Abstract: A gauge for use in determining the extent to which the cutting blade of an extensible-blade keratotomy knife projects beyond the end of a cutting depth limiting foot, comprises a base, a knife-holding cradle slidable on the base, a clamping screw for clamping the cradle to the base in any selected position within a range, a block fixed to the base, and a gauge element slidable on the base and adjustably movable with respect to the block toward and away from the cradle. With a knife mounted on the cradle, and the gauge element at a predetermined reference position, the cradle is moved until the foot of the knife abuts one face of the gauge element. The gauge element is then moved away from the foot by an adjusting screw, until an edge of the gauge element underlies the tip of the knife blade. The top face of the gauge element and the base surface on which the gauge element slides are of contrasting colors.