Fig. 3. Goldmann applanation tonometry. When the cone tip is pressed against the anesthetized cornea, a small circular area is flattened. The applanated area appears as a dark circle surrounded by a narrow ring of fluorescent tear film. Opposing prisms in the tip of the cone split the image, so that the viewer sees two dark half circles, each with a narrow fluorescent outer border. The force on the cone is adjusted until the inner corners of the fluorescent half rings just touch; at this optical end point, the applanated area is correct: a circle with diameter = 3.06 mm.