Spectra of Helium, Sfc. in the Ultra- Violet. 301 



•6 ampere is probably due to lines lying about X800. Since 

 •2 ampere corresponds to frequencies at or little above the 

 threshold frequency for the target, the sharp drop in the 

 curve at that value is easily explained. The increase in 

 the effect at *4 ampere seen in the curves for the higher 

 driving potentials is probably due to an increase in the 

 intensity of the radiations lying around X800. The position 

 of the maximum makes it further evident that there are 

 some lines between X 800 and X 1200. These conclusions 

 are in accord with the observations of Lyman *, who could 

 detect no lines in the helium spectrum between X2000 and 

 A, 1250, although he found some 27 lines between X1250 and 

 X600 with relatively strong ones about XX1215, 1175, 1040, 

 977, 833, and 720. It is, however, possible that any of 

 these lines may be due to either hydrogen or helium, since 

 most if not all of them above X900 occur in the hydrogen 

 spectrum with as great or greater intensity than in helium. 

 The principal lines occurring in helium only are XX 833, 

 720, and 600. Summing up the curves of these lines with 

 the intensities given in Lyman's paper together with a 

 curve for a line about X400 and one or two between X1000 

 and A, 1200, one gets a resultant curve very similar to that 

 obtained in these experiments. A similar resultant could, 

 however, be obtained from a quite dissimilar set of curves 

 if the intensities were arbitrarily fixed at suitable values. 



After the helium had been finally pumped out and the 

 apparatus properly washed with air and subjected to an 

 extended heat treatment, it was refilled with hydrogen. 

 Curves obtained with this gas are shown in fig. 7. Here 

 again the form and limits of the curves are very little 

 altered at different driving potentials and different pressures 

 between the limits already referred to. The maximum of 

 these curves lies well to the left of the helium maximum, and 

 the limiting frequencies are seen to correspond to wave-lengthe 

 of 830 units to 948 units, with a probable value of X 900. 

 This radiation would be liberated by impacts with an energy 

 between 13 and 14 volts, which is in close accord with Bohr's 

 calculated ionization potential of 13'6 volts, from which the 

 limiting wave-length would be X 910. All of the experi- 

 mental values for the ionization potential of hydrogen, how- 

 ever, lie around 10'4 volts, which would give a limiting- 

 wave-length of 1190 units. Lyman has obtained no 

 hydrogen lines below X 900, but he has found a number of 



* l Spectroscopy of Extreme U ltra- Violet ' 1914. and Astrophysical 

 Journal, March 1916. 



Phil. Mag. S. 0. Vol. 34. No. 202. Oct. 1917. Y 



