  Critical opalescence is a characteristic experimental signature of a second
order phase transition in solid state physics. A new, experimentally accessible
measure of opacity and of attenuation length in heavy ion reactions is
suggested, as a combination of HBT radii and nuclear modification factors. This
opacity is maximal when $\sqrt{s_{NN}}$, the system size and centrality
correspond to the critical point of QCD. To characterize the phase transition
at this critical point, the critical exponent of the correlation function can
be determined by measuring the L\'evy index of stability of the Bose-Einstein
or HBT correlations. The exponent of the correlation length can be determined
from fits to the multiplicity distribution in various pseudorapidity intervals,
also as a function of colliding energy, system size, centrality and (chemical)
freeze-out temperature. These two critical exponents determine the remaining
four critical exponents and the universality class of this second order phase
transition. As a control experiment, the determination of the critical exponent
of the specific heat capacity is proposed, from event-by-event fluctuation
measurements. To measure opacity precisely, well calibrated high transverse
momentum probes are needed, such as given by the excitation function of
$\gamma$ + jet correlation functions.
