  The Newtonian approximation for the gravitational field equation should not
necessarily involve admission of non-relativistic properties of the source
terms in Einstein's equations: it is sufficient to merely consider the
weak-field condition for gravitational field. When a source has electromagnetic
nature, one simply {\em cannot} ignore its intrinsically relativistic
properties, since there cannot be invented any non-relativistic approximation
which would describe electromagnetic stress-energy tensor adequately, even at
large distances where the fields become naturally weak. But the test particle
on which gravitational field is acting, should be treated as non-relativistic
(this premise is required for introduction of the Newtonian potential
$\Phi_{\rm N}$ from the geodesic equation).
