 

 to the helium lines are those given by Runge and Paschen, and some 

 of these lines were used as lines of reference. In general, the iron 

 spark spectrum was the standard of reference. 



The tubes when first examined showed the lines of the first spectrum 

 of hydrogen vividly, and the earlier photographs of the spectrum of 

 the negative pole contained not only the violet lines of hydrogen, but 

 also the ultra-violet series as far up as A. 377. In order to get impres- 

 sions of the fainter rays, exposures of half an hour or more were 

 required, and a succession of photographs had to be taken so as to get 

 different sections of the spectrum into the middle of the field, where 

 measurement of the deviations would not be impeded by the double 

 refraction of the calc spar. As the light of the negative pole only was 

 required, the electric discharge was made continuously in one direction 

 only, with the result that the hydrogen lines grew fainter in each 

 Btiocessive photograph, and soon disappeared altogether. Along with 

 the ultra-violet rays, the less refrangible rays of hydrogen also dis- 

 appeared, so that no trace of the C or F line could be seen, nor yet of 

 the second spectrum, so long as the current passed in the same direc- 

 tion as before. lieversal of the current soon made the F line show 

 again, so that it seems that the whole of the hydrogen was driven by 

 the current to the positive pole. The conditions under which this 

 ultra-violet series shows itself are a matter of interest. It appears 

 here in the midst of a brilliant spectrum due to gases other than 



