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374 PHYSICS: A. J. DEMPSTER 
This is peculiarly fortunate at such an epoch when similar societies in 
Europe must be greatly reduced in membership and activity. 
Having secured the approbation and support of the National Academy 
of Sciences for the coming year, through a further grant from the J. 
Lawrence Smith fund, it is hoped that the results for 1916 will surpass 
those for the previous year, and indeed a good start has been made in 
that direction. We still need and desire the help of other persons inter- 
ested in such work and a cordial invitation is again extended to them. 
THE LIGHT EXCITATION BY SLOW POSITIVE AND 
NEUTRAL PARTICLES 
By A. J. Dempster 
RYERSON PHYSICAL LABORATORY, UNIVERSITY OF CHICAGO 
Received by the Academy, June 12, 1916 
The speed at which an electron begins to excite light, and the char- 
acter of the light emitted by slow electrons has formed the subject of 
several recent papers ;i the corresponding problem for slow positive 
rays has not been carefully investigated. The only papers on the sub- 
ject are those by Stark^ and Wehnelt^ who detected a luminosity prob- 
ably due to positive rays with potential differences as low as 50 volts 
on the tube. 
The light excitation by positive and neutral rays of greater energy 
is included in the many papers on the Hght emission of canal rays. These 
particles have energies corresponding to a fall of potential of from 500 
volts up, depending on the pressure, the low speeds necessarily being 
at a relatively high pressure. The Doppler effect in these canal rays 
in hydrogen shows a dark space between the displaced and undisplaced 
lines, and this may be explained by assuming that those particles in 
the rays which have less than a certain speed (corresponding to 50-80 
volts fall of potential for the hydrogen atom) are unable to excite light. 
The method used in the present experiments was to ionize hydrogen 
by electrons from a Wehnelt cathode and to allow the positives thus 
made to pass through a sHt in a plate behind the cathode (P in the 
figures) into a second chamber where their light emission and the de- 
flection of their path by electric fields could be studied undisturbed by 
the light and the electric fields in the main tube. In one tube (A) a. 
plate was placed in the second chamber at right angles to the beam of 
rays so that by charging it positively the positive rays could be stopped 
and turned back; in a second arrangement {B) the rays were allowed to 
enter an obHque retarding field so that the paths of the positive particles 
