1897.] on Cathode Bays. 429 



are connected with the terminals of a Wimshurst machine. When 

 sparks pass between these terminals currents pass through the wire 

 which induce currents in the bulbs, and cause a ring discharge to 

 pass through them. Things are so arranged that the ring is faint in 

 the larger bulb, bright in the smaller one. On making the wires in 

 these bulbs cathodes, however, the discharge in the small bulb, which 

 is filled by the dark space, is completely stopped, while that in the 

 larger one becomes brighter. Thus the gas in the dark space is 

 changed, and in the opposite way from that in the rest of the tube. 

 It is remarkable that when the coil is stopped the ring discharge on 

 both bulbs stops, and it is some time before it starts again. 



The deflection excited on each other by two cathodic streams 

 would seem to have a great deal to do with the beautiful phosphor- 

 escent figures which Goldstein obtained by using cathodes of different 

 shapes. I have here two bulbs containing cathodes shaped like a 

 cross ; they are curved, and of the same radius as the bulb, so that if 

 the rays came off these cathodes normally the phosphorescent picture 

 ought to be a cross of the same size as the cathode, instead of being 

 of the same size. You see that in one of these bulbs the image of 

 the cross consists of two large sectors at right angles to each other, 

 bounded by bright lines, and in the other, which is at a lower pres- 

 sure, the geometrical image of the cross, instead of being bright, is 

 dark, while the luminosity occupies the space between the arms of the 

 cross. 



So far I have only considered the behaviour of the cathode rays 

 inside the bulb, but Lenard has been able to get these rays outside 

 the tube. To this he let the rays fall on a window in the tube, made 

 of thin aluminium about y^o th of a millimetre thick, and he found 

 that from this window there proceeded in all directions rays which 

 were deflected by a magnet, and which produced phosphorescence 

 when they fell upon certain substances, notably upon tissue paper 

 soaked.in a solution of pentadekaparalolylketon. The very thin alu- 

 minium is difficult to get, and Mr. McClelland has found that if it is 

 not necessary to maintain the vacuum for a long time, oiled silk 

 answers admirably for a window. As the window is small the phos- 

 phorescent patch produced by it is not bright, so that I will show 

 instead the other property of the cathode rays, that of carrying with 

 them a negative charge. I will place this cylinder in front of the 

 hole, connect it with the electrometer, turn on the rays, and you will 

 see the cylinder gets a negative charge ; indeed this charge is large 

 enough to produce the well known negative figures when the rays fall 

 on a piece of ebonite which is afterwards dusted with a mixture of 

 red lead and sulphur. 



From the experiments with the closed cylinder we have seen that 

 when the negative rays come up to a surface even as thick as a milli- 

 metre, the opposite side of that surface acts like a cathode, and gives 

 off the cathodic rays ; and from this point of view we can understand 

 the very interesting result of Lenard that the magnetic deflection of 



