of the Plane of Polarization of Light. 2>21 



VIII. 



In the preceding I have given a general view of the electro- 

 magnetic rotation of iron, cobalt, and nickel upon transmitted 

 and upon reflected light. A more exact investigation of the 

 dispersion due to rotation, of the dependence of rotation upon 

 the thickness of the layers in reflection, &c, must be reserved 

 for a subsequent occasion. Other questions suggest them- 

 selves : as, for example, whether the rotation attains its 

 maximum with the induced magnetic moment, whether it is 

 possible to recognize a rotation produced by a thin metallic 

 film,, which is permanently magnetized instead of being tem- 

 porarily magnetized, &c. 



I will here, in conclusion, only collect together the facts 

 now known of the electromagnetic rotation of the plane of 

 polarization. We will, as usual, designate a rotation in the 

 direction of the hypothetical amperian molecular currents, 

 which represent at the place in question the magnetic field 

 actually present as positive, and an opposite rotation as negative. 



1. Most isotropic solid bodies, fluids, and those gases which 

 have been examined rotate the plane of polarization in the 

 positive direction. 



2. A concentrated solution of ferric chloride produces a 

 negative rotation. The negative rotation of other magnetic 

 salts is recognized by the diminution in the positive rotation 

 of the solvent. 



3. Oxygen, which is comparatively powerfully magnetic, 

 produces positive rotation, as shown by Herr Rontgen and 

 myself. 



4. The plane of polarization of light which traverses iron, 

 cobalt, and nickel is rotated in the positive direction. 



5. According to Kerr, negative rotation is produced upon 

 normal reflection from a magnetic pole. The same has been 

 shown above to hold good for cobalt and nickel. 



6. Upon passing through, as well as upon reflection from, 

 iron, the rotational dispersion of the light is anomalous — that 

 is, red rays are rotated more powerfully than blue ones. 



7. The complicated phenomena which take place upon 

 oblique reflection from the pole-surfaces, or side-faces of a 

 magnet, may, as shown above, be brought together by the 

 assumption that, in reflection, the light traverses a very thin 

 film of the metal, and that negative rotation is produced by 

 this layer. 



