Shape of the Kathode in Geisslei^s Tubes. 463 



8. The phosphorescent figures which appear when spherical 

 electrodes are employed, as might have been expected, become 

 replaced by others when the originally plane kathodes are 

 formed, not into spherical surfaces, but into cylindrical or 

 conical surfaces. I will content myself in this cursory report 

 with mentioning that, with cylindrical kathodes, the phospho- 

 rescent images are different according to the position which 

 the axis of the cylinder has with reference to the axis of sym- 

 metry of the kathodes. Thus, for example, with a square 

 kathode formed to a cylindrical surface we obtain different 

 figures according as the axis of the cylinder is parallel to a 

 side of the square or to one of its diagonals. In the same way 

 there are marked differences in the images of cross-figures 

 according as the axis of the cylinder is parallel to one of the 

 arms of the cross, or bisects the angle between the arms. 



9. With reference to the explanation of the phenomena so 

 far described, it is only wise to maintain a certain reserve 

 towards a class of phenomena as yet very imperfectly known. 

 A doubt as to the success of the attempt to bring a large 

 portion of these phenomena into any simple relationship with 

 known causes, could only be supported by the observations 

 described in No. 10, on the images given by plane kathodes. 

 All that I hope to do is to give some indications, derived from 

 experiment, on the direction in which the explanation of many 

 of the phenomena observed with star-shaped kathodes is to be 

 sought. As to the explanation of most of the phenomena 

 given by the above-mentioned polygonal kathodes, I do not 

 here venture upon any hypothesis. The star-shaped kathodes 

 are of course also polygons with reentrant angles; and their 

 separation from the polygonal surfaces appears unnatural at 

 first sight. Nevertheless the phenomena which we have to ex- 

 plain with star-shaped kathodes are for the most part just those 

 which are closely connected with the presence of reentrant 

 angles, and which may be approximately explained by taking 

 into account only the edges of the kathode. The phenomena 

 due to the surfaces themselves, in those forms of kathode 

 where the surface of the kathode is relatively unimportant 

 as compared with the extent of its bounding edge, are insig- 

 nificant in comparison with the phenomena due to the curves 

 of its circumference ; and the former remain unexplained here 

 just as with the polygons with convex angles, where the phe- 

 nomena of the surface constitute the chief part of the pheno- 

 mena observed. Only the surface-phenomena produced at 

 small exhaustions can be approximately accounted for with 

 both kinds of polygons That the images of a star-shaped 

 kathode which appear at relatively small exhaustions result 



