468 Dr. E. Goldstein on the Influence of tlie 



we might foresee ; hut it is not possible to predict, a priori, 

 the phenomena produced by plane kathodes, not cut out, 

 nor having concave edges, and these constitute an entirely 

 new class of phenomena. I make here the general remark 

 that, while it is true that the images given by plane kathodes 

 alter somewhat with decreasing density, they do so much less 

 than the figures obtained with curved kathodes, and often 

 the changes consist only in the acquisition of richer detail 

 and more definite contour, or perhaps somewhat larger dimen- 

 sions. If these images are produced by kathodes of relatively 

 small surfaces (1 to 1^ square centimetre), the kathodes 

 must be placed tolerably near to the wall of the vessel. The 

 larger the surface of the kathode is, the greater the distance 

 of wall at which it first appears. If with a given kathode we 

 go further off than a certain distance, we obtain a uniform y 

 illuminated phosphorescent surface, whose luminosity slowly 

 and gradually decreases from the centre outwards. 



If a plane square be employed as kathode, we obtain fig. 29 

 on the wall of the spherical discharge-tube of 9 centim. diameter. 

 The relative size and position of the kathode (2§ centim. distant 

 from the wall) is marked by the dotted lines. We obtain 

 therefore a star of eight arms, four of whose rays correspond 

 in direction to the diagonals of the kathode, and the others to 

 its central lines. The centre of the figure is formed by a 

 feebly illuminated square space, upon which the star appears; 

 a luminous zone surrounds the dark central space, formed of 

 four arcs, convex outwards. 



The four rays corresponding to the central lines of the 

 kathodes have their maxima of light within this outer zone ; 

 the four diagonal rays, which are narrower than the others, 

 are uniformly luminous, except that all eight rays are more 

 luminous at the centre of the whole figure. The whole figure 

 is considerably larger than the kathode-square, the darker 

 central square space being larger than the kathode. 



The figure given by a rectangle 2 centim. by 1 centim., with 

 its longest sides horizontal, on the wall at a distance of 1£ 

 centim., is represented in fig. 30. The main figure is thus a 

 narrow line of light, corresponding to the central line of the 

 rectangle, which forks at each end, and is surrounded by a 

 broad band. The lower ground is an oblong, rounded at the 

 small ends. 



A plane circular disk, as mentioned in the introduction, gives 

 no special figure on the illuminated ground obtained in all the 

 figures, unless we regard the bright central point of the image 

 as such a figure. The rays produce a circular disk, which is 

 not sharply defined, with the feebly illuminated ground, and 



