  A test on quantum mechanics proposed long ago by Karl Popper is reconsidered
with further detail and new insight. An ambiguity in the proposal, which turns
out to be essential in order to make the test conclusive, is identified and
taken into account for the first time. Its implications for recently performed
photon experiments [such as in D. V. Strekalov \textit{et al.}, Phys. Rev.\
Lett. {\bf 74}, 3600 (1995)] are briefly analyzed.

Quantum Physics;Optics