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Recent Progress in Theoretical Physic
tion possible for the magnetic rotation of the plane of polarized
light is that, in magnetization there must be molecular electrical
currents, and that the components of these currents can bedynam-
ically compounded with the angular velocity acquired by an ele-
ment of the medium, during the passage through it of a ray of
circularly-polarized light.
  On making in formula (2), m = 4 and neglecting 27r Or
                  0                           0 p,          vpA,
                                   VP,
because it is very small, being essentially the amount of the rota-
tion of the plane of polarization after passing through a thickness
of the medium only equal to half a wave length of the light em-
ployed, we have formula (1).
   Before showing the manner in which formula (2) is derived by
Maxwell, from Thompson's explanation of the magnetic rotation
of the plane of polarized light, it may be best to recall one or two
elementary propositions relating to polarized light, and also to
circular motion.  In the first place, experiment shows that two
rays of light circularly polarized in opposite directions, and
of the same intensity, become, when united, a plane polarized
ray, the plane of polarization of which will depend upon whether
the periods of the component circular vibrations are the same or
not. If, from any cause, the phase of one of the circularly-polar-
ized rays is accelerated, then the plane of polarization of the re-
sultant ray, is turned round through an angle equal to half the
angle of acceleration of the phase.
   So also in certain cases, such as reflection from metallic sur-
faces, or total. reflection in glass at certain angles, as in Fresnel's
rhombs, or in the passage of light through thin lamin, of double
refracting crystals, as in quarter-wave laminae of mica, two plane
vibrations may give rise to one circular one, right banded or left-
handed, according as one or the other plane component is ad-
vanced in phase by a quarter of a complete oscillation.
   This is only what might be expected from the well-known
 theorem in pure motion, that " two uniform circular vibrations
of
 the same amplitude, having the same periodic time and in the
 same plane, but revolving in opposite directions, are equivalent,
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