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 
V = 
where the potential of the cathode is taken as zero and V„ is the potential of the 
cathode glow, V is the potential at distance x from the cathode and ^ 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 IIarrts,§ 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 Yoshida,|| 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. Accad. Lincei,’ vol. 23, 117, 1914. 
I ‘Roy. Soc. Proc.,’ vol. 84, p. 526, 1910. 
§ ‘Phil. Mag.,’ vol. 30, 182, 1915. 
II ‘ Mem. Kyoto Imp. Univ.,’ vol, ii., 6, 1917. 
