To the top of the limb is attached a fixed external brass body tube with an external dimension of 41 mm. An internal optical body tube slides in this external tube, and has a length of 210 mm and external diameter of 38.6 mm. This inner tube is moved up and down by means of a rack which is screwed into its inner surface with its teeth protruding along a longitudinal slot in this tube, and moved by a pinion attached to a single knurled knob on the right side of the outer stationary tube.