Shape of the Kathode in Geisslers Tubes. 469 



brighter zone at the edge ; the centre point is, of course, bright. 

 An ellipse whose axes measure 10 mm. and 20 mm., on the 

 other hand, gives a comparatively complicated figure (fig. 31) 

 at a distance of wall of 1 cm. in a vessel of 9^ cm. diameter. 

 In all these figures the ground expands the more the density 

 of the gas is reduced. 



If we use kathodes made up of several of these simpler 

 forms joined together, we of course obtain much more com- 

 plicated images. The images obtained with the comparatively 

 simple form of kathode fig. 32 a may serve as example. 

 A square out of whose edges smaller squares have been cut 

 (fig. 32 b) shows the central portion of the phosphorescent 

 image completely] but, to save space, the figure gives only 

 two of the streams of light which project from the four sides. 



To enter at present further into detail in describing a large 

 number of these phenomena, which are often characterized by 

 surprising beauty, would be without scientific interest, since 

 the simpler cases already described sufficiently represent 

 whatever is new and characteristic amongst phenomena of 

 the kind, viz. : — 



(1) The fact that such figures are produced. 



(2) The circumstance that the magnitude of the images 

 varies with the density of the gas, and exceeds the magnitude 

 of the kathode itself at high exhaustions. 



This latter phenomenon, to which I have devoted a separate 

 series of experiments, may be supposed to occur in either of 

 two ways : either the direction of the rays varies with the 

 change of density, the pencil emitted by a plane becoming- 

 more divergent the smaller the density of the gas becomes ; 

 or the direction of the rays remains constant, and as the den- 

 sity of the gas decreases the previously unobserved rays of 

 those elements of the surface which are situated obliquely to 

 the outward-directed rays become strengthened. 



Experimental trial gives as result that the first-named 

 reason (variation in direction of rays with variation of den- 

 sity) is to be preferred. Of the different methods of proof 

 employed I will mention only one here. 



If we cut slits in a plane disk, the spaces in the disk show 

 themselves in the phosphorescent image, for which the disk 

 acts as a kathode, as narrow dark lines. A number of con- 

 centric and equidistant semicircular cuts were made in a disk, 

 so that the outside one lay near the edge of the disk (fig. 33). 

 In the phosphorescent image there appear, even at the highest 

 density at which it is visible, the same number of dark semi- 

 circular lines, showing that even at the highest density the 

 phosphorescence produced by the elements on the edge of the 



