A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 587 



Let R be the resistance in electromagnetic measure of a plate of the 

 substance whose thickness is x, breadth b, and length /, then 



R- l 

 ~bx' 



(107) Most transparent solid bodies are good insulators, whereas all good 

 conductors are very opaque. 



Electrolytes allow a current to pass easily and yet are often very trans- 

 parent. We may suppose, however, that in the rapidly alternating vibrations 

 of light, the electromotive forces act for so short a time that they are unable to 

 effect a complete separation between the particles in combination, so that when 

 the force is reversed the particles oscillate into their former position without 

 loss of energy. 



Gold, silver, and platinum are good conductors, and yet when reduced to 

 sufficiently thin plates they allow light to pass through them. If the resistance 

 of gold is the same for electromotive forces of short period as for those with 

 which we make experiments, the amount of light which passes through a piece 

 of gold-leaf, of which the resistance was determined by Mr C. Hockin, would 

 be only 10~ w of the incident light, a totally imperceptible quantity. I find that 

 between -g J 7 and 10 1 00 of green light gets through such gold-leaf. Much of this 

 is transmitted through holes and cracks ; there is enough, however, transmitted 

 through the gold itself to give a strong green hue to the transmitted light. 

 This result cannot be reconciled with the electromagnetic theory of light, unless 

 we suppose that there is less loss of energy when the electromotive forces are 

 reversed with the rapidity of the vibrations of light than when they act for 

 sensible times, as in our experiments. 



Absolute Valves of the Electromotive and Magnetic Forces called into play in the 



Propagation of Light. 



(108) If the equation of propagation of light is 



9_ 



F=Aco9~(z-Vt), 



A 



742 



