  Any pure quantum state can be equivalently represented by means of its wave
function psi(q) or of the Fermi function g_F(q,p), with q and p coordinates and
conjugate momenta of the system under investigation.We show that a Gaussian
wave packet can be conveniently visualized in phase space by means of the curve
g_F(q,p)=0. The evolution in time of the g_F=0 curve is then computed for a
Gaussian packet evolving freely or under a constant or a harmonic force. As a
result, the spreading or shrinking of the packet is easily interpreted in phase
space. Finally, we discuss a gedanken prism microscope experiment for measuring
the position-momentum correlation. This gedanken experiment, together with the
well-known Heisenberg microscope and von Neumann velocimeter, is sufficient to
fully determine the state of a Gaussian packet.
