Armand Imbert (1850-1922) and Adolf Fick (1829-1901) both demonstrated, independently of each other, that in ocular tonometry the tension of the wall can be neutralized when the application of the tonometer produces a flat surface instead of a convex one, and the reading of the tonometer (P) then equals (T) the IOP," whence all forces cancel each other.
This principle was used by Hans Goldmann (1899–1991) who referred to it as the Imbert-Fick "law", thus giving his newly marketed tonometer (with the help of the Haag-Streit Company) a quasi-scientific basis; it is mentioned in the ophthalmic and optometric literature, but not in any books of physics.
[2][3] Any deformation necessarily increases surface area, which is impossible if the membrane is inelastic.
[5] Therefore, in Goldmann tonometry, readings are normally taken when an area of 3.06mm diameter has been flattened.
At this point the opposing forces of corneal rigidity and the tear film are roughly approximate in a normal cornea and cancel each other out allowing the pressure in the eye to be inferred from the force applied.