3H SCIENCE PROGRESS. 



' pulse,' as I will call it, in order to distinguish it from a periodic 

 undulation, would spread in all directions, so that . . . according 

 to the view here put forward, the Rontgen emanation consists 

 of a vast and irregular succession of isolated and independent 

 pulses, starting respectively from the points and at the times at 

 which the individual charged molecules projected from the 

 cathode impinge on the target." J. J. Thomson was then about 

 to give to the world the results of his brilliant researches on the 

 mass and charge of the cathode rays, and we now know that for 

 " molecules " we must substitute the much smaller electrons or 

 corpuscles. Thus, very much on the analogy of the generation 

 of a sharp crack or pulse of sound by the sudden stoppage of 

 a whip-lash, Stokes conceived the origin of a Rontgen pulse. 

 Furthermore, "... we know of no reason beforehand forbidding 

 us to attribute an excessive thinness to the pulses which the 

 charged molecules excite in the ether"; and to this pulse width, 

 small compared even with that of the wave length of light, 

 Stokes attributed some of the differences between ordinary light 

 and Rontgen rays. 



J. J. Thomson has elaborated Stokes' original theory both 

 analytically and by the method of Faraday tubes. Briefly, the 

 developed theory postulates that a Rontgen ray is a thin pulse 

 of intense electric and magnetic force, which travels outwards 

 with the speed of light in all directions from the place where 

 the motion of a charged particle is arrested or changed. Its 

 thickness depends on the abruptness of arrest : more energy 

 is associated with thin pulses than thick ones, not only because 

 the thinner pulses are produced by the stoppage of more rapidly 

 moving cathode particles, but also because a larger proportion 

 of the kinetic energy of fast-moving particles is converted into 

 radiant energy owing to the shorter time of impact. (We here 

 assume that the distance in which the particle is brought to rest is 

 approximately the same whatever its velocity ) The electric inten- 

 sity in the pulse is, of course, ever weakening as the pulse widens. 



Since in a Rontgen tube the cathode rays are all travelling in 

 the same direction, it is to be expected that the electric forces 

 in the resulting pulses will lie in planes passing through that 

 direction, and not in planes perpendicular to it — that is to say, 

 the Rontgen rays should be polarised in a plane at right angles to 

 the direction of motion of the cathode rays. This one-sidedness 

 fits in with Barkla's result (p. 311) that, in a plane perpendicular 



