We thank Dr Tesser for his observations. He is correct that the precise diameter of 3.06 mm allows the easy conversion of 1 g of force (1 pond) to 10 mm Hg. However, we do disagree that a range of diameters would allow a balance between the effect of corneal elasticity and the effect of the capillary attraction of tears. In their paper,1Goldmann and Schmidt show Figure 8 (page 232) which clearly demonstrates a linear relationship between applanation diameter and corneal rigidity forces for diameters greater than 3mm with a slope different than 0. The graph also shows that the behaviour is different for each eye. Therefore, the corneal rigidity and capillary attraction forces balance each other only where this line for an individual eye crosses 0 at exactly 3.06 mm applanation diameter.
Kniestedt C, Kanngiesser H, Stamper RL, Nee M. Applanation Area in Goldmann Applanation Tonometry—Reply. Arch Ophthalmol. 2006;124(11):1664. doi:10.1001/archopht.124.11.1664-b
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