Abstract: A tonometer which is used to measure the tension of eyeballs has a first testing device with a first outwardly bulging deformable wall which can be pressed against the eyeball (either directly or by way of the eyelid) and a second flexible wall which is deformable by the flexible or rigid wall member of a second testing device. The housing of the first testing device is filled with a gaseous or liquid fluid whose pressure is monitored by one or more gauges or risers, and a further instrument is provided on or in the second testing device to indicate the position of the wall member. The scale of the further instrument is calibrated in units of pressure in such a way that the tension of the eyeball can be read directly off the scale when the gauges or risers indicate that the deforming work upon one of the flexible walls is the same as that upon the other flexible wall.