Fluorescence produced by Ultra-Schumann Rays. 



455 



dark background for the jet ; in having a special mounting 

 for the truncated copper rivet with the slit forming t\ie one 

 electrode, and the copper wire forming the other, so that 

 these could be readily removed and replaced to allow for 

 cleaning of the plate the transmission of which was being- 

 tested. Moreover, the entire jet-apparatus was mounted in a 

 position inverted with respect to that shown in fig. 1, so that 

 the spark was above, while the jet pointed down. The new form 

 and mounting are represented diagrammatically in fig. 2 in the 

 text. J is again the jet-chamber, T the tube delivering the 



Fi 



w r » 





L3 





N. 



gas, also serving as dark background, TTj the window through 

 which the jet is photographed. R the rivet having a slit in it 

 in the plane of the drawing, B the wire forming the other 

 electrode. R is driven through a thin copper plate A; 

 A and B are both fastened to an arm which rotates about a 

 horizontal axis in the plane of the drawing, so that both may 

 be readily raised, by raising this arm, and then replaced. 

 E is the plate or lamina whose transparency is to be tested ; 

 A' is a thin brass plate upon which E rests, and serves also 

 to prevent light scattered by the edges and surfaces of E 

 from reaching the spectroscope. The spectroscope is on the 

 left as in fig. 1. 



The first experiments in transmission were made wi 

 small disk of clear colourless fiuorite '57 mm. thick. Fig. 8 

 (PI. VI.) shows the transmission of the rays exciting the fluor- 

 escence of the water band. Long wave-lengths are now on the 

 right. It will be noticed that the vertical streamer is much 



