Prof. J. J. Thomson on Cathode Rays. 



301 



when the cathode rays were passing through it ; the divisions 

 on the plate enabled the path of the rays to be determined. 

 Under the action of the magnetic field the narrow beam of 

 cathode rays spreads out into a broad fan-shaped luminosity 

 in the gas. The luminosity in this fan is not uniformly 



Fiff. 4. 



distributed, but is condensed along certain lines. The phos- 

 phorescence on the glass is also not uniformly distributed ; it 

 is much spread out, showing that the beam consists of rays 

 which are not all deflected to the same extent by the magnet. 

 The luminosity on the glass is crossed by bands along which 

 the luminosity is very much greater than in the adjacent parts. 

 These bright and dark bands are called by Birkeland, who 

 first observed them, the magnetic spectrum. The brightest 

 spots on the glass are by no means always the terminations of 

 the brightest streaks of luminosity in the gas ; in fact, in some 

 cases a very bright spot on the glass is not connected with the 

 cathode by any appreciable luminosity, though there may be 

 plenty of luminosity in other parts of the gas. One very 

 interesting point brought out by the photographs is that in a 

 given magnetic field, and with a given mean potential-difference 

 between the terminals, the path of the rays is independent of 

 the nature of the gas. Photographs were taken of the dis- 

 charge in hydrogen, air, carbouic acid, methyl iodide, i. e., in 

 gases whose densities range from 1 to 70, and yet, not only 

 were the paths of the most deflected rays the same in all 

 cases, but even the details, such as the distribution of the 

 bright and d;irk spaces, were the same; in fact, the photographs 

 could hardly be distinguished from each other. It is to be 

 ncted that the pressures were not the same ; the pressures in 

 the different gases were adjusted so that the mean potential- 

 differences between the cathode and the anode were the same 

 in all the gases. When the pressure of a gas is lowered, the 

 potential-difference between the terminals increases, and the 



