Page 254

254      Wisconsin Academy of Sciences, Arts, and Letters.
by magnetism, " not as giving a 2mechanical explanation of the phe-
nomena, but as showing that the phenomena may be explained
by equations, which equations appear to be such as ruight possi-
blv be deduced frcm some plausible mechanical assumption,
although no such assumption has been made."
   This explanation of what rotatory polarization is, as it occurs in
bodies which of themselves rotate the plane of polarization, may
       Fig. 16.     help to an understanding of      Fig. 17.
                     the manner in which an        /~      "
                     electric current, circulating
                     around a medium through
                     which circularly polarized
                     light is passing, may possi-
                     bly affect the velocity of
either circular component of the polarized light, and thus, accord-
ing as the direction of the current is with a circular componentt
as in Fig. 16, or aqainst it, as in Fig. 17, produce a right-handed
or a left-handed rotation, according to the direction in which the
current circulates around the medium.
   Of this latter, Sir Wm. Thompson, in 1856, made the important
observation,' which Prof. Clerk Maxwell has elaborated into the
  "The magnetic influence -on- light, discovered by Faraday, depends
on the
direction of -motion -of moving particles. For instance, in a medium possess-
ing it, particles in a straight line parallel to the lines of magnetic force,
dis-
placed to a helix round this line as axis, and then projected tangentially
with
such velocities as to describe circles, will have different velocities, according
as their motions are round in one direction (the same as the nominal direc-
tion of the galvanic current in the magnetizing coil) or in the contrary
direc-
tion. But the elastic reaction of the medium must be the same for the same
displacements, whatever be the velocities and directions of the particles;
that
is to say, the forces which are balanced by centrifugal force of the circular
motions are equal, while the luminiferous motions are unequal. The absolute
circular motions being therefore either equal or such as to transmit equal
cen
trifugal forces to the particles initially considered, it follows that the
lumi-
niferous motions are only components of the whole motion; and that a less
luminiferous component in one direction; compounded with a motion ex-
isting in the medium when transmitting no light, gives an equal resultant
to
that of a greater luminiferous motion in the contrary direction compounded
with the same non-luminous motion. I think it is not only impossible to
conceive any other than this dynamical explanation of the fact that circularly-