310 Prof. A. Kundt on the Electromagnetic Rotation 



and also those of Kerr. I shall further on briefly review 

 the quantitative results obtained. 



I have, however, succeeded in rendering Kerr's experiments 

 decidedly more complete by another method ; since, having 

 prepared thin transparent films of iron, cobalt, and nickel, I 

 have examined the rotation of the plane of polarization pro- 

 duced npon passage of light through these films. The result 

 is as follows: — 



Transparent films of iron, cobalt, and nickel in the magnetic 

 field rotate the plane of polarization of transmitted light very 

 powerfully. The rotation produced by iron for the mean rays 

 of the spectrum is more than 30,000 times as great as that pro- 

 duced by glass of equal thickness. The rotation takes place 

 with all three substances in the direction of the magnetizing 

 current. 



After a few brief remarks upon the mirrors and transparent 

 films employed, I give in what follows, first of all, observa- 

 tions npon the electromagnetic rotation upon transmission 

 through metals ; then my experiments upon the rotation which 

 is produced by reflection of light from metals ; then are in- 

 cluded, and added at suitable points, general discussions of the 

 experiments and the conclusions drawn from them. 



I. The Mirrors and Transparent Metallic Films employed. 



In the experiments upon reflection, I at first employed 

 circular steel mirrors of 35 millim. diameter and 6 millim. 

 thickness, of a very high polish. The mirrors were either 

 placed between the poles and suitably fastened there, or 

 attached laterally to the clamps uniting the poles. Any pos- 

 sible distortions were examined by observing in a telescope 

 the images of horizontal and vertical millimetre-scales reflected 

 in the mirrors. Any distortions or changes in the position of 

 the mirrors would be recognized by small irregularities and 

 displacements in the images given by the mirrors. In the 

 series of experiments with mirrors, such as described later on, 

 no irregularities were detected by means of the scale and tele- 

 scope by which the optical effects observed could have been 

 explained. Then I have employed mirrors consisting of 

 speculum-metal, which had been electroplated with iron or 

 nickel. And, finally, I employed plate-glass mirrors, coated 

 electrolytically with very thin transparent films of iron. 



If a glass surface is to be electroplated with a metal, the 

 glass surface must first of all be made conducting. I tried to 

 silver glass in the usual way and then to deposit the iron upon 

 this layer of silver ; but so far these attempts have not led to 

 any result. So soon as the current was closed, and iron or 



