A novel close spaced Fabry-Perot interferometer has been developed which contains the fluid under study between the mirrors. The interferometer-cell has been used to make the first successful measurement of the anomalous refractive index of a binary fluid mixture near its consolute critical point. The cell has also been used to determine the coexistence curve for a binary mixture and the associated critical index B.Intensity autocorrelation spectroscopy of laser light scattered from Brownian particles suspended in a binary mixture has been used to study the effective shear viscosity of the fluid near its critical point. A theory has been developed which includes a momentum dependent shear viscosity and takes into account effects due to surface interactions between the particles and the fluid. This theory explains our observations and is consistent with the results of earlier tracer diffusion measurements in a critical mixture.