The Goldmann applanation tonometer consists of an applanating probe and weight loaded lever system, which is placed on a slit-lamp mount. The end of the probe is brought into contact with the anaesthetised cornea. When contact is made, the clinician views the applanated area (which is made visible by the instillation of fluorescein). The force is then adjusted until the applanated area has a diameter of 3.06 mm. The prisms in the probe have the effect of shifting the upper half of the field of view to the left, and the lower half to the right, so that the centres of the two halves are separated by a distance of exactly 3.06 mm. The applanated area is therefore seen as two semicircles. When the applanated area is exactly 3.06 mm the inner edges of the semicircles will be in contact and the intraocular pressure can be recorded.