that the silhouette of the phosphorescent substance appears in black on the proof. If a coin is interposed between the phosphorescent 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 (properly crystallized) gives a splendid fluorescence with the Röntgen 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 decreasing 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 luminous. If so, the light waves in question, although they are distinctly 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 sensitive 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 Röntgen rays behave like ordinary light." Further experiments were made to ascertain the percentage of the rays
