  The constructive method of determining amplitudes from on-shell pole
structure has been shown to be promising for calculating amplitudes in a more
efficient way. However, challenges have been encountered when a massless
internal photon is involved in the gluing of three-point amplitudes with
massive external particles. In this paper, we use the original on-shell method,
old-fashioned perturbation theory, to shed light on the constructive method,
and show that one can derive the Feynman amplitude by correctly identifying the
residue even when there is an internal photon involved.
