496 M. A. Bertin on Circular Magnetic Polarization, 



Action of the two poles of the apparatus. — The formula 

 ^ = Ar*, which represents the action of a single coil, gives us 

 also that of two electro-magnetic coils facing each other with 

 the poles of opposite names, as in the case of our apparatus. 

 If, in fact, these two coils are at a distance d, the flint-glass of 

 e thickness, placed at a distance x from the first, will be di- 

 stant d—e—x from the second; and as the two actions add to 

 each other, the total rotation will be 



■ = c(^~){r--^7^-'-'). 



Even the form of this expression shows that if we only vary 

 the distance x in taking three consecutive rotations z, z\ »", 

 observed in the same Jlint-glass placed successively at the distances 

 .r, ^ + a, ^ + 2a, the sum of the two extreme rotations will be to 

 the intermediaiy rotation iti a fixed relation equal to ?•« + ;■-«; 

 that is to say, that 



2 + 3'' 



:/•« + ;—«. 



This conclusion is confirmed by experiment, as is seen in the 

 following tables : — 



