M. A. Berlin ofi Circular Magnetic Polarizatio7i. 493 



will be equal upon all, and the rotation observed will be pro- 

 portional to the thickness of the substance. This, in fact, is 

 the law discovered by Prof. Faraday, on employing horse- 

 shoe electro-magnets not furnished with keepers. 



The law of the variations with the thickness is evidently 

 connected with that of the variations with the distance ; but 

 this is not better known than the first. It is therefore to 

 the simultaneous examination of these two laws that my at- 

 tention was necessarily directed, and I made it as soon as I 

 had RuhmkorfPs great apparatus at my disposal. 



Law of the thickness and of the distafice. 



The action of the two reels or coils of the apparatus being 

 no more than the sum of the rotations produced by each of 

 them, I had at first, in order to simplify the problem, to study 

 the action of a single coil upon a substance of known thick- 

 ness, placed upon the axis at a fixed distance. 



Action of a single pole. — One of the coils being removed, I 

 placed the flint-glass upon which I wished to experiment in con- 

 tact with the remaining coil, then I removed it a certain distance, 

 which I valued by the passage of its support over a divided 

 scale. Now, on increasiyig the distance of the fiint-gla%s from 

 the coil in arithmetical -progression^ the rotations of the plane of 

 polarizatio7i decrease in geometrical progression. In proof of 

 this I will enumerate only three series of experiments in which 

 I varied the distances ; at first 1 millimetre, then 5 millimetres, 

 and lastly 10 millimetres. The relations of the successive 

 rotations are — 



In the first case . . 0'97587=r^ 

 In the second case . 0*88504 = r^, 

 In the third case . . 0-78233 = r*'^. 



1. ExperimeJits isoith the flint-glass of Mr. Faraday, thickness 

 38 "9 millims. 



