Fig. 5. McKay-Marg applanation tonometry. In this method, the applanation force is sensed electronically through a sensor attached to the central cylinder (black). As the instrument is applied to the eye, force on the central cylinder increases (small graph) until the full area of the cylinder face is flattened against the cornea. Just beyond this point, some of the force is transferred to the surrounding inert portion of the instrument (gray), which produces a small dip in the force recording from the central cylinder (*). At this point, a fixed area has been applanated and the force required has been determined (marked by the dip in the recording). A second end point is similarly recorded (**) as the instrument is withdrawn from the eye. This test method has been rendered both convenient and portable in a small handheld unit—the Tonopen. One tonometry/manometry comparison study found an inadequate level of accuracy with this instrument, however. (Eisenberg DL, Sherman BG, McKeown CA et al: Tonometry in adults and children. A manometric evaluation of pneumatonometry, applanation, and Tonopen in vitro and in vivo. Ophthalmol 105:1173, 1998.)