  We provide an update of the determination of the electromagnetic self-energy
contribution to M_p - M_n based upon Cottingham's Formula. A technical
oversight in the literature is uncovered: the application of the Cottingham
Formula requires the use of a subtracted dispersion integral; an argument to
evade the subtraction function was presented; the argument was based on false
assumptions about the scaling violations of the parton model, a point first
mentioned by J. C. Collins. We elucidate this point and utilize low-energy
effective theory to relate the unknown subtraction function to the nucleon
isovector magnetic polarizability. This allows us to provide the first reliable
determination of dM = 1.30(03)(47) MeV [1].
