French Repeting or reflecting circle
Jecker à Paris nr 50. in box complete
Type of object:
Circle/Quintant
Time period:
Britain rules the waves + France
Place:
Paris
Date:
1819
Maker / Author:
Francois Antoine Jecker, apprentice of Jesse Ramsden
Publisher / Printer:
idem
Dimensions:
28 cm or 11 inch diameter with box
Material:
Brass and glass
Graduation:
0° till 720°and 155° till 145°
Inscription:
No 50 Jecker, Paris
Provenance:
Description
The repeating circle, Borda type, consists of a circular varnished brass frame, with a detached wooden handle. The index arm carries the index glass and the telescope arm carries the horizon glass. The double-ended tangent screw and clamping screw are located on the back of the index arm. A second double-ended tangent screw and clamping screw are at the ocular end of the telescope arm. The instrument has five detached shades, two red, two orange, and one green, and three detached horizon shades ( one missing), two green, and one red. Index- and horizon-glass adjustment is made by square-headed screws and a detached key. Attached to the circle is a magnifier on a 60mm swivelling arm. There are also two threaded telescope brackets with perpendicular adjustment made by a rising-piece and two milled knobs. No extra telescopes. Screwdriver and small magnifying glass missing. Accompanying the circle is a sight-tube is 82 mm in length, an extra drawtube for the magnifier, which is 21 mm long, two red and one green shaded eyepieces, five additional detached shades, two rectangular brackets and an adjusting key. The instrument has a varnished brass limb with an inlaid silver scale from 0° to 720° by 20 arcminutes. Brass finder arc from 155° to 0° to 145° by 1°, with two brass stops. The repeating circle has three silver verniers measuring to 1 arcminute, with zeros at the right. The repeating circle is contained in a square fitted mahogany box.
Additional information
The reflecting circle –essentially an octant extended to a full circle–was designed for the purpose of determining longitude by measuring the distance between the moon and a nearby star (the so-called lunar distance method). Johann Tobias Mayer, the German astronomer who introduced the form, also developed the tables of the moon’s motions that were needed for longitude determinations. Mayer presented a detailed description of this instrument to the Board of Longitude. Tests of Mayer’s instrument in England in the 1750s led to the invention of the sextant.