22G 



than the willemite screen, and an exposure of three minutes 

 shows curves on the plate which cannot be detected on the 

 screen *. 



One method I have employed to detect the positive rays 

 by photography is as follows : — The photographic plate is 

 inserted in the lid of a light-tight box, A, fig. 1, so that when 



Fisr. 1.— Plan. 



"fe cA-o-re**? U*t> 



Fig-. 1. — Elevation. 



L_ 



N* 



m& 



WZZjL 



7ZZK. 



f 



1 



the lid is down the plate is protected from light. The bottom 

 of the box is fastened down to the floor of the discharge-tube, 

 while the lid is fastened to a rod, B, which is fixed to a glass 

 tap, C, which rotates in a glass tube ; tube and tap are 



* Since this was written I have seen a paper by Koenigsberger & 

 Kelching ( Verli. Deutsch. Physik. Gesellsch. xii. p. 995, 1910), who have 

 also used the method of putting the plate inside the tube. 



