RECENT ADVANCES IN SCIENCE 557 



but also the radiation corresponding to the ionisation poten- 

 tial of a gas may be expected to be highly absorbable by the 

 gas. Thus to detect such radiation it is therefore desirable 

 to avoid the long path through the gas which is demanded by 

 the vacuum grating spectrograph. Further, in dealing with 

 helium it is necessary to have an arrangement in which the 

 gas can be maintained at a very high degree of purity, and on 

 this account the authors of this paper criticise Lyman's most 

 recent work inasmuch as under the conditions of experiment 

 it must have been extremely difficult, if not impossible, to keep 

 the helium free from impurities, especially hydrogen. In the 

 method used by Richardson and Bazzoni, the radiation excited 

 by the ionisation of the gas is allowed to fall on a metallic 

 target and the velocity of the photoelectric electrons emitted 

 is measured. In determining the frequency of the impinging 

 radiation the truth is assumed of the well-known equation 



1 mv 2 = hv — Wy 



where m and v are the mass and maximum velocity of emis- 

 sion of the photoelectrons, h is Planck's constant, and w a known 

 constant for the metal target. Hence v f the radiation frequency, 

 is calculated. The following results were obtained. By means 

 of large electrons currents under potentials up to 800 volts, 

 helium ionised by electronic impact emitted a radiation the 

 upper limit of which extended to a line in the neighbourhood 

 of 420 Angstrom units ; hydrogen, a radiation terminating at 

 a wave length close to 900. Mercury vapour gives a spectrum 

 extending to about 1,000 units. These limits seem to be 

 identical with values of the frequencies calculated by Bohr on 

 the basis of his theory of the atoms of hydrogen and helium. 

 Incidentally, the gap between the Schumann region of the 

 spectrum and the softest X-rays has been still further narrowed 

 by this attainment of a line near 420. 



In the August number of the Physical Review, Messrs. 

 Davis and Goucher describe experiments also bearing on the 

 excitation of radiation by electron impact in hydrogen and 

 mercury vapour, and state that their results, while not incon- 

 sistent with the Bohr theory of the atom, are of greater com- 

 plexity than would be inferred from it. 



In the September number of the Physical Review, Mr. H. 



