Abstract: An applanation tonometer for sensing the intraocular pressure of the eye. The tonometer includes coaxial inner and outer probes which are placed in contact with the eye. The difference in longitudinal forces applied by the eye to the inner and outer probes is measured and the intraocular pressure is determined therefrom. The total pressure applied to the eye by the tonometer is not measured as only the differential pressure on the two probes is needed. The tonometer may include a feedback mechanism to oppose relative longitudinal movement between the probes, thus improving the linearity, accuracy and stability of the instrument. The differential-pressure sensing circuitry includes a sample-and-hold circuit for receiving the measured result and supplying it to an indicating meter. The circuitry also includes compensating means for compensating for the force effects of gravity on the masses of the device.