Before revision: The problem is that a sphere formed by such a membrane and filled with incompressible liquid (water) cannot be indented or applanated even when the pressure inside is zero, because a sphere contains the maximum volume with the minimum surface area.Koster W. "Beiträge zur Tonometrie und Manometrie des Auges".
After revision: A sphere formed from an inelastic membrane and filled with incompressible liquid cannot be indented or applanated even when the pressure inside is zero, because a sphere contains the maximum volume with the minimum surface area.Koster W. "Beiträge zur Tonometrie und Manometrie des Auges".