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THE POPULAR SCIENCE MONTHLY 



the more refrangible rays that call forth fluorescence. For, if we illu- 

 minate them with light which has passed through a red glass, no trace 

 of fluorescence is visible. But, if the red be exchanged for a blue glass, 

 the fluorescence becomes as strongly marked as Avith the direct solar 

 light. A remarkable phenomenon is presented in the splendid bright- 

 green light Avhich is emitted by uranium glass under the action of 

 blue illumination. 



The highly-refrangible rays which possess in so high a degree the 

 power of exciting fluorescence are contained in large proportion in the 

 light emitted by a Geissler's tube filled with rarefied nitrogen. In 

 order to expose fluorescing fluids to the influence of this light, the ar- 

 rangement represented in Fig. 3 may be employed with advantage, 

 A narrow tiibe is surrounded by a wider glass tube, into which the 

 fluid is introduced by a side opening which can be closed if reqiiired. 

 Another form of Geissler's tube is represented in Fig. 4, which con- 



Ficf. 3 Geisslee's Fluorescence Tcbe. 



Fig. 4. Geissleb's Tube with IlRANiuai 

 Glass Spheres. 



tains in its interior a number of hollow spheres composed of uranium 

 glass. Where a beam of reddish violet nitrogen light traverses the 

 tube, the uranium glass balls shine with a beautiful bright-green fluo- 



rescent light. 



The electric light passing between carbon-points is rich in rays of 

 high rcfrangibility, indeed the ultra-violet end of its spectrum reaches 

 even farther than that of the solar spectrum. In the light of the mag- 

 nesium-lamp the ultra-violet rays are also abundant, and both sources 



