326 Pr 'of. A. Kundt on the Electromagnetic Rotation * 



The polarizing angle of the glass (i + r=90°) was found to 

 be56°-4. 



A similar series of experiments was made, with the two 

 poles of the electromagnet pushed close together and the glass 

 plate placed against the side surface of the closed magnet. 



The rotations were similar to those given above. There is 

 consequently a complete analogy between the phenomena 

 obtained with the glass plate and the previously given obser- 

 vations of reflections at the side-faces of a magnet. 



In that case, as in this, when the plane of incidence and the 

 plane of polarization coincide all rotations have the same sign. 

 If, however, the plane of polarization is at right angles to the 

 plane of incidence, then, with metals as with glass, the sign 

 of the rotation changes — for glass at the polarizing angle 

 (56°*4), for iron at about 80°, for nickel at about 60°. 



Nevertheless we have the difference that, with the magnetic 

 metals the direction of rotation is the opposite to that in the 

 case of glass. If it is not allowable to assume that it is proved 

 by this analogy that, upon reflection at metallic surfaces, the 

 light penetrates to a small depth into the metal, and that the 

 rotation takes place in the thin layer which the reflected rays 

 traverse, yet the phenomena of rotation observed upon reflec- 

 tion at the side of a magnet admit of being brought under a 

 simple and uniform expression. The fact that the light rotated 

 upon oblique incidence on metals does not preserve its con- 

 dition of plane polarization, but becomes elliptical, has been 

 shown above. 



Instead of bringing our glass plate up to the side-face of a 

 magnet, we may bring it in front of the pole-surfaces so that 

 the lines of force are at right angles to it. The amount of the 

 rotation of the plane of polarization when the magnet is ex- 

 cited may be calculated as a function of the angle of incidence 

 and position of the plane of polarization by means of Fresnel's 

 formula, as explained above. The result is that, when the 

 plane of polarization of the incident light lies in the plane of 

 incidence, the rotation always has the same sign, viz. positive ; 

 but when the plane of polarization is at right angles to the 

 plane of incidence, then from 0° up to the polarizing angle the 

 rotation is positive, and thence on to 90° it is negative. 



These phenomena are therefore exactly analogous to those 

 of rotation at the pole-surfaces of a magnet, with the difference 

 that we must ascribe a negative power of rotation to the 

 magnet. 



