On the Source of the Rontgen Rays in Focus Tubes. 435 



surface are considerably the most efficient in producing Rontgen 

 rays. Similarly those portions of the stream that impinge on the 

 anti-cathode surface very much on the slant are correspondingly in- 

 effective in producing Rontgen rays. 



(5) At the degrees of exhaustion most suitable for producing 

 Rontgen rays, and with concave cathodes of the usual dimensions, 

 the cathode stream proceeds almost entirely from a small central 

 portion of the cathode surface, the remaining portion of the 

 surface being apparently practically inoperative. That this is 

 so was very conclusively established by photographs taken with 

 the tube shown in fig. 2. In the manufacture or subsequent 

 exhaustion of this tube three very minute fragments of glass by some 

 means attached themselves on to the concave surface of the alu- 

 minium cathode, and remained fixed there during the experiments. 

 The cathode itself was 29 mm. diameter, and the radial distances of 

 the three glass fragments from the centre were respectively about 

 9 mm., 4 mm., and 2'5 mm. In all the pin-hole photographs of the 

 anti-cathode of this tube in which the enlargement of the active area 

 was sufficient, the shadows of the two glass fragments nearest to the 

 centre of the cathode are clearly visible, while in none of them is 

 there any appearance of the third and outer fragment. It, therefore, 

 is evident that the whole of the cathode stream that was effective in 

 producing Rontgen rays came from an area of the cathode surface 

 less than 18 mm. diameter, or less than two-thirds of the full 

 diameter of the cathode. Further, in each case the shadows of the 

 two inner glass fragments appeared outside of the central nucleus, 

 showing that the whole of the more intense portion of the cathode 

 stream proceeded from a portion of the cathode surface less than 5 rnm. 

 in diameter. These results confirm the writer's observations made 

 with carbon cathodes.* 



(6) The different portions of the cathode stream proceeding from 

 different portions of the cathode, cross at the focus and diverge in 

 a cone that retains any special characteristics of the convergent cone. 

 The relative positions of the two inner glass fragments on the 

 cathode, and the positions and enlargement of their shadows 

 on the anti-cathode for different distances between the latter and the 

 cathode, were found to show this very clearly. 



(7) Though by far the greater portion of. the Rontgen rays given 

 by a focus tube proceed from the active anti-cathode area, still, a 

 very appreciable quantity is also given off by all those portions of 

 the glass of the tube that show the green fluorescence. 



Using a somewhat large pin-hole, this is easily observed by turning 

 the tube so that the more powerful rays from the anti-cathode cannot 

 reach the pin-hole, when a Rontgen ray image of the whole of the 

 * ' Eoy. Soc. Proc.,' vol. 61, pp. 9293. 



