Nipher — Properties of Light-struck Photographic Plates. 159 



ducting material forming the track is quickly torn out and 

 dispersed. In a few cases these points or ball discharges have 

 been seen to move quickly along the old tracks, but as a rule 

 they do not appear to be capable of motion. Such motions 

 as have been seen were away from the cathode knob, and they 

 consisted in most cases rather in a disappearance of a glowing 

 ball-like mass at one point, and its reappearance at an adjacent 

 point. Only one or two negatives gave results of any sig- 

 nificance. In most of them the film as a whole had become 

 either too good or too poor a conductor. 



It is apparent that the gradual formation of a channel of 

 somewhat lower resistance, is a material feature in the ball 

 discharge. It is probable that the breaking down of insulat- 

 ing material by stresses due to high potentials will yield val- 

 uable results. Whether these ball discharges on the photo- 

 graphic plates are the same as those reported in connection 

 with lightning, it is perhaps too early to decide. They are 

 certainly similar. It is very probable that optical illusions 

 are to be credited with some of the descriptions given of these 

 phenomena. The remarkable photographs taken by Sidney 

 Webb and recently published,* show that during lightning dis- 

 charges, tracks stream out from arc lights. The line wire is 

 a conducting channel. The arc itself is a point of weakness 

 in the gaseous medium, by reason of its high temperature. 

 At low temperatures, nitric and nitrous fumes when mixed 

 with air, increase its resistance to the passage of sparks be- 

 tween discharge knobs ; but at high temperatures the result 

 is likely to be different. Certain it is that these photographs 

 taken by Mr. Webb, show that just such discharges are formed 

 in the air as are known to exist along the track of a ball dis- 

 charge on the photographic plate. 



In many cases where spark intervals have been specially 

 adjusted, and a continuous violet brush discharge was seen 

 passing along the track leading to a ball, persistent appear- 

 ances resembling what has been described as bead lightning 

 have been observed. These beads were really incipient ball 

 discharges that were about to branch out from the main track. 

 Such branches are seen in almost every negative secured. 



* Nature, Feb. 8, 1900. 



