86 



John Brown, Esq., read a Paper on 



FIGURES PRODUCED BY ELECTRIC ACTION 

 ON PHOTOGRAPHIC DRY PLATES. 



A rapid photographic dry plate is laid, film upwards, on a sheet 

 of tinfoil connected to one terminal of the secondary of an induc- 

 tion coil, whose ordinary discharging points are set about three 

 centimetres 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 rests on the centre of the film. A single 

 discharge from the coil is caused by moving its mercury break 

 by hand, and the plate is then placed in the developer. When 

 the terminal wire at the centre of the plate is negative, the re- 

 sulting figure is like that represented at A, which shows the 

 typical negative form. When the same terminal is made 

 positive, and a discharge caused under otherwise precisely 

 similar conditions, the figure is quite different, as represented 

 at J3, which is the typical positive form. The distinctive cha- 

 racter of the two kinds of discharge is remarkable and interest- 

 ing. If wires from both poles are brought down on the plate, 

 laid as before on foil, and the coil discharged, the typical figure 

 is produced under each wire. If the foil be omitted, or if actual 

 spark discharge occur between the terminals, or if the terminals 

 be placed on the back of the plate, other figures of a different 

 character in each case are produced. An account of these ap- 

 pears in the Philosophical Magazine for December, 1888, 

 p. 502. Mr. J. Joly, of Trinity College, Dublin, has been struck 

 by the resemblance between these figures and the Oldhamia 

 markings on the pre-silurian rocks. 



In making experiments on the hypothesis that these might 

 have an electric origin, Mr. Joly has produced some remarkable 

 figures (differing from those described above) by using, instead 

 of the photographic dry plates, plain glass plates covered with 

 lycopodium and other dry powders. A connection with the 

 Oldhamia markings has not, however, been established. 



Photographic reproductions of these figures are to be found 

 in the following plates. 





