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LVIII. On Figures Produced by Electric Action on Photo- 

 graphic Dry Plates. By J. Brown *. 



[Plates VI. & VII.] 



HILE photographing the electric discharge from an in- 

 duction-coil it occurred to rne to try what effect would 

 be produced on the plate by the discharge when taking place 

 directly on the sensitive film itself. 



A rather rapid photographic dry plate was laid film 

 unwards on a piece of sheet metal connected to one terminal 

 of the secondary, whose ordinary discharging points were set 

 about 3 centim. apart to act as a by-pass to the spark and 

 prevent it striking over the edge of the plate. The end of a 

 wire from the other terminal rested on the centre of the film. 

 A single discharge from the coil was caused by moving its 

 mercury-break by hand, and the plate was then placed in the 

 developer. 



When the terminal wire at the centre of the plate was 

 negative, and particularly if no discharge passed over the 

 edge of the plate, the result was like that represented at A 

 (Plate VI.), which shows the typical negative form consisting 

 of beautiful sharply defined symmetrical palm-like fronds on 

 irregular stems branching out from the centre where the 

 wire rested, together with a mass of less distinct irregular 

 straggling lines also branching outwards, but not reaching so 

 far as the fronds. 



When the wire terminal was made positive and a discharge 

 caused under otherwise precisely similar conditions, the figure 

 was quite different, as in B, and consisted on the plate of dark 

 irregular branchings sharply defined except near the centre, 

 where apparently the luminosity of the spark has caused a 

 nebulous edge to the branch. These branchings are accom- 

 panied by light irregular straggling radiations, similar to those 

 on the negative plate, but having a rather more distinctly 

 centrifugal direction and extending beyond the well-defined 

 branches, from which they seem to be to some extent off-shoots. 

 The experiments were repeated several times and gave similar 

 characteristic results. If, however, the metal sheet were 

 omitted, and wires from both positive and negative terminals 

 brought down on a plate insulated on a paraffin block, neither 

 the palm- fronds on the negative, nor the dark markings on the 

 positive appeared, but only the lighter irregular branchings, 

 and these were in much greater quantity round the positive 



* Communicated by the Author. 



