Cipher — Properties of Light-struck Photographic Plates. 157 



short discharge tracks exactly resembling those of the ball 

 discharge, were found originating in irregularities of the film. 

 This suggested the experiment, the details of which are shown 

 in the adjoining cut. A disk, b, c, was armed with two 

 needle points, one of which was directed towards the point a, 

 from which the ball discharges issue. The other was directed 

 in an opposite direction. The positions of the knobs of the 

 machine are shown in the cut. A very luminous ball dis- 

 charge passed very slowly from a to b, requiring about a 

 minute to traverse one inch. At b the luminosity disappeared, 

 but it appeared at once at c and drifted around towards the 

 -f knob. It reached the edge of the plate at cZ, and remained 



d 



P l 



Diagram Showing the Arrangement of Discharge in Pl. XV., Fig. 8. 



there for several minutes. A little rivulet of violet discharge 

 passed along the whole line of the track and was especially 

 strong near a. After developing it was found, what has been 

 seen in several other plates, that the film was attacked along 

 this part of the track, as is shown by dendritic formations 

 extending outward from the main track but which do not 

 show in the half-tone reproduction. When the machine was 

 stopped and the flow along this line ceased, it was found on 

 starting the machine that this track had ceased to act as a 

 conductor. No glow appeared at d. Another ball discharge 

 appeared. It was drawn out and to one side of the old track 

 by means of the teaser, and then was allowed to traverse its 

 own path. It found its way by a new path, to b and c, and 

 finally to the edge of the plate. There it also persisted, but 

 the machine was soon stopped, and the glow ceased. This 

 was repeated many times. The result is shown in Fig. 8. 



