  When a macroscopic concentration gradient is present across a binary mixture,
long-ranged non-equilibrium concentration fluctuations (NCF) appear as a
consequence of the coupling between the gradient and spontaneous equilibrium
velocity fluctuations. Long-ranged equilibrium concentration fluctuations (ECF)
may be also observed when the mixture is close to a critical point. Here we
study the interplay between NCF and critical ECF in a near critical mixture
aniline/cyclohexane in the presence of a vertical concentration gradient. To
this aim, we exploit a commercial optical microscope and a simple, custom-made,
temperature-controlled cell to obtain simultaneous static and dynamic
scattering information on the fluctuations. We first characterise the critical
ECF at fixed temperature $T$ above the upper critical solution temperature
$T_{c}$, in the wide temperature range $T-T_{c}\in[0.1,30]$ $^{o}$C. In this
range, we observe the expected critical scaling behaviour for both the
scattering intensity and the mass diffusion coefficient and we determine the
critical exponents $\gamma$, $\nu$ and $\eta$, which are found in agreement
with the 3D Ising values. We then study the system in the two-phase region
($T<T_{c}$). In particular, we characterise the interplay between ECF and NCF
when the mixture, initially at a temperature $T_{i}$, is rapidly brought to a
temperature $T_{f}>T_{i}$. During the transient, a vertical diffusive mass flux
is present that causes the onset of NCF, whose amplitude vanishes with time, as
the flux goes to zero. We also study the time dependence of the equilibrium
scattering intensity $I_{eq}$, of the crossover wave-vector $q_{co}$ and of the
diffusion coefficient $D$ during diffusion and find that all these quantities
exhibit an exponential relaxation enslaved to the diffusive kinetics.
