THE X EAYS. 



By W. 0. EoNT&EN. 



I. — UPON A NEW KIND OF EAYS.^ 



1. If the discliarge of a great Kuhmkorff induction coil be passed 

 through a Hittorf vacuum tube, or a Leuard's, Crooke's, or similar appa- 

 ratus containing a sufi&ciently high vacuum, then, the tube being covered 

 with a close layer of thin black pasteboard and the room darkened, a 

 paper screen covered on one side with barium-platinum cyanide and 

 brought near the apparatus will be seen to glow brightly and fluoresce 

 at each discharge whichever side of the screen is toward the vacum tube. 

 The fluorescence is visible even when the screen is removed to a distance 

 of 2 meters from the apparatus. 



The observer may easily satisfy himself that the cause of the fluores- 

 cence is to be found at the vacuum tube and at no other part of the 

 electrical circuit. 



2. It is thus apparent that there is here an agency which is able to 

 pass through the black pasteboard impenetrable to visible or ultra 

 violet rays from the sun or the electric arc, and having passed through 

 is capable of exciting a lively fluorescence, and it is natural to inquire 

 whether other substances can be thus penetrated. 



It is found that all substances transmit this agency, but in very dif- 

 ferent degree. I will mention some examples. Paper is very trans- 

 missible.^ 



I observed fluorescence very distinctly behind a bound book of about 

 1,000 pages. The ink presented no appreciable obstacle. Similarly 

 fluorescence was seen behind a double whist pack. A single card held 

 between the fluorescent screen and the apparatus produced no visible 

 efiect. A single sheet of tin foil, too, produces hardly any obstacle, and 

 it is only when several sheets are superposed that their shadow appears 



'Translation of paper by Professor Rontgen in the Sitznngsber. der Wiirzburger 

 Physik-Medic. Gesellscb. Jabrg. 1895, as reprinted in Annalen der Pbysik und Cbemie 

 (Neue Folge) 64, 1, 1898. 



" By tbe transmissibility of a substance I designate tbe ratio between tbe bright- 

 ness of a fluorescent screen beld bebiud the body and that which the screen woukl 

 have under the same conditions in the absence of the interposed substance. 



137 



