Shape of the Kathode in Geissler's Tubes. 459 



The type of successive changes in decreasing density of gas 

 with polygonal kathodes may be illustrated by the series of 

 images given by a concave spherical square (fig. 15). Figs. 

 c and d, for the sake of greater distinctness, are represented 

 on a scale somewhat larger. If the kathode-polygon possesses 

 an odd number of sides, then the luminous polygonal images 

 corresponding to figures 15 b-d are reversed with reference to 

 the polygonal kathodes, corresponding with the observation 

 made with star-shaped kathodes with an odd number of rays. 

 It is these luminous figures so far described and figured 

 which first strike the observer, in the phosphorescent images 

 of the corresponding kathodes ; upon closer observation we see 

 that the other surfaces of the glass vessel are also not devoid 

 of luminosity, but show phosphorescent surfaces of feeble 

 luminosity at different points. The boundaries of these sur- 

 faces, in the case of star-shaped kathodes, are prolongations of 

 the luminous curves which bound the chief figures; with 

 polygonal kathodes they are extensions of the star-rays 

 observed in the luminous figure. Partly because, from what 

 has been said, a sufficient preliminary account of the way in 

 which these less-luminous portions complete the figures already 

 figured and described is now possible to the reader, and partly 

 to economize space, I abstain from further description of 

 these outlying portions until the separate observations have 

 been completely described. The image obtained in a spherical 

 vessel of about 9 centim. diameter, employing as kathode the 

 cross figured in fig. 11 a, may serve as a good example : it is 

 represented in figs. 16 a and 16 e, in the pointed phase and in 

 the dark-cross phase. 



3. Besides the forms obtained by simple variation of the 

 number of sides and arms of polygons and stars composed of 

 rectangles, I have further examined the images given by 

 numerous other forms of kathode — some simpler, some more 

 complex — in order to separate as far as possible that which is 

 general from that which is special. Thus, for example, of 

 simpler forms were examined : — rectangle, rhombus, rhomboid, 

 isosceles right-angled triangle, &c. of compound forms; crosses 

 composed of obliquely compounded rectangles, and crosses 

 formed of isosceles triangles (the latter either with the 

 vertices or with the bases outwards) ; further, figures such as 

 are obtained by cutting out of squares segments of circles or 

 smaller squares at the four sides. 



So far as the effect of the form of the kathode, and of the 



variation in density of gas, manifests itself with these images, 



I must defer a description of the phenomena observed until 



I give a more complete explanation of the whole. The 



2H2 



