  The $t$-channel contribution to the difference of electromagnetic
polarizabilities of the nucleon, $(\alpha-\beta)^t$, can be quantitatively
understood in terms of a $\sigma$-meson pole in the complex $t$-plane of the
invariant scattering amplitude $A_1(s,t)$ with properties of the $\sigma$ meson
as given by the quark-level Nambu--Jona-Lasinio model (NJL). Equivalently, this
quantity may be understood in terms of a cut in the complex $t$-plane where the
properties of the $\sigma$ meson are taken from the $\pi\pi -> \sigma ->
\pi\pi$, $\gamma\gamma -> \sigma -> \pi\pi$ and $N\bar{N} -> \sigma -> \pi\pi$
reactions. This equivalence may be understood as a sum rule where the
properties of the $\sigma$ meson as predicted by the NJL model are related to
the $f_0(600)$ particle observed in the three reactions. In the following we
describe details of the derivation of $(\alpha-\beta)^t$ making use of
predictions of the quark-level NJL model for the $\sigma$-meson mass.
