Theory of X Rays and Photoelectric Rays. 553 



of quality of the X rays with distance over distances of more 

 than a metre or so. Indeed, absolute parallelism would lead 

 us into difficulties with regard to the interference o£ light 

 from distant bodies like the stars, since, unless the streaks 

 from each electron were practically infinite in number, no 

 electron in the star would succeed in sending more than one 

 streak to our planet at most, so that we could not get inter- 

 ference. The slightest spreading, however, releases us from 

 our difficulty. In fact, all that is needed is sufficiently little 

 spreading to account for X-ray phenomena over the dis- 

 tances to which they have been tested, and sufficiently great 

 spreading to insure that when the source of light is so far 

 from the observer that the number of streaks coining from 

 each electron is too small to account for interference, the 

 spreading of the streaks makes up for the difficulty. 



It may be suggested that the electromagnetic solution of 

 the problem of an accelerated electron shows that the pulses 

 are spherical, but 1 would submit that this is not necessarily 

 the case. If the proof of the problem be examined, it will 

 be found that it is only in virtue of the simplicity of the 

 assumptions with regard to the electron itself that the spherical 

 pulse is obtained. Quite apart from this, however, it is quite 

 conceivable that the atom in which the electron is accelerated 

 exerts some kind of focussing action on the radiation. In 

 fact, when we think of the atom with its spherical boundary, 

 its electrons inside it, and possibly with its positive elec- 

 tricity gathered together into little nuclei which as a whole 

 constitute the hypothetical positive sphere, the picture of 

 our electron inside it is, in the case of such short wave- 

 lengths as those of X rays, electromagneticalry not very 

 unlike that of a source of light inside a glass globe, which 

 to the electromagnetic eye is nothing more than conglome- 

 ration of electrons. 



Summary. — (1) The action of an electromagnetic train on 

 the free electrons in a metal is considered. The order of 

 magnitude of the want of symmetry in the expulsion of the 

 /3 rays is calculated. The question of the energy of the 

 pulses is examined, and a method of avoiding the assumption 

 of filamentary pulses is discussed. The main facts are summed 

 up on p. 543. 



(2) The case where the electrons are ejected from the 

 atoms is considered. A formula is deduced which corre- 

 sponds to a case where the electrons are ejected from the 

 atom entirely in the direction in which the waves are tra- 

 velling. The formula agrees with observations for X rays, 

 the fact that some /3 rays come out against the wave-train 



