  It has recently been proposed, with the help of numerical investigations,
that fluorescence correlation spectroscopy at variable observation area can
reveal the existence of a meshgrid of semi-permeable barriers hindering the
two-dimensional diffusion of tagged particles, such as plasmic membrane
constituents. We present a complete theory confirming and accounting for these
findings. It enables a reliable, quantitative exploitation of experimental data
from which the sub-wavelength mesh size can be extracted. Time scales at which
fluorescence correlation spectroscopy must be performed experimentally are
discussed in detail.
