  The existence of the limit of a sample scattering intensity, as the
scattering vector approaches zero, requires and is ensured by the property that
the mean value of the scattering density fluctuation over volume $V$
asymptotically behaves, at large $V$s, as $\nu V^{-1/2}$, $\nu$ being an
appropriate constant. Then, the limit of the normalized scattering intensity is
equal to $\nu^2$. The implications of this result are also analyzed in the case
of samples made up of two homogeneous phases.16 pages, 3 figures
