  A recent proposal to explore vacuum electrodynamics using the speed of
propagation of an electromagnetic pulse through an ambient constant magnetic
field is examined. It is argued that the proposal should be modified so that
the background magnetic field, the direction of propagation and the transverse
projection of the electric field (with respect to the direction of propagation)
are not coplanar. The implications of invariance under Gibbons'
electric-magnetic duality rotations are determined in this context.
