Schiotz tonometer is commonly used to measure IOP, it measures the depth of corneal indentation by the plunger while tonometer is loaded with a given weight. Before placing the plunger barrel of a tonometer on the cornea, 4% xylocaine is used to anaesthetise the cornea. Tonometry is the method of estimation of intraocular pressure. Applanation tonometry records the force necessary to flatten the cornea. It is based on the principle which states that the flattening of a sphere made by counter pressure is equal to hydraulic pressure inside the eyeball. Ref: Comprehensive Manual of Ophthalmology By Ahmed E page 116.