146 Prof. A. LI. Hughes on Characteristic 



ppacings between its planes sufficiently large- to measure 

 appreciably longer wave-lengths. The spacing available sets 

 an abrupt limit to the study o£ X-ray spectra in the direction 

 or! longer wave-lengths. It is unlikely, therefore, that the 

 spectrum can be extended more than a few Angstrom units 

 by the method of crystal analysis. From the ultra-violet 

 end, steady progress has been made in identifying shorter 

 and shorter wave-lengths by Lyman and by Millikan, who 

 used diffraction gratings. The furthest line yet photo- 

 graphed in the ultra-violet spectrum is A 202 *, a line in the 

 spectrum of nickel (Millikan). In Millikan' s work the wave- 

 length of the lines in the extreme ultra-violet is only about 

 one-seventieth of the distance between the rulings on the 

 grating, as contrasted with from one-third to one-sixth when 

 the grating is used in the visible. While there is no factor 

 in the diffraction grating which sets an abrupt limit to the 

 extension of the oltra-violet spectrum, it is evident that the 

 closing of the gap between ultra-violet light and X-rays by a 

 diffraction grating will be attended with increasing difficulties. 

 The gap yet to be explored extends from X 202 to X 12. 



The principle underlying the method of this research is as 

 follows : — Experiments by Beatty and others have shown 

 that when an element is bombarded by electrons, whose 

 energy is increased by steps, the characteristic radiation is 

 called out when the energy exceeds a certain critical value. 

 This, superposed on the general radiation, causes a more or 

 less abrupt increase in the curve connecting the intensity 

 (measured by ionization) of the radiation with the energy 

 of the electrons impinging on the element. In this work 

 the radiation emitted by carbon (and boron) bombarded by 

 electrons was allowed to fall on a metal plate, and the total 

 photo-electric current leaving it measured as a function of 

 the energy of the impinging electrons. A discontinuity 

 in the slope of the curve was taken to point to the excitation 

 of the characteristic radiation. The wave-length of the 

 radiation is connected with the energy of the electrons by 

 the well-known quantum relation 



Ye = hv = hc/\, 



where V is the potential accelerating the electrons, e the 

 charge on the electron, h Planck's constant, v and A, the fre- 

 quency and wave-length of the radiation, and c the velocity 

 of light. When Y is expressed in volts and 7 in Angstrom 

 units, we have 



V = 12331/\. 

 * Millikan, Astrophysical Journal, lii. p. 47 (1920). 



