  A simple Ginzburg-Landau theory with a single, scalar order parameter is used
to study the microscopic structure of microemulsions and sponge phases. The
scattering intensity in both film and bulk contrast, as well as averages of the
internal area $S$, the Euler characteristic $\chi_E$, and the mean curvature
squared $<H^2>$, are calculated by Monte Carlo methods. The results are
compared with results obtained from a variational approach in combination with
the theory of Gaussian random fields and level surfaces. The results for the
location of the transition from the microemulsion to oil/water coexistence, for
the scattering intensity in bulk contrast, and for the dimensionless ratio
$\chi_E V^2/S^3$ (where $V$ is the volume) are found to be in good quantitative
agreement. However, the variational approach fails to give a peak in the
scattering intensity in film contrast at finite wavevector, a peak which is
observed both in the Monte Carlo simulations and in experiment. Also, the
variational approach fails to produce a transition from the microemulsion to
the lamellar phase.
