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Mr. A. A. C. Swinton. 



be stated that the vacua in each case were comparatively low, as 

 vacua as high as those employed when the anti-cathode was outside 

 the focus, gave no results at all. As will be seen, however, the 

 diameter of the luminescent ring is affected by the degree of 

 proximity of the anti- cathode to the cathode, being larger when the 

 distance is small than when it is great ; while in every case there is 

 a decided tendency towards hollowness, though usually with some 

 slight internal luminescence and with a bright central spot, while in 

 one case, when the anti- cathode was very close to the cathode and 

 the vacuum was comparatively high, the ring is seen completely 

 hollow, and there is no central spot. 



A convenient form of tube for showing the apparent hollowness of 

 both the divergent and convergent cone of cathode rays is shown in 

 fig. 8, where the anti-cathode disc B, made of electric-light carbon, 

 is supported upon a small carrier which slides upon the bottom of 

 the tube, and is connected to the anode terminal D by means of two 

 aluminium wires, each of which have a ring at their extremity 

 through which they respectively pass. As the carbon, under the 

 action of the cathode rays, gives off hydrocarbon vapour, it is neces- 

 sary, as already mentioned, to try all these experiments with the 

 tube connected to the mercury pump ; but with this connection made 

 through a slightly flexible mercury joint it is possible by inclining 

 and gently tapping the tube to bring the anti-cathode to any desired 

 position either near or far away from the cathode. For experiments 

 upon the divergent cone, it is not necessary that the anti-cathode 

 screen should be connected to the anode terminal, and, consequently, 

 the sliding aluminium wires inside the tube are not required. They 

 are, however, necessaiy when observations are to be made on the 

 convergent cone between the cathode and the focus, as the anti- 

 cathode screen when placed within the focus must be connected to 

 the anode, or it appears to get negatively charged and acts itself as 

 an additional cathode, throwing cathode rays in all directions. 



It may here be mentioned that the fatigue of the carbon already 

 alluded to renders necessary some precautions in carrying out the 

 above-mentioned experiments, as otherwise the observer may be 

 misled into thinking that a beam of cathode rays is effectively 

 hollow when this is not the case, owing to the centre of the carbon 

 covered by the beam having been fatigued by some previous experi- 

 ments. By taking the precaution, however, of deflecting the 

 cathode beam by means of a magnet on to various portions of the 

 carbon screen, such errors may be avoided. It should also be noted 

 that these hollow effects appear only to be obtained with fairly short 

 focus cathodes, such as are usually employed in X-ray focus tubes, 

 that is to say, with cathodes whose diameter is large as compared 

 with their radius of curvature, so that the rays converge and diverge 



