1. A method for determining the distance of a base to, or the parallelism of a base having a direction of movement relative to, a row of objects located along an axis, comprising the steps of: projecting light at an oblique angle to the axis from the base located at one side of the axis so as to illuminate a limited plurality of objects in a row; imaging the light from the illuminated objects at a location on the base at an oblique angle different from the illuminating oblique angle; detecting the imaged light from the illuminated objects; generating a signal representative of the frequency of the images produced by said objects; and analyzing said signal to determine the distance to the row or the parallelism of the base relative to the row.