Ocular rigidity is the term used to describe the relationship between pressure and volume changes in the eye. The rigidity of the eye is provided mainly by the sclera and cornea, which, however, are not completely unyielding but undergo an appreciable amount of stretching and contracting as a result of variations in the intraocular pressure. The application to the cornea of an impression tonometer, such as that of Schiøtz, produces a rather large change in pressure due to the weight of the instrument and the resulting displacement of intraocular fluid. Therefore, if the rigidity of the ocular coats can be shown to vary to any extent from eye to eye serious errors in interpretation of tonometer readings may result. This is true because tonometer conversion scales are based on an assumed average normal value for rigidity.
Friedenwald1 determined that the data on rigidity of the eye obtained by Koster,
McBAIN EH. Tonometer CalibrationII. Ocular Rigidity. AMA Arch Ophthalmol. 1958;60(6):1080-1091. doi:10.1001/archopht.1958.00940081100015