Transparent Bodies by the action of Magnetism. 505 



another ; but he said nothing of the manner in which the passage 

 from one of these extremes to the other is effected. Subsequent 

 observers have confirmed these tworesults, but have added nothing 

 to them. M. Bertin made some experiments by placing a piece 

 of flint or heavy glass upon the base of a cylindrical electro- 

 magnet at different distances from the axis ; in this manner he 

 observed that the rotations of the plane of polarization varied 

 with the position of the transparent substance, but he discovered 

 no law, and it is, in fact, evident that with the complex circum- 

 stances under which he experimented, the elementary law of these 

 phenomena could not be discovered. M. Pouillet, M. Edmond 

 Becquerel, and M. Wiedemann in all their experiments have di- 

 rected the rays of light parallel to the action of the magnetic 

 force. 



In this second part of my investigation I propose to com- 

 plete our knowledge of the phenomenon discovered by Mr. Fa- 

 raday, by determining, in a general manner, what takes place 

 when tbe angle formed by the direction of the rays of light with 

 the direction of magnetic action varies from 0° to 90°. It will 

 not be necessary to insist upon the inherent interest of this re- 

 search ; it ' is evident that it must precede all discussion of the 

 theories which have been, or may be proposed to explain these 

 phsenomena. 



The apparatus usually employed is not well suited to experi- 

 ments of this kind ; in them the rays of light must necessarily 

 have the same direction as that of magnetic action. RuhmkorfPs 

 apparatus is an example of this; it was, in fact, constructed with 

 a view to give the greatest possible intensity to the phenomena 

 when the directions above mentioned were coincident. Never- 

 theless, at first I endeavoured to make use of RuhmkorfPs appa- 

 ratus by giving a variable direction to the luminous rays by means 

 of two successive reflexions from plane mirrors parallel to one 

 another. A few experiments were sufficient to show that this 

 modification of the apparatus was not adapted to give results 

 worthy of confidence. On this account I experimented in a 

 manner similar to Mr. Faraday, by placing the transparent sub- 

 stance so as to cause the rays of light to pass a little on one side, 

 or a little above the polar extremities of the electro-magnet : as 

 in my former experiments, however, the apparatus was arranged 

 so that the space occupied by the transparent body was a field of 

 uniform magnetic intensity. 



For this purpose I procured a strong horse-shoe electro-magnet 

 composed of two cylinders of soft iron, each O20 metre high 

 and 0075 metre in diameter, and each surrounded by 250 metres 

 of copper wire 2*5 millimetres in diameter ; the two were fixed to 

 the extremities of an iron bar Ali m, .'J5 long, 0"' - 07 broad, and 



