  In the theoretical analyses of impurity effects in superconductors the
assumption is usually made that all quantities, except for the Green functions,
are slowly varying functions of energy. When this so-called Fermi Surface
Restricted Approximation is combined with the assumption that impurities can be
represented by delta-function potentials of arbitrary strength, many reasonable
looking results can be obtained. The agreement with experiments is not entirely
satisfactory and one reason for this might be the assumption that the impurity
potential has zero range. The generalization to finite range potentials appears
to be straightforward, independent of the strength of the potential. However,
the selfenergy resulting from scattering off finite range impurities of
infinite strength such as hard spheres, diverges in this approximation at
frequencies much larger than the gap amplitude! To track down the source of
this unacceptable result we consider the normal state. The elementary results
for scattering off a hard sphere, including the result that even an infinitely
strong delta-function potential does not lead to scattering at all in systems
of two and more dimensions, are recovered only when the energy dependencies of
all quantities involved are properly taken into account. To obtain resonant
scattering, believed to be important for the creation of mid-gap states, the
range of the potential is almost as important as its strength.
