10 Trans. Acad. Sci. of St. Louis. 
across the film of a photographic plate P, Fig. 1, sup- 
ported at its edges on insulating supports. This wire was 
held in proper tension by means of brass springs, from 
which silk cords passed to the wire, and its position with 
respect to the film of the photographic plate was adjusted 
by means of hard rubber supports on either side of the 
plate having adjusting screws of insulating material. Be- 
low the center of the plate a distance of about 1.5 cm. 
was the pointed end of a copper wire, which was ground- 
ed on the water pipe, G,. The resistance R was so ad- 
justed that a spark discharge would not pass from the 
wire above the film around the plate P to the grounded 
wire below, but would be on the point of doing so. This 
adjustment was made for the exposures in the positive 
and also in the negative line. 
Plate II. shows a 5X7 inch photographic plate across 
which 5 spark discharges from the negative terminal were 
passed. The fine wire which carried the discharge was 
in contact with the film. This wire was surrounded by a 
glow of light, but the resistance between the plate and 
the ground was not sufficient to force discharges over the 
film. To have made this resistance greater would have 
brought about a spark discharge around the plate, when 
the pointed ground conductor was put in position, al- 
though it was not in position during this exposure. The 
effect produced by introducing this ground wire G, is 
shown in Plate III. This plate was otherwise exposed 
exactly as the former plate. The ground wire terminated 
1.5 em. below the center of the plate. The result in Plate 
ITI. may be explained as follows: 
1. By the two-fluid hypothesis. The negative discharge 
through the wire in contact with the film, is attended by 
a positive discharge from the ground wire to the lower 
face of the plate. This positive discharge is spread over 
an area coincident with the blackened area which the neg- 
ative discharge is shown to cover in Plate III. The glass 
plate on which the photographic film is spread is in a con- 
dition like that of the glass wall of a Leyden jar which 
