166 Some Experiments until Rontgen's Rays, 



5. The phenomenon of diselectrification by #-rays is very 

 readily demonstrated. For this purpose I have found a very 

 convenient instrument to be an electroscope consisting of 

 two strips of aluminium leaf (which is lighter than gold leaf) 

 suspended in a thin-glass jar entirely covered with a fine 

 metal gauze. It is charged with a dry pile, and a metal 

 cap is then placed over the charging knob, so that it is 

 entirely electrostatically screened from external electrical 

 influences. Positive and negative electrifications are both 

 readily discharged, even at the distance of several feet from 

 the bulb. 



6. On the first announcement of the diselectrifying pro- 

 perties of <£-rays, I attempted to obtain electric dust-figures 

 as shadows of metallic objects by applying the ^?-rays to dis- 

 charge electrified surfaces of glass or ebonite upon which 

 mixed powders of red lead and sulphur were then dusted. 

 These were obtained almost at the first trial ; but to produce 

 them satisfactorily requires a little care. 



The object whose shadow is to be obtained— a pair of 

 scissors, for example — is laid upon a thin sheet of aluminium 

 placed to stand on four feet at the height of about 20 millim. 

 over the sheet of ebonite or varnished glass upon which the 

 shadow is to be thrown. This sheet of ebonite is first carefully 

 diselectrified by passing it over an alcohol flame, and then laid 

 upon an earthed sheet of foil upon the table. The aluminium 

 tray with the scissors upon it is placed over the ebonite. A 

 guard-box of lead with a rectangular hole in its top is placed 

 over all. Then the aluminium tray is charged electrically 

 by a small influence-machine which has one pole put to 

 earth and the other connected to the aluminium tray. In 

 this state of things the ebonite plate lies in an electro- 

 static field, but is not electrified upon its upper surface. 

 The ..r-rays are now caused to fall upon the aluminium tray, 

 through which they pass save when obstructed by the metallic 

 object, and, discharging the tray, virtually carry down the 

 electrification upon the surface of the ebonite in straight 

 lines, leaving the shadowed portions unelectrified. The 

 influence-machine is disconnected, the aluminium tray re- 

 moved, the sheet of ebonite lifted off the table, and the mixed 

 powders are forthwith dusted over its surface, revealing the 

 shadow. Both positive and negative shadows can be obtained, 

 Several alternate dispositions are possible. 



These observations were made early in February before the 

 announcement by M. Righi of some similar cases of production 

 of shadows by #-rays. 



7. I have also made some observations upon the reflexion 



