Whenever one uses the Schiötz tonometer, one inevitably asks oneself the question: "Is my reading reliable?" It being assumed that the technic used is correct, there still remain two factors to be considered: the eyeball and the instrument. Since the tonometer measures merely the degree of impressibility of the cornea, the readings will vary slightly with such factors as the rigidity and thickness of the cornea and the size and shape of the eyeball. However, these individual variations are of minor importance and, at any rate, are unavoidable.
The accuracy of the tonometer itself plays decidedly the most important rôle in determining the reliability of the readings. Since the tonometric deflections have to be translated into millimeters of mercury by referrence to Schiötz' graphs, it is, of course, essential that the tonometer one uses should be an exact replica of the original one with which the graphs were
POSNER A. A CHECKING STATION FOR TONOMETERS. Arch Ophthalmol. 1935;14(3):453–457. doi:10.1001/archopht.1935.00840090139009
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