Q82 Mr. G. A. Schott on Radiation from Moving Systems 

 In the cases under consideration we must consider 



(a) A force Ucosa, o£ zero frequency and class, 



giving rise to no forced wave. 



COS (h 



(b) Forces of the types A — — >Usina, where d> = cot + 8 



2iri sm< /> 



+ • These are of frequency a and class — 1, and 



therefore give rise to no forced waves. 



cos 

 This is clearly true of any disturbance of type . k(j>. 



It follows that spectrum lines cannot generally be excited 

 by the variable motion of a system, whether changing in 

 velocity or orientation, or both. 



§ 23. An exception occurs when the system is subject to 

 a force of nearly the same frequency as one of its free 

 vibrations, bat only when, owing to the motion, the amplitude 

 of the force varies at a rate comparable with the difference of 

 the two frequencies. 



For instance, two neighbouring systems, e. g. two ions, act 

 on each other by resonance ; the difference between the 

 frequency of a free vibration of the first ion and that of the 

 force of corresponding type due to the second is small, being 

 merely the Doppler effect due to the relative motion of the 

 ions. It is easy to see that in this case a/a is of order r/r, 

 w T here r is the distance between the ions, while the difference 



of frequency, 8j?, is of the order p _ ; thus 'ajahp is of order 



\j2irr. This is particularly large during a collision. Thus 

 we conclude that the free vibrations of an ion, or of any 

 other moving system of electrons, are excited especially in 

 consequence of the variations of its motion due to collisions 

 with other systems. Since the corresponding waves are 

 sufficiently intense and of suitable frequency we can in this 

 way get observable spectrum lines. 



§ 24. The chief conclusions from our theoretical dis- 

 cussion are as follows : — 



(1) The radiations due to disturbances existing in a 

 system of electrons are augmented in intensity by the 

 motion of the system (§§ 16, 17). 



(2) The work done by radiation pressure on the system 

 represents only a very small fraction of the energy 

 radiated (§§ 16, 17). 



