Some Experiments with Cathode Rays. 



81 



only slightly concave, focus further in proportion beyond their 

 centres of curvature than do deeply concave cathodes, for the same 

 vacuum 



Apparent Hollowness of the Divergent Cone of Bays. 



When the divergent cone is thrown upon a thin platinum disc, as 

 in the ordinary focus tube, and sufficient electric power — say, from a 

 10-in. Ruhmkorff coil — is employed, the platinum quickly attains to 

 a red heat. With platinum, either the whole disc becomes uniformly 

 heated, or in the event of the diameter of the cone of rays where it 

 strikes the platinum being small compared with the area of the 

 platinum, that portion of the platinum covered by the base of the 

 cone becomes uniformly heated to a higher temperature than the 

 remainder. This is as much as can usually be seen with platinum, 

 though rather more is sometimes visible with aluminium ; but if 

 instead of either metal the disc is made of ordinary electric-light 

 carbon, I have found that the luminescent portion of the carbon, 

 instead of comprising the whole disc, or consisting of a uniformly 

 heated circle, will in some cases take the shape of a brilliantly lumi- 

 nescent and apparently white-hot ring, with a well-defined dark, and 

 seemingly quite cold, interior. As the dimensions of the cone of rays 

 are iucreased or decreased by decreasing or increasing the vacuum, 

 the luminescent ring will be found to increase or decrease corre- 

 spondingly in diameter, at the same time being brighter when small 

 than when large. Further, when the ring is very small it will 

 usually have a very brightly luminescent central spot, with a dark 

 intervening portion between this spot and the ring, and when the 

 vacuum is further increased the ring will gradually close in upon the 

 spot until only the latter remains. 



Figs. 1, 2, 3, and 4 show diagrammatically these hollow effects, as 

 produced by spherical aluminium cathodes, 1*125 in. diameter and 

 0*708 in. radius of curvature, for four different degrees of vacuum, 

 1 being the lowest and 4 the highest exhaustion. The upper portion 

 of each of these figures represents the general appearance of the 

 cathode discharge between the spherical concave aluminium cathode 

 C at the top, and the carbon anti-cathode B at the bottom, as accu- 

 rately as it is possible to represent evanescent coloured appearances 

 in monochrome. The other appearances, due to the dark space and 

 fluorescence of the glass, are omitted for the sake of simplicity. 

 Beneath each of the elevational views of the cathode discharge will 

 be found a plan view of the carbon anti-cathode, showing for each 

 condition of vacuum the effect of the cathode discharge upon the 

 carbon anti-cathode, in forming a brightly luminescent hollow ring, 

 gradually decreasing in diameter as the vacuum is increased, until 

 it centres on a point, as already mentioned. 



