  We discuss several natural metrics on spaces of unbounded self--adjoint
operators and their relations, among them the Riesz and the graph metric. We
show that the topologies of the spaces of Fredholm operators resp. invertible
operators depend heavily on the metric. Nevertheless we prove that in all cases
the spectral flow is up to a normalization the only integer invariant of
non-closed paths which is path additive and stable under homotopies with
endpoints varying in the space of invertible self-adjoint operators.
  Furthermore we show that for certain Riesz continuous paths of self--adjoint
Fredholm operators the spectral flow can be expressed in terms of the index of
the pair of positive spectral projections at the endpoints.
  Finally we review the Cordes--Labrousse theorem on the stability of the
Fredholm index with respect to the graph metric in a modern language and we
generalize it to the Clifford index and to the equivariant index.
