io8 POPULAR SCIENCE MONTHLY. 



that tlie silhouette of the phosphorescent substance appears in 

 black on the proof. If a coin is interposed between the phospho- 

 rescent substance and the paper, its image appears on the proof. 

 A thin sheet of glass may be interposed to preclude the possibility 

 of chemical action." 



It is announced by Mr. Edison that calcium tungstate (prop- 

 erly crystallized) gives a splendid fluorescence with the Rontgen 

 rays, far exceeding that of platino-cyanide. 



A rather ingenious explanation of the X rays is offered by Mr. 

 J. W. Gifford. He likens the Crookes tube to a vibrating tuning 

 fork, which, if sounding simple A, would set an A violin string 

 vibrating not only A but its octave and the fifth to its octave, 

 and quite a host of other overtones or harmonics of rapidly de- 

 creasing wave length which would seem to have no theoretical 

 limit. The waves of long period from a Crookes tube would pass 

 through wood, paper, or the human body, without much resistance, 

 but would be absorbed or reflected by the denser metals. But if 

 objects capable of taking up their vibrations lay in the path of 

 these long rays they would set them vibrating like the violin 

 string, and might in the same way produce overtones which did 

 not before exist. These overtones may include waves of such 

 short lengths as to cause the objects themselves to become lumi- 

 nous. If so, the light waves in question, although they are dis- 

 tinctly instrumental in darkening a photographic plate exposed 

 to them, have nevertheless not passed through, and could never 

 pass through the obstacles easily traversed by the electric waves 

 which gave them origin. 



Prof. Ogden N". Rood, of Columbia College, has quite recently 

 published in Science an account of some important work on the 

 reflection of these " rays." The mirror used was a new sheet of 

 ordinary platinum foil. Great care was taken to prevent any 

 rectilinear emanations from the discharge tube reaching the sen- 

 sitive plate, which was contained in an ordinary plate holder, 

 being covered with two sheets of aluminum, each 0*17 millimetre 

 in thickness, and the draw slide, and over the whole was fastened 

 a netting of iron wire. " After an exposure of ten hours it was 

 found that a good image of the netting had been produced on the 

 vertical strip of the plate exposed to the reflected rays. This 

 image had various deformations, the vertical lines representing 

 the netting being, as a general thing, most distinct; in some 

 places, however, the horizontal lines had the upper hand, and 

 there were a few spots where both were equally distinct. These 

 facts and the character of the deformations point very strongly 

 to the conclusion that in the act of reflection from a metallic 

 surface the Rontgon rays behave like ordinary light." Further 

 experiments were made to ascertain the percentage of the rays 



