  Form factors in the nucleon-photon vertex with one off-shell nucleon are
calculated by dressing the vertex with pion loops up to infinite order. Cutting
rules and dispersion relations are implemented in the model. Using the
prescription of minimal substitution we construct a \gamma \pi N N vertex and
show that it has to be included in the model in order that the Ward-Takahashi
identity for the \gamma N N vertex be fulfilled. The vertex is to be applied in
a coupled-channel K-matrix formalism for Compton scattering, pion
photoproduction and pion scattering. The form factors show a pronounced cusp
structure at the pion threshold. As an illustration of a consistent application
of the model, we calculate the cross section of Compton scattering. To provide
gauge invariance in Compton scattering, a four-point \gamma \gamma N N contact
term is constructed using minimal substitution.
