Fig. 14. Schematic drawing of Mackay-Marg handpiece. The plunger of the Mackay-Marg handpiece is connected to a strain gauge, which measures force. When the tip first contacts the cornea, the force increases (B) from baseline (A). When the cornea is flattened, the baseplate takes up some of the force from the plunger, causing a slight, momentary drop in the force registered by the strain gauge (C). As the handpiece is pressed further against the eye, the force of the plunger continues to increase (D). The point of applanation is indicated on the tracing at the trough (C). (Hilton GF, Shaffer R: Electronic application tonometry. Am J Ophthalmol 62: 840, 1966.)