The calibration of tonometers to give pressure in millimeters of mercury requires the determination of the pressure while the tonometer rests on the eye (Pt). With the Schiøtz tonometer Pt is considerably higher than the pressure in the undisturbed eye (Po) because the weight of the tonometer produces a large rise in pressure due to the fact that an appreciable quantity of fluid is displaced by the tonometer indentation (Vc). The relationship between the change in pressure from Po to Pt and the change in volume in the eye produced by the tonometer is determined by the rigidity of the ocular coats (E). If the pressure-volume relationship (rigidity) is known and Pt and Vc can be determined, it is then possible to calculate Po from the rigidity formula.
In an earlier paper the results of making measurements of Pt on
EARLE H. McBAIN. Tonometer CalibrationIII. Volume of Indentation and P0 Determination. AMA Arch Ophthalmol. 1960;63(6):936–942. doi:10.1001/archopht.1960.00950020938007