  We observed a substantial increase of the scintillation light output of lead
tungstate (PbWO$_4$) at a small incidence angle with respect to two main
lattice axes. This reflects the acceleration of electromagnetic shower
development that occurs in the crystalline Strong Field. We measured the
scintillation light generated by $120$-$\mathrm{GeV}$ electrons and
$10$-$100$-$\mathrm{GeV}$ $\gamma$ rays on thick samples. This result deepens
the knowledge of the shower development mechanisms in crystal scintillators and
could pave the way to the development of innovative accelerator- and
space-borne calorimeters.
