The instrument consists of a sighting arm with a threaded collar at one end for attaching to tripod. A clamping screw is fitted to hold the collar securely in the required position. The collar supports a fork, into which a mirror is pivotted. The mirror has a small unsilvered spot at its centre and provision is made for fine horizontal adjustment by means of a milled knob. A rod is hinged to the upper edge of the mirror and this passes into a female screw tube to allow for adjustment of its length. The tube is hinged to a lever at its lower end, which forms a Morse key. The lever is hinged to the stem of the instrument and is held against a fixed stop by a spring attached to the collar. A milled disc, fixed near the lower end of the tube, is for tilting the mirror as may be required for signalling. A socket, with clamping screw, is provided at the forward end of the sighting arm to house a supplementary mirror, or the sighting vane extension arm.