272 Mr. F. T. Trouton on the Acceleration 



increased velocity. As mentioned before, the acceleration of 

 the rate of propagation of the electric component of the dis- 

 turbance must differ by t/4 from that of the magnetic com- 

 ponent in order to provide the necessary readjustment of their 

 relative phases. 



The question now arises if, on the whole, the elementary 

 disturbance to be supposed in Huy gens' construction* may 

 not fairly be considered as being of the same character as that 

 produced by a Hertzian "vibrator/'' Such an elementary 

 disturbance as is there assumed, in the absence of neighbour- 

 ing ones, would have apparently uncompensated ends or poles 

 equivalent to the opposite electrifications on a " vibrator/' 

 Of course the only way we know of producing an elementary 

 disturbance is by the presence of matter, so that it seems 

 natural to take the disturbance assumed in Huygens' con- 

 struction to be similar in its effects. It might perhaps be 

 objected to this that a "vibrator" is a conductor, and that 

 there are conduction-currents to be dealt with ; but as against 

 this the fact is to be considered that, as we shall see, a non- 

 conductor such as glass can act by reflexion so as to be the 

 source of such an elementary disturbance. 



Some experiments were described in i Nature ' (August 22, 

 1889) in which this acceleration in phase was actually observed 

 to take place in the reflexion from a small surface. It was 

 there described how, in Hertz's experiment of "loop and 

 nodes/' if a small sized reflector be employed, the magnetic 

 node, instead of being at J X from the reflector, is found to 

 be nearly J- X further out. This evidently corresponds to a 

 change of phase of J period in the reflected wave, for the 

 distance J X has to be twice traversed. 



The way in which this change in phase is brought about is 

 perhaps most conveniently considered by supposing the small 

 reflector to be itself resonant, or equivalent to a second 

 vibrator having the requisite phase with reference to the inci- 

 dent wave. And indeed that something such as this occurs 

 may even be noticed by touching the mirror with a small 

 piece of metal ; for bright sparks can be drawn from the two 

 edges running at right angles to the electric force, while little 

 or no effect is obtained from the central parts. 



Let the magnetic component of the wave incident vertically 

 on the reflector, and supposed a parallel beam, be taken as 



P = Acos27r(-j-+£); 



* In the case of sound, the divergency of the direction of motion of 

 the particles from an element of surface affords the analogous reason for 

 the acceleration. 



