  Spatially resolved, long-slit echellograms at different position angles of
the bright, low excitation planetary nebula NGC 40 indicate that, the higher is
the gas excitation, the faster is the radial motion, thus confirming the
overturn of the Wilson law already suggested by Sabbadin and Hamzaoglu (1982).
New reduction procedures, giving the radial trends of the electron density and
of the ionic and chemical abundances, were applied to NGC 40; they show that: -
the radial matter distribution has a sharp ``bell'' profile with peaks up to
4000 cm-3; - the ionization structure is peculiar, indicating the presence of
chemical composition gradients within the nebula: the innermost regions,
hydrogen depleted, are essentially constituted of photospheric material ejected
at high velocity by the WC8 nucleus. Moreover, detailed H+, O++ and N+
tomographic maps, giving the spatial ionic distributions at four position
angles, are presented and discussed within the interacting winds evolutionary
model.
