Images in the Field of a Hittorfs (6Voote') Tube. 313 



dimensions of the central spot diminish when the height of 

 the cylinder is increased. Comparing this phenomenon with 

 that described in (4) we see that the yellow spot should he 

 regarded as the image of an aperture. On covering the 

 cylinder by a metal disk the central yellow spot disappears 

 and we obtain red traces in the central part of the image. 

 The diminution experienced by the spot when a glass plate 

 is brought under the ebonite screen would apparently lead us 

 to seek the aperture towards which the rays proceeding from 

 certain points in the tube converge in the optical image of 

 the circular section of the cylinder nearest to the tube and 

 obtained by regarding the glass as a mirror. 



The central spot of the image of a cylinder placed beneath 

 the ebonite plate is red ; its size remains the same whatever 

 be the height of the cylinder provided that its diameter 

 remains the same. The phenomena are not altered if the 

 lead cylinders are replaced by glass ones. 



The following additional experiments correspond to those 

 described above : — on placing on the ebonite brass weights of 

 500 grams, 200 grams, and 5 decigr., it is only the last, 

 which represents a thin piece of metal, that gives a red image; 

 the first two do not give distinct images. In the image of a 

 coin placed above the screen we find red portions correspond- 

 ing to the points which w r ere in contact with the screen, and 

 black and yellowish portions corresponding to the depressions 

 of the coin. If this last is placed beneath the ebonite, the red 

 colour is replaced by yellow. We obtain in this manner the 

 design on the faces of the coin. 



In all these experiments the duration of the action of the 

 Hittorfs tube has an influence on the clearness and intensity 

 of the image ; prolonged action imparts a red or yellow colo- 

 ration (according to the circumstances) to the neutral bands. 



By obtaining the images of objects by our method and by 

 the photographic method we have proved that the yellow 

 colour corresponds to the parts of the figure directly attacked 

 by the #-rays ; the red colour to the images of the objects, 

 and the neutral bands to the shadows which surround, for 

 example, the images of the cylinders. 



Preliminary experiments have shown us that analogous 

 phenomena are obtained on replacing the Hittorfs tube by a 

 metallic point connected to the conductor of an electric 

 machine. Like phenomena, under slightly different con- 

 ditions, have been obtained by means of electric discharges by 

 M. Augusto Righi (Memorie della Accademia delle scienze 

 delPIstituto di Bologna, (4) iii. 1881, pp. 291 to 304, and 

 pp. 461 to 496) ; there is in this memoir a sketch of a 



