1898.] on Some New Studies In Cathode and Eontgen Badiations. 581 



This concentration or focnssing of the cathode rays by means of a 

 magnetic field, which has been studied by Biikeland and by Fleming, 

 can be also shown by means of another tube, the interior of which is 

 free from any obstruction. This tube, when excited in the ordinary 

 manner, shows, as you will observe, the nsual green fluorescence 

 nearly all over its surface, but especially at the rounded end opposite 

 the cathode. I suspend this tube over one pole of a powerful electro- 

 magnet, placed with its axis in line with that of the tube as shown in 

 Fig. 1. As more and more electric current is passed round the 

 electro-magnet, and the magnetic field 

 becomes stronger and stronger, it will 

 be obs'-rved that the beam of cathode 

 rays becomes more and more conceu- 

 trat(3d to a point opposite the pole of 

 the magnet, until at length when the 

 magnet is fully excited the whole of 

 the green fluorescence in the tube has 

 now entirely died out, and the cathode 

 stream can be seen as a bluish cone, 

 the b !se of which is the cathode disc, 

 and the apex is a very small point 

 <^xactly over the centre of the magnet 

 pole. It is not possible to keep the 

 tube in this condition for more than a 

 few seconds, as the heat produced on 

 the glass where the cathode rays are 

 concentrated is so intense as to quickly 

 perforate the latter. Indeed, by slowly 

 moving the tube it is possible to en- 

 grave on its interior surface any de- 

 sired figure, the action of the cathode 

 rays being sufficient to erode the glass. 

 Fig 2 is a photograph of the globular 

 end of a tube, upon the interior glass 

 surface of which, as can be seen, a 

 square with diagonals has been 

 roughly engraved by this means. 



Whether the action is due directly to the bombardment of the atoms 

 which form the cathode rays breaking off little pieces of glass as a 

 volley of minute bullets would do, or whether it is a secondary effect 

 due to heat, is perhaps uncertain. The result in any case is that 

 where the concentrated cathode rays impinge upon the glass, the 

 latter is eroded and visibly roughened. 



A concentrated cathode discharge can also be obtained by em- 

 ploying as cathode a si)herically concave aluminium cup, so arranged 

 relatively to the glass of the tube that the rays are, given off only from 

 the hollow side, this being the arrangement now universally used in 

 tubes for the production of the Eontgen rays. It is a method origi- 

 VoL. XV. (No. 92. ) 2 Q 



PiQ, 1.— Cathode rays fooussod to 

 a point by means of a magnet. 



