FINDER·CIRCLES FOR EQUATORIALS.—A very ingeniousand what may prove a most useful addition to an equatorial are the so-called star-dials or finding-circles, a brief account of which is contributed to the current number of the Zeitschrift für Instrumentenkunde (4 Heft, April 1894). Every worker with the equatorial will no doubt at some time have found out that the present mode of setting the instrument on some object, as, for instance, a star, is not always very convenient, and in addition employs comparatively far too much time. The object of these finding-circles 15 to reduce this time very considerably, and a use of three years has shown that its aim has been successfully attained. The instrument to which it has been applied is the 12-inch of Georgetown College Observatory, Washington. On the pillar of this instrument are the two hand-wheels, by means of which the telescope is moved in right ascension and declination, and also two microscopes for reading the R.A. circle. Both axes of the telescope carry the usual circles for orientation, each being graduated in fine divisions on silver and large white divisions on a dark background. The finding-circles are situated just above the hand-wheels mentioned above, and fixed to the pillar, looking like a pair of aneroid barometers or steam gauges; they are arranged as follows:—The circular divided disc, with the declination divisions arranged round its circumference, is fixed firm in its case, and the index is so geared to the telescope that any movement of the latter is recorded on the dial; this gives one directly the declination. In the case of the other dial, that for right ascension, the disc is divided into two circles of twelve hours each, and instead of being fixed is moved by clockwork, sidereal time being shown on its face by means of another index; this latter index responds also to the movement of the telescope, but is quite independent of the first one. It will at once be seen that with these two dials so conveniently placed the telescope can be at once oriented, while the question of hour-angle is entirely eliminated.
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Historical Studies in the Physical Sciences (1975)