1896.] on Electric Shadows and Luminescence, 209 



is electrified can be ascertained by dusting over it mixed powders 

 of red lead and sulphur (or red lead and lycopodium). With the 

 aid of Mr. Miles Walker, who has worked with me all through this 

 matter, I have succeeded in producing, on this plan, well-defined 

 shadows which will now be demonstrated to you. A clean sheet of 

 ebonite freed from all traces of previous electrification by being 

 passed through a spirit flame is laid on a properly prepared metal 

 table. On it stands a small tray of thin aluminium supported on 

 four insulating legs. In this tray is placed the object whose shadow 

 is to be cast, for example a pair of scissors or an object cut out 

 in sheet lead. Over this again is placed a leaden cover with an 

 opening above the tray : the leaden cover being designed to cut 

 oif electrostatic influences which might interfere. The tray is 

 then electrified by a small influence machine, and while it is so 

 electrified X-rays are sent downwards from a Crookes tube placed 

 above. They pass down through the aluminium tray and carry 

 its electrification to the ebonite sheet, which therefore becomes 

 electrified all over except in the parts which are shielded by the 

 scissors or other metallic object. The sheet of ebonite is then re- 

 moved from the leaden enclosure, the aluminium tray lifted off, 

 and the mixed powders are dusted over, adhering to the surface of 

 the ebonite and revealing the otherwise invisible electric shadow. 

 Fig. 10 is a shadow taken in this way. It is but right to mention 

 that Professor Eighi, of Bologna, has independently obtained electric 

 dust shadows in a very similar way since these experiments of mine 

 were begun. 



This will be a convenient place to mention a new effect of X-rays 

 which I have recently observed and which is set down in the table. 

 When X-rays fall upon a metal object electrified by an influence 

 machine, they produce some curious changes in the nature of the 

 discharge into the air. If the body is already discharging itself from 

 some edge or corner in an aigrette or brush discharge (visible in 

 darkness only) the size and form of the aigrette is much altered. 

 Under some circumstances not yet investigated, the incidence of 

 X-rays causes the aigrette to disappear; under others the X-rays 

 provoke its appearance. 



Since the publication of Roentgen's research the most notable 

 advance that has been made has been in the direction of improving 

 the tubes. Eoentgen himself has mostly employed a pear-shaped tube 

 with a flat circular kathode near the top, producing a beautiful fluor- 

 escence of the lower part of the tube. He carefully verified the 

 circumstance that the X-rays originate at that portion of the glass 

 surface which receives the impact of the kathodic discharge. They 

 appear in fact to be generated at the place where the kathode 

 discharge first impinges upon the surface of any solid body. It is 

 not necessary that the substance which is to act as emitter of the 

 X-rays should become fluorescent. On the contrary, it appears that 

 the best radiators are substances that do not fluoresce, namely the 



Vol. XV. (No. 90.) p 



