The eye is a sphere-like organ whose encasement is an elastic, tensile, semirigid structure. The problem of measuring the intraocular pressure, therefore, becomes complex owing to these structural characteristics and their variability. If the eye were an inelastic nonrigid structure so that its envelope did not resist distortion, and the tonometer weight were very small so that it displaced practically no volume, Vc, then the weight, W, would simply be supported by the intraocular pressure, P, over the plunger area, A (W=PA) (Fig. 1).
Two methods of measuring this pressure have been employed, the indentation type of Schiøtz, Gradle, and McLean and the aplanation technique of Goldmann.
In any tonometer there is a finite change of volume, Vc, which distends the eye and brings into play elastic tensile forces and a rise in internal pressure from Po, the pressure in the eye before the application tonometer to Pt, the pressure in the eye with the
OLMSTED KEP. An Evaluation of Tonometric Techniques. AMA Arch Ophthalmol. 1959;62(3):459-464. doi:10.1001/archopht.1959.04220030115016