6S6 



Sir J. J. Thomson on 



o£ these secondary waves is to make the apparent velocity 

 of the light through the medium depend upon the number of 

 electrons in that medium, though all the constituents which 

 make up the resultant wave travel with the velocity of light 

 through a vacuum. 



A detailed analytical investigation of this effect will be 

 given in another paper, but the general principles on which 

 the results depend may be illustrated by considering the 

 special case of a pulse of electric force travelling through a 

 slab of refracting matter bounded by planes at right angles 

 to the direction of propagation of the pulse. Let^us suppose 

 that the electric force in the pulse is parallel to the axis of x 

 and that the pulse is travelling parallel to z and bounded by 

 two parallel planes at right angles to *. Lei the thickness 

 of the pulse be 2d, let the electric force in the pulse before 

 it strikes the slab be constant in the front half d and equal 

 to X, while in the rear half it is also constant but equal 

 to —X. Let us consider the effect produced by this pulse 

 when it strikes the slab oE electrons. When the force X 

 strikes the electrons it will accelerate them, and in conse- 

 quence they will emit secondary waves in which the electric 

 force is in the opposite direction to X. 



Fig. 1. 



Let AB be the slab containing the electrons and suppose 

 the front of the pulse has reached P ; the only secondary 

 radiation which has had time to reach P is that coming from 

 the electrons at B, the part of the slab nearest to P. When 

 a little later a part of the pulse a little in the rear of the 

 front reaches P, the secondary radiation carrying a negative 

 X will have had time to come up from outlying places like 

 and D, and will diminish the electric field in this part of 

 the pulse. Now consider what will happen when the 



