Mr Orange, On certain -phenomena of the kathode region. 229 



The way in which the pencils change with the gas pressure and 

 the separation of the plates is easily summarized qualitatively : 

 The inclination of the rays to the plane of either kathode 



(1) increases with the distance separating the plates, and 



(2) increases with the pressure. 



There is a transition at a certain stage of exhaustion ; at the 

 higher pressures the rays being sharply bounded on one side only, 

 while at lower pressures they form true sheets which are well- 

 defined on both sides. 



[Figs. 1 and 6, PI. VII, Fig. 3, PI. VIII are typical o{ higher pressures, 

 „ 2 and 7, „ „ „ „ rather lower „ 



„ 3 „ is „ „ transitional „ 



4 „ „ „ „ still lower „ 



5 „ Fig. 6, PI. VIII are „ „ low „ ] 

 Figs. 4 and 5, PI. VIII are prints from a single negative. The first 



shows details of the rays near the kathodes very well while the 

 second shows up the general appearance. At this stage the two 

 " oblique " beams on the left as well as the median beam are 

 practically horizontal. 



In seeking an explanation of the pencils, let us first disregard 

 the origin of the rays and consider only their directions and 

 curvature. If we except the comparatively high pressures — at 

 which the rays are ill-defined — it may be observed that the 

 asymptotes of the rays always pass approximately through the 

 central point of the kathode interspace. This would imply that 

 the electrostatic field is determined largely by the existence of 

 free charge in the region containing the kathodes, as distinct 

 from the charge on the kathodes themselves, and, in fact, that 

 in the cases considered there is a region of negative charge 

 corresponding roughly with the brightly luminous part of the 

 interspace. 



It has been shown by various workers that the electric intensity 

 in the ordinary Crookes' dark space is considerable at all points 

 but increases rapidly as the kathode is approached ; Schuster's* 

 experiments indicate that there is a distribution of free positive elec- 

 tricity in the dark space, such that the volume density decreases 

 in geometrical progression as the distance from the kathode 

 increases in arithmetical progression "f. 



In such circumstances, the kathode rays in their progress away 

 from the kathode will acquire a high velocity in the initial stages 

 of their motion, and subsequently will be affected but little by the 

 weaker electrostatic fields which they traverse. 



Consider the rays from the opposed faces of the two kathodes 

 in our experiment. The rays from each must in due course enter 



* Schuster, Proc. Roy. Soc. xlvii. p. 526, 1890. 



t J. J. Thomson, Conduction through Gases, 2nd ed. p. 541. 



