  We re--examine the connection between interferometry and the Wigner
representation for source freeze--out, and continuous emission. At the operator
level, two equivalent representations of the two--particle spectrum are found,
which contradict the standard expression of kinetic theory. The discrepancy is
resolved using two toy models. Further, we revisit interferometry in
scale--invariant one--dimensional hydrodynamics, and argue that recent
experimental results are evidence for a short kaon emission time. Using two
exactly calculable models of two-- and three--dimensional flow, it is shown
that the saddle point approximation, which is reasonable for one--dimensional
flow, is no longer adequate. In these models the scaling law is altered, and we
argue that such qualitative trends, together with other observables, are vital
if one is to draw conclusions about the unknown source parameters.
