PAPERS ON PHYSICS 283 
VISIBLE COLOR EFFECTS IN A POSITIVE RAY 
TUBE CONTAINING HELIUM 
ABSTRACT 
Cuas. T. Knipp, UNIVERSITY OF ILLINOIS 
In a recent number of Science’ the writer described a posi- 
tive ray tube, containing a hollow cathode, in which the dis- 
tinguishing features are the color effects of the cathode and 
canal rays when the residual gas in the tube is either hydrogen, 
helium, argon or neon. For example, when the tube contains 
a trace of helium the cathode beam presents an apple green 
color, while in the same gas the canal ray beam is orange. 
Recently the writer was able to show within the same tube 
by the visible color effect produced, the retrograde rays as 
well. The color of these rays borders on purple. By reference to 
the figure the relative position of the various beams is apparent, 
The cathode rays are bent out of the way by a magnetic field 
perpendicular to the plane of the paper and placed in the po- 
sition of the dotted circle. This comparatively weak field has 
1, Knipp, Science, 42, Dee. 31, 1915. 
