Shape of the Kathode in Geissler's Tubes. 467 



nomenon thus agrees most exactly with the explanation given 

 above. 



If we exhaust up to the density which corresponds to the 

 figure with a dark cross for the uncovered kathode, we obtain 

 the figure 25. By comparison with fig. 24 6, we see here 

 also very plainly that the bounding curves of the dark cross, 

 with their convex sides turned towards each other, are nothing- 

 else than the bounding lines of a " curved point " with con- 

 cave sides turned towards each other, which have become 

 displaced across each other. 



10. The most remarkable phenomena of the kind we are 

 now considering, however, are undoubtedly those produced by 

 plane kathodes. 



That plane kathodes cut into figures would present pheno- 

 mena similar to those of the dark cross was probable, since 

 rays emitted by one edge of an arm of the figure would be 

 repelled by the other edge. Thus, in fact, a four-armed plane 

 (fig. 26 a) gives a figure resembling fig. 26 b, with a distance 

 of wall from 3 to 4 centim. 



The figures 12 c, 12 d, &c, observed with the curved cross- 

 shaped kathode, do not appear with the plane kathode, but as 

 the density decreases the first figure recognizable is the dark 

 cross; if the density is still further diminished the arms in- 

 crease in width, and show in the part nearest the centre a 

 nebulous luminosity, with convex contour line (fig. 27). 



If the distance of the wall be made less than 3-4 centim., this 

 nebulous portion increases in brightness, and contracts, be- 

 coming better defined; and with a distance of wall of about 

 1^ millim. we obtain the figure represented in fig. 28. In each 

 of the four arms, which would otherwise be dark, there appears 

 a beautifully forked line of light, the space between the forks 

 being filled with uniform light with convex contour, while 

 each fork is connected with the others by a slightly luminous 

 arc of light. 



In using a concave spherical cross of the same dimensions 

 as kathode, the radius of curvature being, however, greater 

 than 25 millim., we observe in the arms of the dark cross, so 

 often mentioned, which appears at low pressures, this same 

 luminous fork. We have therefore, as was to be expected, a 

 gradual passage from the forms given by the curved kathodes 

 to those of the plane kathodes, the figures given by the plane 

 kathodes regulating those of the spherical kathodes. Inas- 

 much as no explanation of the images produced by the former 

 is possible, so for the present no explanation is to be given for 

 a number of phenomena obtained with the curved surfaces. 

 The dark cross produced by the plane cross-shaped kathode 



