1896.] on Electric Shadoivs and Luminescence, 205 



tions in the third condenser, to which a vacuum tube can be connected. 

 If, now, by adjusting the distances between the plates of condensers 

 we tune the primary and secondary circuits together, the electric 

 oscillations that result will persist much longer than if the circuits 

 are not so tuned. Though each oscillation may last less than the 

 1 00-millionth of a second, there will be at each spark some 20,000 or 

 30,000 oscillations before they have died out. Wiedemann and 

 Ebert have found that these persistent oscillations are specially 

 adapted to excite luminescence. To illustrate the point I select here 

 an old Geissler tube with a comparatively poor vacuum. When 

 stimulated by ordinary sparks directly from the Apps coil through 

 the platinum electrodes at its ends, it shows the usual features of 

 Geissler tubes : there is a luminous column extending through the 

 central bulb with stratifications along its length, while around the 

 kathode is the usual violet glow. The glass shows no fluorescence. I 

 now charge the connections, uniting the wires from Ebert's apparatus, 

 not to the terminal electrodes of the tube but to two patches of tin- 

 foil stuck upon the outside of the central bulb. Under these 

 conditions the electric oscillations illuminate the central bulb with a 

 glow quite different from that previously seen. Beneath each patch 

 of foil you can discern the bluish kathode discharge, and the glass 

 now shines with characteristic apple-green fluorescence. By moving 

 one plate of one of the condensers in or out I alter the conditions of 

 resonance in the circuit ; and when the tuning is best the fluorescence 

 is at its brighest. Now Wiedemann observed * that the light so 

 generated is capable of exercising a photographic action and of other 

 etifects, but is incapable either of passing through a thin plate of fluor- 

 spar or of being deflected by a magnet. These rays difi'ered therefore 

 both from ultra-violet light and from kathode rays ; hence Wiede- 

 mann pronounced them to consist of a new species which he named 

 " Entladungsstrahlen " or discharge-rays. It is again a matter for 

 research to determine whether Wiedemann's rays are the same as 

 Lenard's, or as Roentgen's rays. Wiedemann's coadjutor Ebert went 

 on with the research and produced on this principle a little " lumi- 

 nescence lamp " having two external rings of foil as electrodes ; 

 and within the vacuum bulb a small pastile of phosphorescent stuff, 

 which, when excited by the oscillations of the tuned circuits, glows 

 with a small bright light. Ebert claims that its efficiency is many 

 times greater than that of the ordinary glow lamp. 



Eeturning now to Roentgen's researches, we will take a glance at 

 the kind of tube (Fig. 8) w^hich he was employing when he made 

 his discovery of the X-rays. Its general resemblance ta previous 

 tubes "f is self-evident. The anode was a piece of aluminium tube 

 through which passed the glass-covered kathode wire, with a small 



=" ' Zeitschrift fiir Elektrochemie,' July 1895, p. 159. 



t It is, in fact, identical with the form described by Hertz in 1883, see 

 ' Wiedemann's Annalen,' xix. p. 810. 



