Scleral Rigidity in Tonography It has been apparent to those working in tonography that one of the uncertainties of the procedure is the determination of the coefficient of scleral rigidity. This quantity, a characteristic of the individual eye, was formulated by Friedenwald1 as follows: in which E=coefficient of scleral rigidity δVs=change of volume of the eye P1=initial intraocular pressure P2=final intraocular pressure Grant2 described the volume change in the eye during tonography. This may be expressed as follows: in which δV=total volume loss E=coefficient of scleral rigidity Pt1 and Pt2=initial and final intraocular pressures with the tonometer in place Vc1 and Vc2=initial and final corneal indentation volumes3 Owing to the difficulty of measurement of E in the individual case, some workers have assumed an average value