Lamp house and semitransparent scale are mounted on a stand with a tripod base. The cylindrical, aluminum lamp-house consists of a 6V electric bulb; a draw tube moving on rack and pinion for focusing of lens; and a cross wire. The lamp house is supported by a bosshead, providing easy, alignment with respect to galvanometer and scale. The scale is mounted on a clamp and can slide sideways for zero adjustments. It is graduated in mm with readings of 0-50cm and 25-0-25cm on both sides of the graduation.