  Two-photon decays probe the structure of mesons and represent an important
contribution to hadronic light-by-light scattering. For the neutral pion, the
decay amplitude tests the effects of the chiral anomaly; for a heavy quarkonium
state, it measures the magnitude of its wavefunction at the origin. We rederive
the expression of the decay amplitude in terms of a Euclidean correlation
function starting from the theory defined on the torus. The derivation shows
that for timelike photons the approach to the infinite-volume decay amplitude
is exponential in the periodic box size.
