  We consider a slow particle with wave function $\psi_t(\vec{x})$, moving
freely in some direction. A mirror is briefly switched on around a time $T$ and
its position is scanned. It is shown that the measured reflection probability
then allows the determination of $|\psi_T(\vec{x})|^2$. Experimentally
available atomic mirrors should make this method applicable to the
center-of-mass wave function of atoms with velocities in the cm/s range.
