552 Dr. W. F. Gr. Swarm on tlie Pulse 



that the narrower the pulse the greater the fraction of ihe 

 energy which becomes absorbed, and we have seen on p. 550 

 that in order that half the energy of the pulse which has 

 passed over the electron shall be absorbed, we must have 



a=- — . Provided that the pulse has a sufficiently small 



m l J 



diameter, the electron will take practically every bit of the 

 energy which the pulse sends to it. It will allow none to 

 pass until it has got enough to enable it to fly away, leaving 

 the beheaded pulse to pass on and produce similar effects in 

 other electrons, until eventually there is nothing of it left. 

 We can see why it is that on this view a definite kind of 

 X ray or photoelectric train may produce a j3 ray of definite 

 velocity without that X ray being looked upon as carrying 

 a defiuite total amount of energy. The question as to whether 

 the /3 ray so ejected will, when it enters another atom, give 

 rise to a pulse of the same frequency as that which liberated 

 it, is one which it would take too long to discuss here. 



It is sometimes thought that the existence of filamentary 

 pulses is inconsistent with the electromagnetic theory. It 

 must be pointed out, however, that no filamentary structure 

 of the sether is necessarily implied, all that is required is 

 that the energy shall travel out in streaks from the vibrating 

 electron; and if it be objected that no electromagnetic solution 

 corresponding to such a state of affairs can be found, we may 

 reply that it' we place a source of light at the focus of a lens, 

 we know that the energy comes out in a parallel beam, and 

 yet we know that if we take cognizance of all the electrons, 

 &c. in the glass which forms the lens, the true electromagnetic 

 solution would bring out the parallel nature of the beam. 

 Truly there is slight spreading at the edges due to diffraction, 

 but this effect only becomes appreciable when the beam is 

 comparable in width with the wave-length of the light. In 

 the case of X rays the wave-length is extremely small, and 

 is comparable with the widths of pulses, which are so thin 

 that they would give up practically the whole of: their energy 

 to the absorbing electrons as they passed them. The diffi- 

 culties with regard to interference phenomena may to some 

 extent be met by supposing that the energy from a single 

 electron is radiated not along a single streak, but along 

 several : in this way it is possible to get light from a source 

 jnvino- interference with its image if there are enouoh 

 streaks to insure that in the case of each electron streaks 

 come to the observer direct, and also after reflexion from the 

 mirror. It may be pointed out that we do not require absolute 

 parallelism in the pulses, for nobody has tested the variation, 



