Abstract: The invention relates to apparatus for automatically guiding a moving object such as a vehicle along a pre-determined path, the apparatus including two detectors which are mounted in a spaced apart relationship on the object, the detectors being responsive to the intensity of electro-magnetic waves such as X-Rays emitted along the path. Comparison means for instance a dividing circuit is connected to the detectors to determine the ratios of the intensities of the magnetic waves and to provide a signal which is acted on by adjustment means that adjusts the direction of movement of the object to bring it back onto the path. The invention provides for irradiation means which is mounted on the object and which causes excitation and emission of secondary waves of pre-determined wavelength from the path.