84 Mr. Sjpottiswoode [Marcli 31, 



magnetic field is strong, it is quite possible to form a magnetic arch at 

 any part of tlie column. In this experiment it will be noticed that 

 for the formation of the arch in mid-column it is necessary that both 

 poles of the magnet should act upon one and the same stria. This, 

 in fact, means that the pole nearest the negative end anchors the 

 stria, and thereby brings it into conditions similar to those of the 

 negative glow. When this is effected the two exhibit similar modifi- 

 cations in the magnetic field. 



In support of this view, we may adduce another and quite indej)en- 

 dent method of anchoring a stria, and of thereby producing a magnetic 

 arch elsewhere than at the negative terminal. It was noticed by 

 Goldstein and others that if the negative terminal of a tube be 

 enveloped by an insulating surface of any form pierced with a number 

 of holes, or if a diaphragm similarly pierced be placed anywhere in 

 the tube, that the pierced surface will act as a negative terminal. He 

 also found that the finer and closer the holes, the more complete the 

 resemblance to the action of a negative terminal. But even when the 

 substance is metallic, and when the holes are neither very small nor 

 very numerous, a perforated diaphragm will so far act like a negative 

 terminal as to serve as a point of departure of a stria. There is, 

 however, this difterence, that the blank space immediately adjoining 

 the diaphragm, as it is usually called, is not generally so large as 

 that at the true terminal ; and the strife thus artificially formed 

 always lie close up to the holes. The diaphragm, in fact, anchors 

 the stria, and renders it susceptible of the same magnetic effect as 

 was shown in the cases studied before. 



The action of a diaphragm in a magnetic field gives rise to many 

 other interesting and remarkable results ; some of which would further 

 illustrate the views now submitted for your consideration. But these 

 must be reserved for another occasion. 



In the foregoing experiments, and in the remarks which have 

 accompanied them, I have endeavoured to illustrate, by reference to 

 gaseous media, the principle enunciated at the outset, that in the dis- 

 placement of the discharge in a magnetic field, the subject of the 

 magnetic action is the material substance or medium which conveys 

 the discharge. I have shown also that, even when the discharge 

 takes place in media so attenuated as to produce the phenomena of 

 striaB, the same principle applies not only to the discharge as a whole, 

 but also to each component stria or unit ; and, lastly, that the 

 apparent diversity of effect on the various striae is due to local cir- 

 cumstances, and not to any fundamental difference between the " nega- 

 tive glow" and the members of the " positive column." 



Seeing now that the magnetic displacement of the luminous dis- 

 charge means displacement of the matter in a luminous condition, and 

 that a crowding of such luminous matter involves an increase of 

 luminosity, may we not infer with a high degree of probability that 

 the striae are themselves aggregations of matter, and that the dark 

 spaces between them are comparatively vacuous. 



