524 Mr. E. Edser on the Reflexion of Electromagnetic 



to {( — c/sin 6') + v cot 7 }, so that the corresponding super- 

 ficial current, in the direction from South to North, is equal 

 to 



— y-^2 {c — v cos $') per cm. 



Thus, the total current C flowing from South to North is 

 given by the equation 



C=t — 2 {B 1 {c + vcosO)— E 2 (c — vcosO')} per cm. . (42) 



At a given instant this current varies with y, and is not 

 circuital ; the condition of continuity is of course satisfied 

 by the displacement current in the aether. 



The value of the superficial screening current can also be 

 determined as follows. It is easily proved that the tangential 

 force that would act on a unit N pole placed near the mirror 

 and moving with it is equal to 



i (E x - E 2 ) + \ (E! cos $ + E 2 cos 0') 

 c c 



= - 2 {E l (c + vcos0)-E 2 (c-vcos0')}, 



' c 



and therefore the current necessary to screen the interior of 

 the conductor from magnetic force has the value given by (42) . 

 The part of the tangential magnetic force which is not 

 derived from the current, and is independent of the motion 

 of the mirror (cf. § 6) is equal to (l/2c)(Ei— E 2 ). Thus, the in- 

 stantaneous pressure pi exerted on unit area of the screening 

 current, and therefore on unit area of the mirror, is equal to 



^(Ej— E s ) x ^ {Ej (c + v cos 0) -E 2 (c-v cos 0)}. (43) 



From (38) and (28) 



Ei (c + vcos0) — E 2 (c—vcos0')=2E 1 (c + vcos0). 

 Similarly 



Thus, at a point on the mirror where the force exerted on a 

 unit stationary charge would be E x due to the incident 

 waves and E 2 due to the reflected waves^-ihe pressure p x 

 exerted on unit area of the screening current is given by the 

 equation 



2E, 2 (c + vcos0) 2 



Pi = 



47TC 2 C 2 — V 2 



