An elbow is rigidly attached to the lower end of the tube. At the intersection is an ac curately polished mirror set at an angle of 45 degrees. At the outer end of the elbow is another mirror, similarly set. The objective is so placed that the light it gathers from the star is reflected by the mirrors through the tube to the eye-piece. The combined movements of the polar axis (the telescope tube) and the objec tive and mirror carried on the elbow enable the observer to bring into view any star in the visible heavens. The polar axis, with its elbow carrying the objective and revolving in sidereal time by means of a driving clock, follows the apparent motion of the star in the usual way. Two of these instruments are in successful use in the Paris Observatory.
It will be evident that the equatorial tele scope with its various modifications as above de scribed, while giving facilities for examining and photographing the heavenly bodies, does not enable the astronomer to determine with re quired accuracy the positions of the stars and planets. These fine measurements are secured only by special forms of telescopes. The Me ridian Circle is one of the most approved in struments for this purpose. From the middle of the tube trunnions extend on either side, car rying finely graduated circles and terminating in accurately ground pivots which are exactly at right angles to the optical axis of the tube. Two piers are so set as to form a rigid and accurate support for these pivots, east and west, carrying the tube so that the movement of the telescope is in the true meridian only. In the exact focus of the telescope a fixed system of cross-hairs or wires is placed. The best ma terials for this purpose are taken from the cocoon of the field spider, the web being only one five-thousandths of an inch in diameter. Finely drawn platinum wires are also used. These vertical spider webs are equally spaced and so adjusted that the central wire is exactly in the optical axis of the telescope as measured east and west. A horizontal wire is adjusted exactly in the optical axis as measured north and south. Parallel to these central wires there are two movable wires, one horizontal and one vertical, each governed by a micrometer screw.
In measuring transits of stars for determin ing right ascension, or for time, the telescope, by means of the graduated circles, is set to the declination of the star required, and when the star appears, its transit across each of the wires is recorded on a chronograph, by the observer tapping an electric key. In determin
ing declinations, the telescope, by means of the graduated circles, is set to the approximate declination of the star to be observed, and when the star appears at the edge of the field, the observer carefully adjusts the telescope until the star seems to be exactly bisected by the horizontal wire as it threads its way across the field. By reading the fine circle the declination of the star is obtained. Other types of tele scopes for similar observations are known as transits, zenith telescopes, mural circles, etc., but the illustrations given will suffice.
Even with all the caution used in the con struction of these delicate instruments, errors are sure to develop, due to refraction, flexure of the tube, variation resulting from changes in temperature and other contributing causes, for all of which allowance must be made in the final reduction of the observations. About the middle of the last century Professor Airy, then Astronomer Royal at Greenwich, designed and constructed a vertical telescope, which he be lieved would eliminate the errors so manifest in his other instruments. He named it the "Reflex Zenith Tube." The principle is shown in Fig. 7. Every part of the instrument is stationary and no part need be touched when in use by the astronomer. The light from the star as it passes the zenith is concentrated by the objec tive upon a surface of mercury in the base of the column, by which it is reflected back through a hole in the objective; the cone of rays then meets a diagonal prism, is reflected at right angles and enters the eye-piece to be observed as in other instruments. Contrary to the ex pectations of the Astronomer Royal, errors in the observed position of the stars were still manifest and the most careful investigations failed to trace them to their source. The in strument was, therefore, discarded. Fifty years. later, Professor Chandler, of Cambridge, Mass., discovered that the pole of the earth "wobbles° slightly, causing a variation in latitude. The results of his observations were compared with the Airy observations of a half century before, and the supposed errors of the eReflex Zenith Tube° were at once traced to the variation in latitude. The old instrument which had been condemned is thus proven to be correct both in theory and practice. It, therefore, represents the latest development in astronomical tele scopes, and a large Reflex Zenith Tube is now in the service of the University of Pennsyl vania.