1. A system for the automatic guidance of a movable object traveling over a supporting surface along a predetermined path, comprising: transmitting means on said object for directing radiant energy forwardly and sideways toward said surface in the form of at least one pair of beams intersecting said surface in two substantially rectangular zones of illumination with substantially coextensive major dimensions in the direction of motion of said object, said zones being relatively offset transversely to said direction of motion and lying alongside a centerline of said path; a series of spaced-apart responders disposed in at least one row parallel to said centerline and close to said surface for reflecting radiant energy of substantially equal intensity from both said beams toward said object as long as said object occupies a normal position athwart said centerline, said zones having a width exceeding that of said row and a length extending over a plurality of responders of said row; receiving means on said object for separately detecting the reflections of said beams from said responders; and steering-control means on said object connected to said receiving means for correcting deviations from said centerline, said receiving means including comparison means for ascertaining differences indicative of said deviations between the intensities of said reflections.