140 DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 



In recent years the radiation from the dark space has become of particular interest 

 in view of the fact that it is in this region that the Stark effect or the electrical 

 resolution of spectrum lines analogous to the magnetic resolution known as the 

 Zeeman effect is observed. 



In this region the quantitative relation between the electric field and the distance 

 from the cathode was first investigated by SCHUSTER,* who expressed his results by 

 the empirical formula 



where the potential of the cathode is taken as zero and V u is the potential of the 

 cathode glow, V is the potential at distance x from the cathode and k is a constant. 



This formula gives the distribution of potential in the dark space, and more 

 recently Lo SuRDO,t from a series of measurements of the electrical separation of 

 spectrum lines in front of the cathode, has verified that it is a satisfactory first 

 approximation. Investigations in this direction have also been carried out by ASTON| 

 and by HARRIS, who measured the deflection of a beam of cathode rays passing in a 

 direction perpendicular to the electric field. 



We do not discuss these observations in detail. Very recently the distribution of 

 potential in narrow tubes has been investigated somewhat exhaustively by TAKAMINE 

 and TOSHIBA,!! who found that, under the conditions of their experiments, the 

 relation between the electric field and the distance from the cathode could be 

 represented, within the limits of experimental error, by a parabolic law. 



In the work described in the present communication, we are concerned with 

 pressures somewhat greater than have been used by these investigators, and with 

 the cathode glow itself in addition to the dark space, and although a knowledge of 

 the precise distribution of potential, from the cathode to a distance at which there is 

 no longer any perceptible luminosity, would be of value, it is not in the first instance 

 essential to a discussion of our results. For this purpose we may, in fact, merely 

 assume that the electric field falls away rapidly with increasing distance from the 

 cathode without the necessity of postulating any exact law. For it would appear 

 that the average velocities of the electrons at different distances from the cathode (in 

 which the effect of collisions naturally plays an important part) are probably more 

 strictly relevant to a discussion of the results. A visual examination through 

 coloured glasses of the cathode spectrum of the tube used in this investigation at 

 once shows that the term " dark space" is, in fact, a purely relative one, and refers 

 only to the integrated effect on the eye of all the radiations emitted. 



* ' Roy. Soc. Proc.,' vol. 47, p. 541, 1890. 



t 'Rendiconti R. Aecad. Lincei,' vol. 23, 117, 1914. 



J ' Roy. Soc. Proc.,' vol. 84, p. 526, 1910. 



'Phil. Mag.,' vol. 30, 182, 1915. 



|| ' Mem. Kyoto Imp. Univ.,' vol. ii., 6, 1917, 



