1864.] 



of the Electromagnetic Field, 



535 



we have called tlie Electric Elasticity of the medium in which the experi- 

 ments are made (?. e. common air). Other media, as glass, shellac, and 

 sulphur have different powers as dielectrics ; and some of them exhibit the 

 phenomena of electric absorption and residual discharge. 



It is then shown how a compound condenser of different materials may- 

 be constructed which shall exhibit these phenomena, and it is proved that 

 the result will be the same though the different substances were so intimately 

 intermingled that the want of uniformity could not be detected. 



The general equations are then applied to the foundation of the Electro- 

 magnetic Theory of Light. 



Faraday, in his Thoughts on Ray Vibrations " *, has described the effect 

 of the sudden movement of a magnetic or electric body, and the propaga- 

 tion of the disturbance through the field, and has stated his opinion that 

 such a disturbance must be entirely transverse to the direction of propaga- 

 tion. In 1846 there were no data to calculate the mathematical laws of 

 such propagation, or to determine the velocity. 



The equations of this paper, however, show that transverse disturbances, 

 and transverse disturbances only, will be propagated through the field, and 

 that the number which expresses the velocity of propagation must be the 

 same as that which expresses the number of electrostatic units of electricity 

 in one electromagnetic unit, the standards of space and time being the same. 



The first of these results agrees, as is well known, with the undulatory 



theory of light as deduced from optical experiments. The second may be 



judged of by a comparison of the electromagnetical experiments of Weber 



and Kohlrausch with the velocity of light as determined by astronomers 



in the heavenly spaces, and by M. Foucault in the air of his laboratory. 



Electrostatic units in an electromao;- 1 n^c\ f\f\f\ *. a 

 ^. ° \ 310,/40,000 metres per second, 

 netic unit J * ' ^ 



Velocity of light as found by M. Fizeau 314,858,000. 



Velocity of light by M. Foucault 298,000,000. 



Velocity of light deduced from aberra- \ 

 tion 



At the outset of the paper, the dynamical theory of the electromagnetic 

 field borrowed from the undulatory theory of light the use of its lumini- 

 ferous medium. It now restores the medium, after having tested its powers 

 of transmitting undulations, and the character of those undulations, and 

 certifies that the vibrations are transverse, and that the velocity is that of 

 light. With regard to normal vibrations, the electromagnetic theory does 

 not allow of their transmission. 



What, then, is light according to the electromagnetic theory ? It consists 

 of alternate and opposite rapidly recurring transverse magnetic disturbances, 

 accompanied with electric displacements, the direction of the electric dis- 

 placement being at right angles to the magnetic disturbance, and both at 

 right angles to the direction of the ray. 



* Phil. Mag. 1S4G. Experimental Researches; vol, iii. p. 447. 

 VOL. XIII. 2 s 



I 308,000,000. 



