Shape of the Kathode in Geissler's Tubes 465 



arc of the upper arm of the cross, and only the lower bound- 

 ing arc of the right-hand arm: the two dark arms themselves 

 cannot be perceived; but the space between the bounding arcs 

 mentioned is uniformly illuminated. 



The two bounding arcs, which are now absent, result there- 

 fore from an action between the upper and right-hand arms, 

 prevented or hindered by the presence of the screen. 



If the plate is withdrawn a little, then, as already men- 

 tioned, the figure 19 c is seen. 



The dark arms which were wanting in the first image are here 

 seen as very narrow strips, and the corresponding two bound- 

 ary curves are of feeble luminosity. The more the screen is 

 withdrawn towards the kathode the broader do the two dark arms 

 become, and the more closely does the luminosity of the two 

 curves approach to the normal luminosity. The mutual action 

 between each two neighbouring arms, to which the production 

 of the dark cross is due, may be supposed to take place in 

 three different ways : — either (1) as an immediate action of 

 each of the arms of the kathode upon the other, which thus in- 

 directly produces an effect upon the course of the electric rays; 

 or (2) as an action of each arm upon the system of electric 

 rays emitted by the other; or (3) as a mutual action of the 

 two systems of rays. 



Any further discussion of the utility of the three hypotheses 

 at present would lead us too far away. I will content myself 

 with remarking that the assumption (1), so far as I see, is 

 opposed to the details of the phenomena of kathodic deflection; 

 but it is not possible to decide certainly between (2) and (3). 



Taking account, however, of the fact that the third assump- 

 tion involves certain accessory assumptions which have not 

 yet been verified by experiment, I shall employ the language 

 of the second hypothesis in seeking for further explanation; 

 which hypothesis, moreover, I have employed throughout in 

 my research on kathodic deflection, in describing the phe- 

 nomena observed. 



If now (fig. 20) rays issue from the edge a of the right- 

 hand arm (1) towards b, then, according to the laws of ka- 

 thodic deflection, these rays will be repelled by the edge b and 

 the whole surface of (2) . The same holds good for the rays 

 emitted by the left arm at c. Let fig. 21 be a rough repre- 

 sentation of the kathode, turned through 90° from its position 

 in fig. 20, the upper half having moved forwards; and let 

 I and r represent the upper edges of the left and right arms 

 respectively, and o the (fore-shortened) upper arm. The 

 repulsion which o exerts upon the systems of rays emitted by 

 r and I will cause the mutual convergence of these two last 



