616 Scientific Intelligence. 



dows normal to the #-axis, next the light entered the ic-tube 

 through a quartz window, after this it passed through the 

 rectangular opening in a diaphragm inside the negative section of 

 the tube, and finally it was absorbed and scattered by the opaque 

 positive end of the tube. The negative portion of the 2/-tube 

 was designed for making visual and photographic observations 

 along its axis and hence it too was closed at the outer end by a 

 quartz window. The positive segment of the ?/-tube constituted 

 a dark cave across the mouth of which the beam under investiga- 

 tion passed. The #-tube was furnished with smaller lateral tubes 

 to enable air and other gases to be pumped into, or out of, the 

 tubular cross. 



The field of view always consisted of a bright ring (due to 

 light scattered from the entrance to the cave-tube) which was 

 usually crossed by a narrow bright band arising from light scat- 

 tered by the gas molecules or by fine dust particles. With 

 ordinary untreated air in the apparatus, a very bright track due 

 to scattering by dust particles was observed. This track had the 

 same color as the arc. When air, which had been dried by phos- 

 phorus pentoxide and filtered by cotton wool, was pumped into 

 the apparatus and allowed to stand until the few remaining dust 

 particles had settled to the walls of the vessel, the beam appeared 

 fainter than in the preceding case and its color was definitely 

 blue. The ring of light, of course, did not vary in tint. A pho- 

 tograph taken with an ordinary plate using only ultra-violet light 

 (filter of cobalt glass and paranitrosodimethyl-aniline) shows 

 the diametral band across the ring very distinctly. This is in 

 striking contrast to a photograph taken with a yellow screen and 

 an isochromatic plate. In the latter negative the transverse 

 band is extremely faint. When the vessel was exhausted, the 

 blue track disappeared, nothing remaining except the ring of dif- 

 fused light. Before finishing this part of the investigation 

 Strutt took the most elaborate precautions to prove that the blue 

 band was not due to extremely minute dust particles. 



To test if the phenomena observed were caused by hypothetical 

 fluorescence of the air, the troublesome arc lamp and the ordi- 

 nary camera were replaced respectively by a Westinghouse- 

 Cooper-Hewitt quartz-mercury lamp and a convenient spectro- 

 graph. Two spectrograms are reproduced in the paper juxta- 

 posed in register one above the other. The upper half was 

 obtained with a three days' exposure to the radiations from the 

 faint blue beam, the lower, with such a length of exposure to the 

 mercury lamp as gave about the same intensity as the preceding 

 for "the middle of the spectrum" (about A4000). The upper 

 spectrum shows no signs of any constituent except the known 

 mercury lines. ' ' Thus the lateral emission is scattered light, not 

 fluorescent light." The maximum of intensity is shifted 

 markedly toward the extreme ultra-violet in the case of the scat- 



