  Electrons obeying the Dirac equation are investigated under the
non-relativistic $c \mapsto \infty$ limit. General solutions are given by
derivatives of the relativistic invariant functions whose forms are different
in the time- and the space-like region, yielding the delta function of $(ct)^2
- x^2$. This light-cone singularity does survive to show that the charge and
the current density of electrons travel with the speed of light in spite of
their massiveness.
