A body, for
instance, may be animated with a certain linear velocity or a speed of
rotation; it may be compressed, or twisted; it may be placed in an
electric or in a magnetic field; it may be affected by an electric
current or by one of heat; it may be traversed by a ray of light
either ordinary or polarized rectilineally or circularly, etc.:--in
each case a certain minimum and characteristic dissymmetry is
necessary at every point of the body in question.
This consideration enables us to foresee that certain phenomena which
might be imagined _a priori_ cannot exist. Thus, for instance, it is
impossible that an electric field, a magnitude directed and not
superposable on its image in a mirror perpendicular to its direction,
could be created at right angles to the plane of symmetry of the
medium; while it would be possible to create a magnetic field under
the same conditions.
This consideration thus leads us to the discovery of new phenomena;
but it must be understood that it cannot of itself give us absolutely
precise notions as to the nature of these phenomena, nor disclose
their order of magnitude.
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