Page 246

-246    Wisconsin Academy of Sciences, Arts, and letters.
   Under the same circumstances, where "heavy glass" would
produce a rotation of 6f, Bisulphide of Carbon would produce a
rotation of 30; flint glass, 20 8'; rock salt, 2f 2'; water, 1'.
   The behavior of a large number of substances under the simul-
taneous influence of magnetism and circularly polarized light of
different colors was examined by Verdet in 1863. He found the
results of his experiment to agree very well with the formula:
               = ncr (i-2i).                 (1.)
where 0 is the angular rotation of the plane of polarization; m a
constant (the coefficient of magnetic rotation of the medium); y
the intensity of the magnetic force resolved in the direction of the
ray; c the length of the ray within the medium;  the wave length
in air, of the particular kind of light employed; i its index of
refraction in the medium.
  For Creosote there was considerable deviation fromthe formula.
On account of the mixed nature of Creosote, being an aggregate of
Carbolic Acid and several other substances, this might have been
expected, even if the above were the true formula representing
the relation between the rotation, magnetic force, wave length,
and refractive index.
  Vercdet has summed up his results as follows:
  Ist. "The magnetic rotations of the planes of polarization for
light of different colors are approximately as the inverse square
of the wave length of the light employed.
  2nd. " The exact law is that the product of the rotation of the
square of the wave length, increases from the least refrangible to
the most refrangible end of the spectrum."
  3rd. " The substances for which this increase is most sensible
are also those which have the greatest dispersive- power."
  The formula (1) may be derived from the following more gen-
eral formula
            dq   4-r 4C4/2         di\       1
          dr     v p _            dA    - 2ICt   i    ...(2.)
which Prof. Clerk Maxwell has shown to be a consequence of Sir
Wim. Thompson's assumption that the only dynamical explana-