JXeio Magneto-Optic Phenomena. 487 



double refraction is positive in a weak field, decreases gra- 

 dually, and ultimately becomes negative under the action of 

 a strong field. It therefore has a point of inversion, 

 corresponding to a field of given intensity, for which the 

 double refraction becomes equal to zero. If the point of 

 inversion is very low, i. e. occurs at a weak field intensity, 

 then the following laws hold with a considerable degree of 

 approximation. The magnetic double refraction is : — 



(1) Directly proportional to the concentration of the 

 liquid. 



(2) Directly proportional to the thickness of liquid 

 traversed in a direction normal to the lines of force. 



(3) Directly proportional to the square of the field 

 intensity. 



(4) Inversely proportional to the square of the wave- 

 length of the light experimented with. 



3. Magnetic JDichroism. — Dialysed iron strongly absorbs 

 light. It is therefore easy to foresee that by reason of the 

 phenomenon of double refraction described above, the ab- 

 sorption of a polarized wave will undergo different modi- 

 fications according to the azimuth of the plane of polarization, 

 when the magnetic field is excited. It is thus that I was able 

 to establish the existence of a series of phenomena to which I 

 propose, with good reason, to apply the term Magnetic Vichro ism. 

 These phenomena are entirely analogous to those presented by 

 dichroic crystals (e. g. tourmaline), with the sole difference 

 that they are only observed when the circuit of the exciting 

 current is closed. The more important results are: — 



In the case of the magnetic dichroism exhibited by dialysed 

 iron, it always happens that when one wave is propagated 

 through the interior of the liquid with a velocity less than 

 that of another wave (on account of different azimuths of the 

 plane of polarization, or different directions of propagation) , 

 it also suffers less absorption. Photometric measurements 

 have shown that the absorption suffered by vibrations propa- 

 gated in a direction parallel to the lines of force is the same 

 as that suffered by vibrations propagated normally, the plane 

 of polarization being parallel to the field. 



4. Bimagnetic dotations. — This phenomenon is exhibited 

 by solutions of ferric chloride prepared from certain hvdrated 

 oxides of iron. 



With the arrangement described in connexion with double 

 refraction, the solution of ferric chloride replacing the dialysed 

 iron, it is observed that when the field is excited between the 

 two crossed nicols at 45° to the field, the light reappears ; and 

 that it again disappears when the analyser is rotated through a 



