  Starting from the linearized fluctuating Boussinesq equations we derive an
expression for the structure factor of fluids in stationary convection-free
thermal nonequilibrium states, taking into account both gravity and finite-size
effects. It is demonstrated how the combined effects of gravity and finite size
causes the structure factor to go through a maximum value as a function of the
wave number $q$. The appearance of this maximum is associated with a crossover
from a $q^{-4}$ dependence for larger $q$ to a $q^2$ dependence for very small
$q$. The relevance of this theoretical result for the interpretation of light
scattering and shadowgraph experiments is elucidated. The relationship with
studies on various aspects of the problem by other investigators is discussed.
The paper thus provides a unified treatment for dealing with fluctuations in
fluid layers subjected to a stationary temperature gradient regardless of the
sign of the Rayleigh number $R$, provided that $R$ is smaller than the critical
value $R_\mathrm{c}$ associated with the appearance of Rayleigh-B\'{e}nard
convection.

Statistical Mechanics