ABSTRACT
This chapter discusses the original field Hamiltonian describing hadrons in erms of partons there are terms giving the coupling of partons with the vector potential of the quantum electromagnetic field. It explains the spirit of minimum electromagnetic coupling that they couple in the simplest way expected from the propagation operators via gauge invariance. As q rises the contribution of such diagrams falls away and only the direct coupling term of photon scattering a parton of the hadron remain important. Note that the state described as "immediately after coupling" is an initial parton state- there still must be interactions from the Hamiltonian before it becomes the "appropriate" outgoing wave function. In the center of mass picture there are terms of coupling in which the photon first divides into partons on the way in, for example one fast one slow, and these slow partons interact or annihilate with the wee partons of the hadron.
