Shape of the Kathode in Geisslers Tubes. 457 



is worthy of remark, as throwing light upon their mode of 

 production. If we cover up the upper portion of a polygonal 

 kathode by a screen placed between the kathode and the 

 centre of curvature, then the upper arms of the star are want- 

 ing in the image. The arms are therefore not produced, as we 

 might have expected, by the radiation from the portions of 

 the kathode diametrically opposite. 



A four-armed cross, fig. 11 a (actual size), forming a portion 

 of a sphere of 40 millim. diameter, gives fig. 11 h on the wall 

 of a vessel 8 centim. in diameter, in which again the position 

 of the kathode is marked by dots. The metallic arms of 

 the kathode thus correspond to the dark arms of the cross 

 in the phosphorescent figure, and the light is concentrated 

 in fields corresponding to the intermediate spaces in the 

 kathode. 



If, again, the kathode has three arms, with angles of 120° 

 between the arms, we obtain a figure with three dark arms. 

 These again fall upon the metallic arms, and the bright fields 

 upon the intermediate spaces between the metal arms *. 



The dark arms of the image, however, are much narrower 

 with the four-rayed figure than with the three-rayed one, if 

 the arms of the kathode are of equal width and equal length 

 in the two cases. 



We obtain analogous figures when the kathode has five or six 

 (uniformly distributed) arms, except that as the number of 

 arms increases the dark fields which correspond to the metallic 

 arms become narrower, not only absolutely but also relatively 

 — that is, in proportion to the width of the bright intermediate 

 fields. 



These two typical forms — the polygon, and the star consisting 

 of rectangles — may suffice as a preliminary indication of the 

 forms of the images which appear when the exhaustion is 

 sufficient. 



2. The images formed by kathodes of this sort alter very 

 much when the density of the gas is altered. 



The image-forms described above are obtained with den- 

 sities of gas about r ^ millim. mercury. We can, however, 

 trace the phosphorescence at pressures only slightly less than 

 1 millim., or even over this pressure, by including in the dis- 

 charge at the same time sparks in free air. We obtain then, 

 for example, for the four-armed cross-shaped kathode, fig. 11 a 

 (radius of curvature 20 millim.), the images 12 a to 12 e in 



* I have made repeated attempts to obtain a result described iu Carl's 

 Repert. 1880, p. 244, where a sort of three- rayed star gave simply an erect 

 image of the kathode, but never with success. 



Phil. Mag. S. 5. Vol. 14. No. 90. Dec. 1882. 2 H 



