Abstract: A surgical knife, particularly for use in radial keratotomy, comprises a blade, such as a diamond, which is movable for adjustment of the depth of cut by an invariant coupling to a micrometer mechanism, without provision for full retraction of the blade, and a removable protective end cap adapted to encompass the blade when the knife is not in use. The invariant coupling of the blade to the micrometer mechanism is via a mechanical linkage which incorporates a rotatable spindle having two axially separate screw-thread connections. The end cap is provided with holes for the passage of sterilizing fluid to and around the blade.