582 SCIENCE PROGRESS 



trons, all having initially the same velocities, the rule just 

 stated is followed only in the case of the upper limiting fre- 

 quency of the X-rays emitted ; and that much of the X-radia- 

 tion has frequencies below the frequency corresponding to that 

 of the electron energy. But the electrons lose*speed in pene- 

 trating the plate and soon possess velocities varying over a 

 wide range from the initial velocity downwards. Consequently 

 it is to be expected that there should be a corresponding 

 diversity in the emitted frequencies, even if an electron, moving 

 with a given speed, can produce X-rays of one frequency only. 



So also when a pencil of X-rays of given frequency strikes 

 a substance, a stream of electrons which is made to issue from 

 the substance has velocities ranging downwards from that 

 which corresponds to the given frequency. But that also is 

 to be expected on the simpler hypothesis, since even if the 

 electrons all have the same speed initially, they have to make 

 their way through more or less of the substance before they 

 get out and will by then have acquired a great diversity of 

 velocity. 



These two statements may be rounded off by adding that 

 when a stream of electrons having the same velocity is allowed 

 to fall on a plate, the " reflected " or " scattered " electrons 

 are found to be diverse in speed, none, however, moving as 

 fast as the originals. So also it may be confidently anticipated 

 that when a stream of X-rays of one frequency is made to fall 

 on a substance, there will be an emission of X-rays of all fre- 

 quencies, because electrons will be set in movement and these 

 will cause the varied X-ray emission, as has been seen. In all 

 these cases, of course, characteristic X-rays may or may not 

 be excited, depending on whether the frequency of the inci- 

 dent X-rays or the energy of the incident electrons is above 

 the critical value for the substance which is struck. If they 

 are, they will show up as bright lines in the spectrum of the 

 emitted rays, whenever a spectral analysis is made. 



We may sum up the whole situation in the statement 

 that whenever there are X-rays there must be electrons in 

 movement, and vice versa, since the one radiation produces 

 the other ; and there must be a certain equilibrium between 

 the quantities of each, which will be attained if the radiation 

 once started is left to itself. 



It is certain that a given electron energy or its equivalent 



