Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/473

 value as found from the first term, namely,. The equation thus becomes

so must be of the form

where evidently represents the dielectric constant of the medium, and  is the coefficient which measures the magnetic rotatory power. In the magneto-optic term we may replace by, the external magnetic force, since this is large compared with the magnetic force of the luminous vibrations. Thus if denote the electric induction, we have

Combining this with the usual electromagnetic equations,

we have

When a plane wave of light is propagated through the medium in the direction of the lines of magnetic force, and the axis of is taken parallel to this direction, the equation gives

and these equations, as we have seen, are competent to explain the rotation of the plane of polarization.