Production of Soft Rontgen Radiation. 621 



placed in front of the plate so as to get a definite image. 

 The plates used were Schumann plates or plates specially 

 prepared for me by the Paget Plate Company; these are 

 not so sensitive to the radiation as the Schumann plates, and 

 require a longer exposure ; when, however, the radiation is 



Fiff. 1. 



strong enough they are more convenient to work with. L 

 and M are two pairs of parallel plates 6*5 cm. long and 1 mm. 

 apart, placed so that any radiation coming from A and 

 striking the plate P must pass between L, while any radia- 

 tion from P must pass through M before it reaches the 

 photographic plates. These plates could be connected to a 

 large battery of small storage-cells, and a potential difference 

 of 1000 volts established between the plates in either pair. 



When the positive rays were striking against the plate an 

 exposure of a Schumann plate for an hour gave a dense 

 photograph of the slit, showing that something was coming 

 down the side tube which could affect a photographic plate. 

 There are many well recognized types of radiation produced 

 in A, and it is necessary to make further experiments to see 

 if the photograph is produced by these or by some new type 

 of radiation. It might be suggested, for example, that the 

 effect had nothing to do with positive rays passing through 

 the cathode to P, but was produced by ultra-violet light or 

 Rontgen rays generated in the discharge-tube, which passed 

 through the aperture in the cathode, and was then reflected 

 from the platinum plate. If this were the case, since neither 

 ultra-violet light nor Rontgen rays are deflected by an electric 

 field, the intensity of the photograph should be the same 

 whether the potential difference between the plates in L was 

 zero or 1000 volts. It was found, however, that the photo- 

 graphic effect almost disappeared when there was a potential 

 difference of 1000 volts between the plates, showing that the 

 effect is due to the positive rays. The effect, though reduced 

 to a small fraction of its former value by putting on 1000 

 volts, is yet not entirely eliminated. The small residual 



