1 3 o 



PARTICULAR CASES OF EQUILIBRIUM. 



mirror, the image of A is a virtual one, and is formed at A' ; the 

 illumination of the space on the right of the plane is the same as 

 if this plane were replaced by a source of light placed at A', and the 

 intensity of this virtual source would be equal to A, if the reflecting 

 power of the plane were equal to unity. 



149. INDUCTION IN A MEDIUM CONSISTING OF Two DIELEC- 

 TRICS SEPARATED BY A PLANE. The principle of images enables us 

 to determine the condition of two unlimited dielectrics, separated by 

 a plane surface, in one of which is the acting mass. 



Let m be this mass placed at the point A (Fig. 37), /^ and ft 2 

 the inductive powers of two dielectrics separated by the plane Q, the 

 acting mass being situate in the former. 



Equilibrium may be established by imagining that on the plane Q, 

 a layer m' is distributed as it would be on an uninsulated conducting 



Fig- 37 



plane under the influence of a mass m', placed at A, or at the sym- 

 metrical point B ; in other words, the plane would act on all points 

 on its left like a mass m' placed at A, and on all points on the right like 

 the same mass m' placed at B. The potential near the point P, 

 taken in the plane Q, is, in the first medium, at P p 



_^_ m_ 

 1 PA*?! 1 



and in the second medium, at 



