Abstract: Apparatus for the optical observation of the evolution of instabilities between two fluids having different densities subject to high initial pressures, of the type comprising an observation chamber with parallel faces containing the two fluids, initially separated by a diaphragm, and wherein it comprises in combination the arrangement of the observation chamber within a tight enclosure equipped with static pressurizing means and whereof one face is constituted by an optical observation window parallel to the two parallel faces of the observation chamber, the diaphragm having a straight cylindrical shape and is coupled to a mechanical extraction shaft, the geometrical axes of the diaphragm and the extraction shaft being perpendicular to the faces of the observation chamber, the extraction shaft sealingly sliding through the wall of the enclosure opposite to the said window, while part of the extraction shaft external of the pressure enclosure is integral respectively with a piston sliding within a cylinder an