Brass dip sector.

Dip sector made by Thomas Jones, London, about 1845. Lacquered brass frame and index arm, polished limb, a hinged wooden handle. Signed on the limb: Thomas Jones 62 Charing Cross London. Marked on the limb: I 1. Inlaid silver scale from -5° to 40° every 10' (from left to right). Silver vernier by 10", zero at the left. The double-ended tangent screw and the clamping screw are on the back of the index arm. No shades. Index-glass adjustment by screw; adjustment of the horizon glass by capstan screws. A detached magnifier. Threaded telescope bracket in two parts, fitted for correcting collimation error by capstan screws; perpendicular adjustment by rising-piece and a milled knob. Telescope in two parts at a right angle, containing a prism; a shaded eyepiece (grey). No box. The observer looks through the telescope and sees reflections of the horizon on either side of him, at an angle of 180°. The horizons are brought into one line by moving the index arm, and the dip or rise of the horizon is read from the scale. I 1 was the Hydrographic Office serial numbering adopted in February 1828, where I was the letter presumably given to dip sectors.