Spectra of Helium, fyc. in the Ultra-Violet. 307 



These correspond to wave-lengths roughly in the neighbour- 

 hood of 200, 150, and 100 A.tJ. respectively. Nothing was 

 detectable with these fields, although the effects of the line 

 near 420 were quite marked in the same discharge. Again, 

 with a clean copper target freed from occluded gases and in 

 a liquid-air and charcoal vacuum, it is recorded that nothing- 

 was found at H = 60, 80, or 102 lines. The last magnetic 

 field corresponds to about 70 A.U. and was made with 

 625 volts driving the exciting electron current. 



Summary. 



The most important results of this investigation may be 

 summarized as follows : — 



The high frequency limits of the spectra of helium, 

 hydrogen, and mercury when stimulated by large electron 

 currents under potentials up to about 800 volts have been 

 determined. The measurements are believed to be most 

 accurate for helium and least accurate for mercury. 



The helium spectrum extends to a limit which is certainly 

 between 470 and 420 and probably near to the latter value. 

 The hydrogen spectrum terminates at a wave-length between 

 830 and 950 Angstrom units and probably close to 900. 

 The mercury spectrum terminates at a wave-length between 

 1000 and 1200 Angstrom units. 



I So far as we are able to ascertain, the observed terminal 

 frequencies are identical with the frequencies calculated 

 from Bohr's theoretical values of the ionization potentials of 

 the respective gases, and they exhibit no obvious relationship 

 to the ionization potentials which have been determined 

 experimentally by Franck and Hertz and others. 



The ultra-violet spectrum has been extended to a wave- 

 length which is certainly shorter than 470 and probably 

 close to 420 Angstrom units. 



The high frequency limits of the spectra referred to are 

 independent of the applied potential up to about 800 volts, 

 provided this potential exceeds a lower limit which has not 

 been determined with any accuracy. 



Wheatstone Laboratory, 

 King's College, Strand, W.C. 



