394 Prof. Thomson, On the Diffuse [Jan. 24, 



working at the Cavendish Laboratory has arrived at these and 

 other results by the electric method. 



The following investigations were undertaken in the hope of 

 throwing some light on the way in which the molecules of a 

 substance might be conceived to give out Rontgen rays. 



Since there is no reflection of the Rontgen rays we cannot 

 suppose that the secondary rays are produced by an action similar 

 to that by which light is ' scattered ' from small particles. 



If however we adopt the theory described by the author in the 

 Philosophical Magazine for Feb. 1898, that the Rontgen rays are 

 thin pulses of intense electric and magnetic intensity such as is 

 shown would be generated by the sudden stopping of the cathode 

 rays, then one way in which the secondary Rontgen rays might be 

 produced is as follows. Let us suppose that the atoms of the 

 substance carry electric charges, then when the pulse of intense 

 electric intensity which constitutes a Rontgen ray falls on these 

 atoms it will suddenly change their velocities, this sudden change 

 in the velocity of a charged atom will generate a secondary pulse 

 of electric and magnetic intensity which on the above theory 

 would constitute a secondary Rontgen ray. The nature of this 

 secondary ray is as follows : if a sphere of radius a with a charge e 

 is suddenly started with a velocity u parallel to the axis of X, 

 then the state of the magnetic field can be shown to be as follows. 

 If P is a point under consideration, the centre of the particle, 

 and OP = r, then H the magnetic force at P is zero when the 

 time t which has elapsed since the particle was started is less 

 than (r-a)/V, where V is the velocity of light through the 

 medium surrounding the sphere: when t > (r — a)jV< (r + a)/V, 

 then 



ew sin 9 



H = 



ar 



where 6 is the angle between OP and the axis of X ; when 

 t>(r+a)/V, 



eiv sin 6 

 -" = i — ' 



the lines of magnetic force are circles with their centres along the 

 axis of X and their planes at right angles to it. Thus between 

 the times t = (r-a)/V and t = (r + a)jV a pulse of intense 

 magnetic and electric force is passing over P. 



Now consider a primary Rontgen ray travelling along the axis 

 of z ; let the electric intensity in the pulse make an angle § with 

 the axis of X, this intensity will give an impulsive velocity parallel 

 to itself to a charged particle in the path of the pulse, let this 



