DEFINITION OF DIELECTRIC. 



97 



Poisson's hypothesis amounts, in short, to supposing that, on the 

 surface of the dielectric, there is a fictive layer the density of which 

 cr satisfies this condition. 



112. This result may be exhibited under another form. On 

 both sides of an element PP' or dS of the surface of the dielectric 

 (Fig. 24) let us draw two tubes of force, and let them terminate 

 in two orthogonal bases dS l and d$\, one in air and the other in 

 the dielectric, and just far enough apart to comprise between them 

 the layer a-dS. The flow of force which enters by the base ^S x is 

 ~F n d -, that which emerges by the base dS\ is 



Fig. 24. 



the rate of variation of the flow is then equal to (F M - F' n ) dS, or 

 from equation (i) which defines the dielectric, to ( i -- \ ; it corre- 

 sponds to a mass of electricity o-d such that 



The effect is therefore the same as if a constant fraction of the flow 

 of force were absorbed or emitted by the fictive layer on the surface ; 



the value of this fraction is i - - . 



I* 



113. REFRACTION OF THE FLOW OF FORCES. The tangential 

 components being the same in the two media, if / and /' are the 

 angles which the forces F and F' make with the perpendicular N to 

 the surface S, the expressions 



F cos/=/>tF' cos/', 

 F sin i F' sin *', 



H 



