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The following notation will be made nse of: 

 A, B, C components of magnetic force. 

 X Y Z ,, ,, electric „ 



21, 33, € ,, ., magnetic polarization. 



£, $>, 3 „ „ electric „ 



r /o x hi h y> )> electric partial polarizations. 



(if>, b/ lf Ch, d/i, H parameters. 

 P external magnetic force, supposed // Z. 

 v velocity of light in vacuum. 



Then the equations of the theory based on the suppositions I have 

 made for an isotropic active body become : 



Zr =v (M_ d J[) (2) 



Ph + W'h + bhfh + ViP\jh = e h X — dhA' , (3) 



5 = Z + -r», (4) 



* = A+ Sdrfh/eh (5) 



By a simple calculation we obtain for the so-called complex 

 refractive index n in the direction of the magnetic field the value 



n = 1/(1 + E) (1 -f @) ± A, (6) 



where 



ph±ChPv 8h(ph±ChPv) 



ph±ChPv 

 v = frequency, 



Ph = 1 -f- ivajt — r s 6/ ( . 



The double sign ± corresponds to the two waves of circular 

 vibrations and of opposite sense of rotation propagated parallel to 

 the field. 



The complex index of refraction n is connected with the real 

 index n and with the absorption index y. by the formula n = n (1 — in). 

 The constants (f/, and Ch measure the natural and the magnetic 

 activity of the crystal. It is seen that even if these two effects are 

 small in general, in the neighbourhood of an absorptionband, where 

 ph is of the same order as c/ t Pv, they do not superpose at all. 



If the natural activity vanishes, then also & and L vanish, and 

 we have 



24* 



