  A novel method for the extraction of form factors of unstable particles on
the lattice is proposed. The approach is based on the study of two-particle
scattering in a static, spatially periodic external field by using a
generalization of the L\"uscher method in the presence of such a field. It is
shown that the resonance form factor is given by the derivative of the
resonance pole position in the complex plane with respect to the coupling
constant to the external field. Unlike the standard approach, this proposal
does not suffer from problems caused by the presence of the triangle diagram.
