  We show that the Einstein covariance principle provides an opportunity to
generate infinitely many solutions of given covariant equation from a known
one. With use of this statement we derive exact expressions for charge and
current densities in a medium in an arbitrary external field. We obtain that in
a homogeneous nondispersive medium the field of a point particle differs at
short-distances from the well known expression. We also demonstrate that in a
linear medium second harmonic generation and optical detection become possible
in the field of radially polarized radiation.
