488 
DR. S. P. THOMPSON ON CATHODE RAITS 
found the curved cathode beam across a tube to become straighter as the exhaustion 
proceeded. From these experiments it appeared :— 
(1.) That the various constituents of a heterogeneous cathode beam are emitted in 
various proportions at different degrees of exhaustion; (2) That in the cathode rays 
emitted at higher degrees of exhaustion there is a greater proportion of the less- 
defectible rays; (3) That the least-defecitble rays are those ivhich most readily 
penetrate through a perforated screen ivhen that screen is itself made cathodic. 
8 Effect of a Negatively-electi'ified Screen on Cathode Rays, Double Fluorescence. 
Dia-cathodic Rays. 
Attention having been directed by previous experiments to the particular effects 
produced—notably in tube [No. (fig. 18)—by the interposition of a screen, which 
was itself made cathodic, other tubes were prepared to aid in examining these effects. 
In tube [No. C 10], (fig. 19), there was inserted a helix, made of a dozen turns of bare 
iron wire, the ends of which were welded to platinum terminals passing through the 
glass. The cathode, K, was a flat disk; and there was an anode in a lateral bulb 
in the exhaust tube. The terminal turn of the helix, at the end nearest the cathode. 
Fig. 19. 
was brought down across the end face of the spiral, so that its cathodic shadow on 
the end wall of the tube appeared as a vertical diametral line across the circular 
outline of the helix. One purpose in constructing the tube thus was to test the 
effect of a longitudinal magnetic field upon the cathode rays. When a current was 
sent through the helix, the ampere-turns being about 140, and the intensity of the 
field nearly 60 C.G.S. units, the shadow of the diametral wire was rotated through 
aoout 20°. When the wire helix was made cathodic, the yellow-green luminescence of 
the glass was (as in the case also of tube [No, G 19], described above) confined to the 
portion of the tube between the cathode and the nearest end of the helix, no yellow- 
green luminescence being now produced on the tube end, and almost none on that 
part of the tube which covered the helix. But there now appeared a blue cone of 
rays proceeding from the inside of the further end of the helix, and extending thence 
to the far end of the tube. The tube-wall at this end now showed the dark 
orange luminescence which has been more than once alluded to in the preceding 
