Spectra of Helium, Sfc. in tlte Ultra- Violet. 285 



shorter than a certain value has not been observed. This, 

 however, can be accounted for if corresponding radiations in 

 other elements are of neighbouring wave-length. 



Similar irregularities were found in the absorptions by 

 paper and water, both within the same region. 



it seems just possible that these are connected with a 

 spectral line or spectral lines of still higher frequency — 

 possibly of an I series. This, however, is a point which 

 would require careful investigation by another method. 



It should also be pointed out that though we have drawn 

 smooth curves — for the most part straight lines — through 

 observed points, this does not indicate that certain slight 

 deviations are not real. We know certain of them to be so. 

 but they are only small, as the figures show quite clearly. 

 For the present they are of only secondary importance. 



We wish to acknowledge our indebtedness to Miss J, 

 Dunlop for some preliminary work on the absorption of 

 high-frequency X-rays. 



XXX. The Limiting Frequency in the Spectra of Helium, 

 Hydrogen, and Mercury in the Extreme Ultra- Violet. By 

 0. W. Eichardson, F.R.S., Wheatstone Professor of 

 Physics in the University of London, and Lieut. C. B. 

 Bazzoni, General Staff A.E.F., formerly Harrison Re- 

 sear ch Fellow of the University of Pennsylvania*. 



ri^HE investigations described in this paper were under- 

 JL taken for the purpose of detecting and measuring the 

 frequency of the shortest vibrations emitted from the various 

 gas atoms under electron impacts. A preliminary notice of 

 some of the results obtained has already been published in 

 4 Nature ' f- From the quantum relation eV — hv a certain 

 frequency v can be calculated for each ga3 from its ionization 

 potential V which should be the maximum frequency ob- 

 tainable from that particular gas when subject to the same 

 type of ionization. This calculation gives a probable 

 minimum wave-length for hydrogen of 909 x 10 ~ 8 cm. and 

 for helium of 422 x 10~ 8 cm. assuming Bohr's ionization 

 potentials (136 volts for hydrogen and 29*3 volts for helium) 

 to dp. correct. In making these computations the value of 

 e is taken to be 4*77 x 10" 10 E.S. unit and of h 6'55 X 10" 27 . 

 Taking the experimentally determined values of 10'4 volts 



* Communicated by the Authors. 



t Richardson and Bazzoni, ' Nature,' vol. xcviii. p. 5 (1916). 



Phil. Mag. S. 6. Vol. 34. No. 202. Oct. 1917. X 



