M, A. Bertin on Circular Magnetic Polarization. 495 



Thus the action of a pole upon any section whatever of a 

 substance depends solely on the distance of this section from 

 the pole, and according to a known law. If, then, in a thick- 

 ness of e millimetres, we consider e sections of 1 millimetre, 

 and represent by c the rotation which each of these sections 

 would produce if it were in contact with the pole, the rotation 

 produced on contact by the thickness e will be the sum of the 

 terms of a geometrical progression, the first term of which is 

 c, the cause r, and the number of the terms e; that is to say, 

 we shall have 



1— r« 

 A^c- , 



whence 



J/ 



-<\^} 



This formula represents the general action of a single pole. 

 It may be proved by comparing the rotations observed at the 

 same distance a? by two thicknesses e and e' of the same flint- 

 glass; for if y and ?/' represent the two rotations observed, it 

 is evident that we ought to have 



y _ l-r^ 



and we are able to compare this rotation with that given by 

 experiment. This comparison confirms the accuracy of the 

 formula, as will be seen by the following table :— 



