A tonometer is described for measuring the intra-ocular pressure of an eye in which a pulse of fluid is projected towards the cornea of the eye to distort the corneal surface by the pressure of the pulse, the distortion in the corneal surface being detected by a change in the reflection of light directed onto the cornea. A fluid delivery system (36, 44, 48) leads to an outlet through which the fluid pulse is applied to the cornea, and a branch (54) leads to a pressure measuring chamber (52), containing a transducer (170) which produces an electrical signal whose value depends on the pressure in the branch. A flow restrictor is removably fitted in the branch (54) and signal processing means (176, 178, 180) (the gain and rise time of which are adjustable by (182) or (186)), is supplied with the output from the transducer. Adjustment of 182, and 186 enable the electrical signal output of the processing means to be adjusted, for calibration. Calibration of the instrument is achieved by selecting an appropriately dimensioned restrictor and by adjusting the gain, and where appropriate the time constant, of the signal processing means, so that the plot of a pressure-time signal curve of the processing means output corresponds to the pressure-time curve for such an instrument when set to operate in line with a clinical standard.