524 Mr M c Glelland, On the figures produced [May 1G, 



Figures 7 and 8 are positive and negative taken by placing 

 the photographic plate on a metal plate connected to the outer 

 coating of a Leyden jar; the outer coating of a second jar was 

 joined to a metal rod which just touched the film. The jars were 

 then charged until a spark passed between their inner coatings. 



Here we have the discharge running out in heavy lines, from 

 each of which at various points branch off small discharges similar 

 to those in the smaller figures. In the positive the potential rose 

 high enough to break down the air to the edge of the plate and 

 on to the metal base, and the greater part of the discharge passed 

 this way. Even the heavier lines in the negative are less bent 

 about than in the positive. 



To test whether the effects on the film are due to the light 

 of the spark a number of experiments were made with thin 

 plates, some transparent to light and some opaque, placed between 

 the film and the discharging point. 



With a thin plate of mica on the film the branching lines of 

 the positive figure are only slightly less distinct than when the 

 film is uncovered. 



Figure 9 is taken with a small circular piece of glass \ mm. 

 thick placed on the centre of .the photographic film, and the 

 discharging point touches the glass. The lines of the positive 

 figure beneath the glass are quite distinct but broader than they 

 otherwise would be; we would expect this broadening as the 

 light is further from the film by the thickness of the glass. 



For figure 10 the same piece of glass was used but half of it 

 was rendered opaque to light by a thin coating of Brunswick 

 black on the side next the film. Here again the figure comes out 

 distinct through the transparent part but not through the opaque 

 part. 



In figure 11 part of the film was covered with a piece of 

 ebonite ^ mm. thick ; the opaque ebonite entirely shielded the 

 film although not much thicker than the glass which had little 

 shielding effect. 



These figures seem to prove that the light of the discharge is 

 necessary to produce the figure on the plate. 



As it is well known very perfect impressions of coins or any 

 metal dies can be obtained by placing them on a photographic 

 plate and passing a discharge to them. Here again the light of 

 the discharge from the coin appears to be sufficient to explain 

 the effect produced. Around the edge of the coin we have a 

 discharge and also minute discharges from the edges of the raised 

 portions of it, where the film is affected. Where the coin touches 

 the film it is not affected. 



Figure 1 2 is taken by weak sparking on a coin laid on a 

 photographic plate ; the edge of the coin is brought out by the 



