REFLECTING circle, an astronomical instrument for measuring angles, It is called reflecting from its property, in com mon with the Hadley's quadrant (of which it is a modification) of observing one of the objects of the angle to be measured by distinct vision, and the other by re flection of plane mirrors. The first in. strument of this kind was invented by Tobias Mayers, in 1770, a celebrated astronomer of Gottingen, who calculated the lunar and solar tables for determining the longitude at sea, for which a reward of 3,0001. was given by the board of lon gitude, In making use of these tables he found that the Hadley's quadrants, though made by the first artists of that time, were not divided with sufficient ac curacy for his purpose; he therefore con. trived the reflecting and repeating circle ; to comprehend which, the reader must turn to our article QUADRANT, by Had ley, from which this instrument differs, principally, in being a whole circle of di visions instead of an octant ; and is so contrived, that when an observation has been made, it is repeated upon a fresh portion of the divisions, then a third time, a fourth, and so on as many times as is necessary ; the observation is then read off, and the product is divided by the number of observations, made so as to take a mean of the errors there may be in any part of the divisions on the circle. This contrivance, though useful, was found so tedious, in taking so many observations, that it was laid aside in favour of the Hadley's quadrant, to which in point of accuracy it was really superior, For the particular description of this instrument we must refer our readers to a work, entitled, " Tabula Motum Solis et Luna," by Tobias Mayer, London, 1770. This instrument received an im. provement from the Chevalier de Borda, at Paris, which rendered its operation much more simple ; hut it was not until the year 1796 that the instrument became much used in the British navy, when it was new modelled by Mr. Edward Troughton, and the objections to the former instruments done away. We have obtained permission from this gentleman to make a drawing of this instrument (see Plate Reflecting Circle) where (fig. 1.) is a plan of the divided side of the in. strument, (and fig. 2) a perspective view of its upper side. A A, in both figures, is a circle of brass, with a narrow ring of silver let into a circular groove in it, as is seen in fig. 1, on which silver the divi. sions are made. B B B are three arms carrying verniers at their ends, they are all cast in one piece, and screwed to a truly turned steel axis, fitting into a tube, which is screwed to the centre of the circle (this tube cannot be seen in the plan, but is denoted by a in fig. 2) the in.
dex glass, b, which is a plane silvered mirror, is fastened to the other end of this axis by three screws, in such a posi• tion that the centre line of the steel axis it is fixed to, if produced, would exactly coincide with the plane of the silvered surface of the mirror, and consequently that the plane of the mirror produced passes through the centre of the circle A A, perpendicular to its plane.
To the upper end of the tube, a (fig. 2) a crooked plate of brass, dot, is fastened, and connects it with two other tubes, e and f, whose lower ends are fixed to the cross bar frame of the circle ; one of these, f, has the mirror, h, called the object glass placed on it ; the other has a telescope, k, fixed to it, directed to the object glass, h. The instrument is held, when in use, by handles adapted to different occasions, of which there are four ; two perpendicu lar and two parallel to the plane of the circle ; of the latter, e, is one on the up per side, supported by a small pillar coming from the interception of two of the bars of the frame, and steadied by entering the tube ; a, the other handle is at the divided side of the circle, and is fastened to the circle at the upper side, in the same manner as e, by a crooked hollow tube, G, going round the circle ; of the other two handles, one, m, is above the circle, screwed into a cock fixed to it, so that it IS perpendicular to the centre of the circle; it comes over the index glass, but does not touch it. The other handle is screwed into the handle of the crooked tube, G, so as to be in the same line %yid; the upper handle m ; o, are three dark glasses, between the index and horizon glasses, turning on a joint, so as to be put out of the way when necessary, Or any one or two of them can be turned in the line of the telescope to darken the light, more or less, in observations of the sun ; p, are three other glasses supported by a small pillar behind the horizon glass, which can also be turned back as is ne cessary. The telescope is screwed into a brass ring, r, this is supported by a square piece of brass, tapped at the cor ners, so as to form a screw ; and by turn ing, a, a nut upon a screw, the telescope can be raised or lowered parallel to itself; there is also an adjustment, to bring the line of collimation of the telescope to be parallel to the plan of the circle.