458 Fluorescence produced by Ultra- Schumann Rays. 



as was here used, the rays exciting at least two, and possibly 

 three of the nitrogen bands, and possibly also those exciting 

 the water band, are transmitted through the quartz. In the 

 original photograph the fluorescent jet of wave-length about 

 2300 A.U., mentioned above as being present in long ex- 

 posures when purified nitrogen is used, is also to be seen. 

 Its position is marked with a short oblique line in the repro- 

 duction. Fig. 9 cannot be used to determine the index of 

 refraction of quartz for the rays exciting the fluorescence, 

 as the photographic plate, lying in the focal plane of the 

 spectroscope, was inclined to the axis of the focus tube, and 

 consequently greatly exaggerated the deviation of the jet. 



To obtain a value of the deviation of the rays by the prism 

 in the jet-apparatus several exposures were made, using the 

 spectroscope merely as a camera (i. e> with the prism of the 

 spectroscope removed). An exposure was then made with 

 smoke in the jet- chamber. The light scattered by the smoke 

 was \filtered through a silver film, and thus the deviation 

 produced by the prism in the jet-apparatus upon light of 

 wave-length 3000 A.U. (the transmission band of silver) 

 was obtained. The index of refraction of quartz for this 

 wave-length is given in tables as 1*57. From this value and 

 the relative deviation for this wave-length and for the light 

 exciting the fluorescence, we calculate for the latter an index 

 of 1*75 x '08, that is, a probable value somewhat greater 

 than the greatest index in the ordinary transmission region 

 of quartz. The object of finding the index of refraction of 

 quartz for these rays was to see if it might not be less than 

 the index for ordinary ultra-violet light. If this had proven 

 to be so, it would have indicated ihat the rays exciting the 

 fluorescence were on the short wave-length side of the ultra- 

 violet absorption band of quartz. But as this has not been 

 found to be true, we must conclude that they are still on the 

 long wave-length side of the absorption region, unless indeed 

 they should be on the long wave-length side of an entirely 

 hypothetical second quartz absorption band, still further out 

 in the ultra-violet. Experiments under way show that the 

 radiation exciting the water band can also be reflected. The 

 reflecting surface used was a cathode deposit of silicon on 

 glass made by Dr. E. O. Hulburt. At 45° incidence the 

 amount of reflexion lies between ten and twenty per cent. 



Summary. 



The fluorescent spectra excited by ultra- Schumann radiation 

 in nitrogen containing some oxygen, in nitrogen free from 

 oxygen, in moistened nitrogen, in dry and moist air, and 



