We have developed a new all-optical method combining white light interference measurements and confocal laser microscopy to directly measure the pressure-volume (PVT) equation of state (EOS) of fluids and transparent solids in the diamond anvil high pressure cell, by measuring the changes in refractive index and sample volume upon pressure increase.  We have conducted proof-of-concept experiments on a variety of samples at ambient temperature, and at high temperatures up to just above 500 K.  Our proof-of-concept data demonstrate that our method is able to reproduce known fluid and solid EOS within error (the error in the volume determinations is now ca. ± 1%).  We also show that we can measure the transmissivity of samples in the DAC with quantitative accuracy in the visible region, and that this can in future allow the accurate determination of the entire complex refractive index of absorbing samples in the DAC.