Subtraction Constants in Dispersion Relations for Compton Scattering and Vector-Meson Dominance

We discuss the requirement of a subtraction constant at infinite energy for the real part of the forward spin-nonflip Compton scattering amplitude from a proton and obtain an upper limit 6.3×${10}^{-4\mathrm{}}$ F/Be${\mathrm{V}}^2$ for such a constant. We show that the model of vector dominance applied to Compton scattering at 3-BeV photon energy is consistent with known photon total cross sections and the application of forward-scattering dispersion relations.