  Interferometry is discussed in terms of the representation of the source. In
particular, scale-invariant 1-d hydrodynamics is revisited, and extended to the
case of unequal transverse masses. It is argued that that kaon emission occurs
over a short time interval. Exact results for models of two- and
three-dimensional flow are presented, which exhibit altered scaling laws. Such
qualitative trends, together with other observables, are vital if one is to
draw conclusions about the source.
