﻿Spectra of Helium and Hydrogen. 289 



a dispersion of 250 A.U. per mm. in the red, 130 A.U. 

 per mm. in the green, and 76 A.U. per mm. in the violet. 

 Later the spectrum was photographed with a concave grating 

 of 1 metre radius ruled with 14,000 lines to the inch. By 

 means of this instrument it was found possible to obtain the 

 first members of series (3) and (4), but no trace of any of 

 the lines represented by equations (2) and (8) even after 

 exposures of 40 hours. A few members of the series 

 represented by equation (2) were obtained in the first place 

 by employing a spectroscope consisting of two glass prisms, 

 which gave a dispersion of 150 A.U. per mm. in the red, 

 54 A.U. per mm. in the green, and 30 A.U. per mm. in the 

 violet. Finally these two prisms were replaced by others 

 giving greater dispersion and capable of resolving 2 A.U. at 

 \ 6500. For the sake of comparison the dispersions given 

 by this prism spectroscope in the red, green, and violet were 

 75, 27, and 15 A.U. per mm. respectively. The time of 

 exposure varied from 20 minutes to 30 hours, depending 

 upon the dispersion of the apparatus and the particular lines 

 studied. The first member of series (3) at 4686 was easily 

 photographed, but the series represented by formula (9) 

 required a very long exposure, and even then only a few 

 members of the series could be detected with certainty. All 

 the photographs were taken with W ratten and Wainwright's 

 panchromatic plates and films. 



The spectrum-tubes employed were of various shapes and 

 dimensions. The diameter of the capillary w r as varied from 

 *8 to 3" 5 mm., and its length from 3 to 8 cm. When the 

 lines represented by equation (9) were studied, a spectrum - 

 tube with a capillary of *8 mm. was chosen, the spark-gap 

 was made 1*2 cm., and the pressure of the helium wa- 

 adjusted to about "5 mm. of mercury. Photographs of the 

 4686 line were taken with spectrum-tubes having capillaries 

 of different bore, and containing helium at pressures ranging 

 from *36 to 2 mm. 



Since the hydrogen lines ol: the Balmer series are in 

 ordinary circumstances much stronger than the odd members 

 of series (9), it was necessary, especially with the relatively 

 small dispersion employed, to obtain a helium spectrum 

 giving little or no trace of the hydrogen lines. In the 

 preliminary experiments spectrum-tubes with aluminium 

 electrodes were employed, but it was found impossible to 

 remove the hydrogen from the electrodes in a reasonable 

 time. Later, thin platinum electrodes of "4 mm. diameter 

 were employed, and a helium spectrum showing no truce of 

 hydrogen was obtained. In this connexion it is interesting 

 ' Phil Mag. S. 6. Vol. 29. No. 170. Feb, 1915. V 



