This document discusses sagittal depth, which is the height or depth of a segment of a circle. It defines the formula to calculate sagittal depth (s) of a lens as s = r − √r2 − y2, where r is the radius and y is half the diameter. It then provides formulas to calculate edge thickness (e) for different lens types (plano-convex, biconvex, plano-concave, biconcave, positive meniscus, negative meniscus) based on the total thickness (t) and sagittal depth(s) of the lens surfaces.