  We make a detailed experimental study of the threshold for the
self-organization of thermal 87Rb atoms coupled to a high-finesse cavity over a
range of atom numbers and cavity detunings. We investigate the difference
between probing with a traveling wave and a retroreflected lattice. These two
scenarios lead to qualitatively different behavior in terms of the response of
the system as a function of cavity detuning with respect to the probe. In both
cases, we confirm a N-1 scaling of the threshold with atom number.
