  Interfaces between media with negative relative refractive index generically
support propagating electromagnetic surface polariton modes with large
wavenumber. The relation of these modes to a recent prediction by Pendry of
``perfect (real) image formation'' by a parallel slab of
negative-refractive-index material is analyzed. The ``perfect image'' theory is
found to be incomplete without inclusion of a large-wavenumber cutoff that
derives from a necessary wavenumber-dependence of the constitutive relations,
and which controls the resolution of the image.
