The eye-piece of the telesccipe must be moved inwards or outwards till the wires in the field of view are distinctly seen; and the objects glass must also be moved by means of the pinion, at, till the station staff, placed at any convenient distance (suppose 100 yards), is also dis tinctly seen. By a few trials, the distance between the eye and the object-glass may be made such that the intersection of the wires will appear to remain constantly at one point on the staff while the observer in looking through the telescope varies the position of Ids eye. It is necessary besides that the intersection of the wires should be precisely in the line of collimation, or the optical axis of the telescope : for this purpose the point of intersection should be directed to some well defined mark at a considerable distance. The telescope must then be turned on its axis; and if the intersection remains constantly on the mark, that adjustment is complete; otherwise it must be rendered so by means of the screws, cd, /tc., on the telescope; those screws being placed at the extremities of two diameters at right angles to one another, on being turned they move the plate carrying the wires in the directions of those diameters. In order that the correction may be made, the apparent displacement of one of the wires, in consequence of the telescope being turned half round on its axis, should be observed, and the screws turned till half the displacement is corrected ; the liko observation and correction may then be made for the other wire : a few repetitions of each adjustment will probably be necessary before the error is wholly removed.
The level constructed by the late Mr. Troughton differs from that which has been above described in having the spirit-tube sunk partly in the telescope; and the latter, being incapable of a movement about its axis, does not admit of a separate adjustment for the intersection of the wires.
Mr. Gravat, who made considerable improvements in the mechanism of these instruments, recommends the following method by which the error in the positions of the cross-wires and spirit-tube may be weer pose, it is placed within a brass case having a long opening on the side which is to be uppermost, and is attached to a telescope; the telescope tamed and corrected :—Let three pickets be driven into the ground in a line and at equal distances from one another, and let the spirit-level be set up successively in the middle between the first and second, and between the second and third pickets; then, having by the screws of the instrument adjusted the spirit-tube so that the bubble of air may retain the same place while the telescope is turned round on the vertical axis, direct the object-end of the telescope successively to the station-staves held up on the different pickets, read the several heights, and take the differences between those on the first and second, and on the second and third staff. Now the staves being at equal distances from the instrument, it is obvious that any error which may have existed in the line of collimation, or from the spirit-tube not being parallel to that line, will be destroyed, and the differences between the readings on the staves are the differences in the levels of the heads of the pickets ; but unless the adjustments are perfect, this will not be the case if the instrument be set up at any point which is unequally distant from all the pickets; therefore from such point direct the telescope to the staves, and take the differences of the readings as before. On comparing these differences with the former, a want of agreement will prove that the intersection of the wires is not in the optical axis : and the error may be corrected by means of the screws belonging to the wire plate. After the agreement has been obtained, should the bubble of air not stand in the middle of the tube, it may be brought to that position by the screw 6, at one extremity of the case, and the instrument is then completely adjusted. (Simms, ' Treatise
on Mathematical Instruments.') The spirit-level is usually provided with a clamp, e, and a screw, r, by which when the axis of the telescopo has by hand been brought near the object, the coincidence may be accurately made by a slow and steady motion about the vertical axis.
The spirit-tube or level which is employed for the adjustment of transit telescopes or astronomical circles is contained in a case with feet or with loops at its extremities, in order that it may either rest above or be suspended below the horizontal axis of the instrument to be levelled ; also the upper part of the case is furnished with a graduated scale, the divisions of which are numbered on each side of a zero point, this point being usually placed near each of the two extremities of the air-bubble when the tube is in a horizontal position. Having set up or suspended the spirit-tube, the two particular graduations at which the extremities of the air-bubble rest are marked ; and half the sum, or half the difference of these numbers, according as the extremities of the bubble are in the same or in opposite directions from the two zero-points, being taken, gives the distance of the centre of the bubble from the middle between those points. The level being then reversed, the graduations at which the air-bubble rests are again marked, and half the RIM or half the difference is taken as before. A mean of the two distances thus found is the true distance of the centre of the bubble from the middle point on the scale ; and the screw which elevates or depresses one end of the axis of the telescope being then turned, till either extremity of the bubble has moved, in a direction contrary to that in which the centre of the bubble had moved from the middle of the scale, through a number of divisions equal to that mean distance, that axis will be brought to a horizontal position. This method is used in preference to that of successive trials, in order to avoid the trouble of making several reversions of the whole instrument.
The levelling-staff till lately in general use for finding the relative heights of ground is a rod consisting of two parts, each six feet long, which, by being made to slide on one another, will indicate differences of level nearly as great as twelve feet. The face of the rod is divided into feet, inches, and tenths, or into feet with centesimal subdivisions ; and a vane, or cross-piece of wood, perforated through the middle, is moved up or down upon the rod by an assistant till a chamfered edge at the perforation is seen by the observer at the spirit-level to coincide with the horizontal wire in the telescope. The height from the ground to the chamfered edge of the vane must be read by the assistant; and it being out of the power of the observer to detect any mistake in the reading, it becomes very desirable that the graduations on the rod should be sufficiently distinct to allow the heights to be read at the spirit-level itself. The rod proposed by Mr. Cravat for this purpose is divided into hundredths of a foot by stripes which are alternately black and white, and are numbered at every foot in the usual way with figures great enough to be seen on looking through the telescope ; the tenths of a foot are indicated by lines longer than the others. A similar staff has been proposed by Mr. Sopwith and Mr. W. P. Barlow; and the former gentleman, besides the number of every foot, has given a number to every first, third, fifth, and ninth decimal. Mr. Barlow's rod is also divided into centesimals of a foot ; but the marks, instead of being stripes whoso edges are parallel to one another, have the form of triangles : each tenth mark, however, is in the form of a lozenge, or double triangle, for the sake of greater distinctness.