The nodal slide assembly consists of an optical bench provided with four uprights. The one upright carries a bulb placed in a metallic cover having a circular aperture, which illuminates a cross slit in the adjacent upright. The third upright carries the nodal slide. Nodal slide is essentially a horizontal metal support capable of rotating about a vertical axis; and lens or lenses can be mounted upon the support. The metallic support can be fixed or it can be moved back and forth by means of a screw so that the relative position of the two lenses can vary with respect to this upright. ... The support can be rotated in a horizontal plane. The fourth upright carries a plane mirror which can be rotated about a horizontal axis perpendicular to the bed of the bench.