400 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



cylindrical tubes. In one, A, the back of the terminal was protected as 

 in Figure 1 ; in the other, B, the back was not protected. The same 

 phenomenon was observed. Tube A came up nearly to the X-ray 

 stage, while the other was apparently far below this stage. 



Figure 2 is a photograph of the state of the two tubes. It is evi- 

 dent that the mere appearance of the discharge between the terminals 

 is no criterion of the state of exhaustion unless one carefully considers 



the forms of the tubes and the extent of 

 wall surface submitted to the bombardment 

 of the cathode rays. The difference which 

 I describe is probably due to the walls of 

 the prolongation of the vacuum tubes, A 

 being more protected from this bombard- 

 ment than those of tubes B. 



The forms A apparently showed the canal 

 rays as perfectly as the forms B, when the 

 iron terminal was made the cathode ; and 

 these rays did not seem to be modified by 

 the protection of the edges of the orifice in 

 the iron tube by the glass tube. The canal 

 rays, therefore, come entirely from the space 

 between the anode and the cathode. 



A solenoid (S, Figure 1) was next slipped 

 over the prolongation of the tubes. This 

 prolongation, therefore, formed the core of 

 the solenoid, and the rays passing through 

 the orifice in the terminal could be sub- 

 mitted to a longitudinal magnetic field. 

 By a proper adjustment of the position 

 of the solenoid the cathode beam passing 

 through the orifice in the iron terminal or 

 the anode could be brought to a sharp 

 focus on the end of the prolongation tube. 

 This was also the case in tube B ; but in 

 the latter there was also a phosphores- 

 cent ring surrounding the focus of the central beam which was due 

 to bringing to a focus the rays which passed over the periphery of the 

 circular iron anode. The phenomenon of focussing or convergence 

 of the rays is due to these rays seeking the weakest part of the 

 magnetic field. The field formed by the iron disc terminal outside 

 the solenoid, together with that of the short solenoid, had two 

 channels in which the field was weakest : one through the orifice at 



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Figure 1. 



