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, Gr 3 . 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 3 is 

 shown in Plate HE. This plate was otherwise exposed 

 exactly as the former plate. The ground wire terminated 

 1.5 cm. below the center of the plate. The result in Plate 

 III. 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 Ley den jar which 



