Images in the Field of a Hittorfs (Crookes') Tube. 309 



With this end in view we replace the photographic plate 

 or the fluorescent screen by a plate of ebonite. The electric 

 field is maintained for two or three minutes, after which the 

 action of the tube is arrested and the ebonite plate withdrawn 

 from its position. By a quick movement the objects on the 

 plate are thrown to the ground and we proceed to develop the 

 images by sprinkling over the plate a mixture of sulphur and 

 minium. As is well known, the sulphur adheres to those 

 portions which are positively, the minium to those which are 

 negatively electrified. Thus the colour of the spots shows 

 the electric condition of the shadows, and the comparison of 

 their configuration with that of the objects indicates the 

 modifications introduced in the mode of action of the field. 



The tube used in our experiments had the shape of a pear 

 with flattened base (fig. 1) ; k is the kathode, a the anode. 



' Fig.L 



In the central part of the base there was a fluorescent spot 

 about a centimetre in diameter ; and at some distance there 

 was a less intense fluorescent zone concentric with the spot. 

 At a few centimetres below the tube is placed a plate or 

 screen of ebonite b, which rests ordinarily on the rim of a 

 glass vessel 13 cm. in diameter and 21 cm. in depth. The 

 manner of supporting the plate is quite immaterial provided 

 that the support is clear of the central portion of the plate. 

 Thus the screen, or at any rate its central portion, is sur- 

 rounded by air on both sides. The objects are placed either 

 above or below the plate ; in the latter case they are held 

 by projecting arms which are bent over the rim of the vessel. 

 The images were developed on both faces of the screen ; we 

 have also employed two screens placed one on the other and 

 developed the images on the four faces of the screens. 



