Abstract: The invention contemplates a system of two-dimensional grid markings for use in accurate two-coordinate identification of the instantaneous relative position of two closely spaced parallel surfaces, one of which may be on a slide-mounting base or plate, and the other of which may be on a movable slide to be viewed, as by microscope, at one or more selected regions in the field of the slide. For the disclosed embodiment, each of two orthogonally related grid markings on the base is orthogonally oriented with respect to a different one of two orthogonally related grid markings on the slide, and parallel lines of intersecting grids are in the same direction. Photoelectric means is disclosed for response to light modulation attributable to the respective x-y components of any given slide displacement, and digital techniques utilize the photoelectric response for measuring and/or control slide displacement.