Fig. 8.—Meridian Circle.
Before the object reaches the meridian, the telescope must be adjusted at the right elevation. The necessary power is transmitted by a chain from a winch at the northern end of the walls to a point near the upper end of the tube. By this contrivance the telescope can be raised or lowered, and an ingenious system of counterpoises renders the movement equally easy at all altitudes. The observer then takes his station in one of the galleries which give access to the eye-piece; and when the right moment has arrived, the star enters the field of view. Powerful mechanism drives the great instrument, so as to counteract the diurnal movement, and thus the observer can retain the object in view until he has made his measurements or finished his drawing.
Of late years reflecting telescopes have been generally made with mirrors of glass covered with a thin film of silver, which is capable of reflecting much more light than the surface of a metallic mirror. Among great reflectors of this kind we may mention two, of three and five feet aperture respectively, with which Dr. Common has done valuable work.
We must not, however, assume that for the general work in an observatory a colossal instrument is the most suitable. The mighty reflector, or refractor, is chiefly of use where unusually faint objects are being examined. For work in which accurate measurements are made of objects not particularly difficult to see, telescopes of smaller dimensions are more suitable. The fundamental facts about the heavenly bodies have been chiefly learned from observations obtained with instruments of moderate optical power, specially furnished so as to enable precise measures of position to be secured. Indeed, in the early stages of astronomy, important determinations of position were effected by contrivances which showed the direction of the object without any telescopic aid.
Perhaps the most valuable measurements obtained in our modern observatories are yielded by that instrument of precision known as the meridian circle. It is impossible, in any adequate account of the Story of the Heavens, to avoid some reference to this indispensable aid to astronomical research, and therefore we shall give a brief account of one of its simpler forms, choosing for this purpose a great instrument in the Paris Observatory, which is represented in Fig. 8.
The telescope is attached at its centre to an axis at right angles to its length. Pivots at each extremity of this axis rotate upon fixed bearings, so that the movements of the telescope are completely restricted to the plane of the meridian. Inside the eye-piece of the telescope extremely fine vertical fibres are stretched. The observer watches the moon, or star, or planet enter the field of view; and he notes by the clock the exact time, to the fraction of a second, at which the object passes over each of the lines. A silver band on the circle attached to the axis is divided into degrees and subdivisions of a degree, and as this circle moves with the telescope, the elevation at which the instrument is pointed will be indicated. For reading the delicately engraved marks and figures on the silver, microscopes are necessary. These are shown in the sketch, each one being fixed into an aperture in the wall which supports one end of the instrument. At the opposite side is a lamp, the light from which passes through the perforated axis of the pivot, and is thence ingeniously deflected by mirrors so as to provide the requisite illumination for the lines at the focus.