576 OPTICAL PHENOMENA. 



Verdet demonstrated that when a salt is dissolved in water the 

 water and the salt each bring into the solution their special rotatory 

 power; the rotation produced by the solution is the algebraical 

 sum of the rotation due to each of the bodies composing it. Thus, 

 water having a positive rotatory power, and ferric chloride a negative 

 rotatory power, a solution of ferric chloride rotates the plane of 

 polarization in one direction or the other, according to its concen- 

 tration. The law may therefore be considered to be general. 



595. MAGNETIC ROTATORY DISPERSION. For the same body 

 the value of the constant <o varies with the wave length. The 

 direction of the variation is the same as for the natural rotation ; 

 in both cases, in fact, the rotation is approximately in the inverse 

 ratio of the square of the wave length. In fact, the products of 

 the rotation by the square of the wave length increases as the 

 wave length diminishes. Both in the case of natural and of 

 magnetic rotation, the substances for which the increase is most 

 marked are just those with the greatest dispersive power. 



Some months after the publication of Faraday's discoveries, Sir 

 George Airy remarked that the phenomena could be accounted 

 for, by adding to the known equations for the vibratory motion 

 of isotropic substances, certain terms proportional to the differentials 

 of the odd orders of the displacements in respect of the time. 

 Among the various formulae at which we arrive, by making special 

 hypotheses as to the nature of the terms to be added to the 

 equation, the following 



, dn\ 



= m ( n- A ) , 

 d\) 



in which m is a constant, and n the refractive index of the substance 

 for a ray of wave length A, gives an almost complete agreement 

 with experiment. The constant m, as we shall afterwards see, 

 will be inversely as the magnetic permeability. 



M. H. Becquerel observed that the quotient of the rotatory 

 power by the product n 2 (n 2 -i) varies very little with different 

 substances, and that this quotient is constant for bodies of the 

 same chemical family. 



596. MR. KERR'S EXPERIMENT. When a ray of polarized light 

 is reflected from the pole of a magnet, its plane of polarization, 

 from Kerr's experiments, experiences a manifest rotation; it is 

 advantageous to make the reflection perpendicular in order to avoid 

 the effects of elliptical polarization. On a positive or north pole, 



