456 Mr. C. F. Meyer and Prof. R. W. Wood on 



wider in this than in the previous photographs, this being due 

 to the use o£ a very much wider slit (*45 mm.) in this part 

 of the work. The gas passing into the jet-chamber through 

 the tube T consisted of nitrogen taken directly from the 

 bomb, and the fluorescent spectrum when the fluorite plate 

 was not in the apparatus consisted of one very broad streamer 

 indicating the water band and the one or two nitrogen bands 

 nearest it all merging into each other. When the fluorite 

 plate was inserted only the water band appeared, showing 

 that the fluorite was more transparent to the radiation ex- 

 citing the water band than to that exciting the nitrogen 

 bands. Three exposures made under approximately the same 

 conditions indicated that an exposure of from fifty to seventy 

 times as long was necessary to obtain an image of the same 

 intensity for the water band when the fluorite plate was in 

 as when it was absent. This indicates that the radiation lies 

 outside of the so-called Schumann region, for which fluorite 

 is quite transparent. The plate reproduced in fig. 8 was 

 exposed for 50 minutes. In making these exposures it was 

 necessary to clean the fluorite plate frequently. It was as a 

 rule wiped with dry cotton every two minutes, and every 

 fifteen or twenty minutes cleaned with nitric acid, water, 

 and alcohol. The time taken for wiping and cleaning is of 

 course not included in the time of exposure. In these ex- 

 posures the amount of nitrogen delivered into the jet-chamber 

 was about a quarter of a litre a minute, the apparatus being 

 washed out for from five to ten minutes before the exposure 

 was begun. 



When purified nitrogen was used the fluorescent spectrum 

 showed the nitrogen band of longest wave-length predominant. 

 Inserting the fluorite plate into the apparatus showed that 

 the radiation exciting this band was also transmitted, but 

 very much less readily than the radiation exciting the water 

 band — perhaps a third or fifth as well. 



The study of the transmission of quartz was attended by 

 more difficulties, and yielded less definite results than that 

 of fluorite. Using nitrogen directly from the bomb, in the 

 spectrum of which the water band predominated, no trans* 

 mission of the radiation exciting the water band has ever 

 been detected, though the conditions under which the ex- 

 posures were made were not entirely favourable. Using 

 purified nitrogen, in which the nitrogen band of longest 

 wave-length came out most strongly, it was found that after 

 transmission through quartz the region of most intense 

 fluorescence was displaced toward the short wave-length 

 side of the spectrum, indicating that it -was probably the 



