AND ON THE ELECTROMAGNETIC THEORY OF LIGHT. 



141 



Let the nature of the dielectric be such that an electric displacement / 

 is produced by an electromotive force P, 



P = - (16), 



where k is a quantity depending on the particular dielectric, which may be 

 called its "electric elasticity." 



Finally, let the current p, already considered, be supposed entirely due 

 to the variation off, the electric displacement, then 



We have now four equations, (14), (15), (16), (17), between the four 

 quantities /J, p, P, and f. If we eliminate p, P, and f, we find 



CLL 47TJLC CtfZr 



Ifwe P ut G- p ( 19 )> 



the well-known solution of this equation is 



/3 = <f> l (z Vt) + (j) 3 (z+ Vt) (20), 



shewing that the disturbance is propagated with the velocity V. 

 The other quantities p, P, and f can be deduced from /8. 



Thus, if /8 = c cos -r- (z - 



A. 



C 27T, 



T , ... 



- 



I have in the next place to shew that the velocity F is the same 

 quantity as that found from the experiments on electricity. 



For this purpose let us consider a stratum of air of thickness b bounded 

 by two parallel plane conducting surfaces of indefinite extent, the difference 

 of whose potentials is E. 



