504 On Figures produced by Electric Action. 



and suggest at least a possibility of some similarity in the 

 causes producing each. 



The question now arose as to whether the result was due 

 to a photographic effect of the luminosity of the spark, or to 

 some more direct action of the discharge on the film — to what 

 might be called an " electrographic " action. 



As evidence for the latter view came the apparent insuffi- 

 ciency of the light, actually visible when the discharge took 

 place, to produce the effect. The lighter branchings were not 

 to me visible at all in either positive or negative, and only an 

 indication of the frond formations in the negative. 



There are also in the positive plate B (Plate VI.) several 

 intervals as if the spark had not been in immediate contact 

 with the film, but had passed over it, leaving only a foggy 

 mark instead of a sharply defined black line. The break or 

 interval would scarcely have been so marked if the whole 

 effect were photographic only. 



However, to further investigate this question the discharge 

 was taken with the two terminals on the back or uncoated 

 side of the plate. The figures which now appeared on the 

 .film were quite different from those described above. In each 

 case there was impressed on the back of the film (next the 

 glass) the cloudy photographic effect of the branching dis- 

 charge on the back of the plate. 



On the front or outer side of the film under the positive 

 terminal the figure reminded one of a photograph of a maiden- 

 hair fern out of focus. That under the negative is a collection 

 of peculiar tadpole-like markings, whose general arrangements 

 correspond in size and shape to the fronds formed by the 

 discharge on the back of the plate. These figures would 

 appear to be due to electricity induced in the film. 



To try further the effect of induction on the film, I cut my 

 initials in tin-foil after the manner of a stencil plate, placed 

 the foil on the film, a piece of gutta-percha tissue on the foil, 

 and pressed all together in an ordinary photographic printing 

 frame. A second piece of foil was placed on the back of the 

 plate, leaving a margin all round, and the foils were joined 

 to opposite poles of the coil with its terminals giving a by- 

 pass of about 1 centim., so as not to have any spark discharge 

 over the plate. The result with the poles connected in either 

 sense, and the coil working for about a minute was, with 

 the stencil foil either positive or negative, an irregular black- 

 ening all round the edge of the foil, including the edges of the 

 cut-out parts as if a discharge had passed out from the edges. 

 In some places the characteristic markings of positive or ne- 

 gative, as the case might be, were visible. There was, however, 



