Shape of the Kathode in Geissler*s Tubes. 471 



axis, and that in greater degree the greater the curvature of 

 the surface. 



12. Still another class of simple forms of kathodes which 

 produce figures are such as may be termed " interrupted;" to 

 which belong, amongst others, prismatic tubes open at the 

 ends and cut off at right angles, then (plane) figures formed 

 by bending wire (e. g. polygons of wire), and so on. 



The images given by such wire kathodes are amongst the 

 most beautiful which can be obtained. In order to render 

 intelligible at least the general mode of their formation, the fol- 

 lowing may be mentioned: — 



If a cylindrical or prismatic tube cut off at right angles to 

 its axis and open at both ends be employed as a kathode, then 

 as the exhaustion proceeds a conical pencil of rays issues from 

 each of the open ends of the tube, the axis of which is coin- 

 cident with the axis of the tube, and which expands more and 

 more into the gas-space the further the exhaustion is carried. 

 Assuming rectilinear rays, the pencil is then so directed as if 

 it issued from a metal plate closing the actual opening of the 

 tube. When the exhaustion is sufficiently great, this pencil 

 reaches to the wall of the vessel and excites phosphorescence 

 there. 



The phosphorescent image of this pencil forms the phos- 

 phorescent figure produced by a tube-shaped kathode. 



In general, there are two images produced corresponding 

 to the two pencils which issue from the two openings of the 

 tube, and which are congruent if, for example, the discharge- 

 vessel be spherical and the middle point of axis of the tube 

 coincide with the centre of the sphere. 



We have a similar result to that obtained with a cylindrical 

 kathode, when the wall of the tube kathode is saddle-shaped, 

 such for example as is formed by the revolution of an arc of 

 a circle about an axis lying on its convex side. 



If we imagine such a tube of very small height, we obtain 

 a case which can also be realized by surrounding a space with 

 a wire of the corresponding form. Just as pencils of rays 

 issued at right angles to the opening of the tube, so they 

 issue from the wire kathode at right angles to its plane, to all 

 appearance as if the empty space surrounded by the wire acted 

 as a kathode. 



The luminous figures obtained from wire kathodes are 

 larger than the space enclosed by the wire, even at small dis- 

 tances of the wall from the kathode. The images may show 

 great changes with change of pressure. I content myself with 



