Confocal microscopy of fluids under static pressure

There are few reliable methods for obtaining equations of state for fluids under extreme conditions. We have used \textit{confocal microscopy} to investigate water and argon under large hydrostatic pressures. Unlike conventional optical microscopy, confocal microscopes collect data point-by-point, enabling three-dimensional image reconstruction. Using this method, we produced three-dimensional images of fluids under large hydrostatic pressures. By combining these images with Fabry-Perot interference measurements, we determined the volume and refractive index, as a function of pressure, in the same experiment.