I will suppose that no instrument-maker will fix to the telescope the nonius, and centre-plates, without steady-pins, as well as screws. Screw the centre-plate of the telescope very fast: put the nonius plate upon the steady-pins, without screws, and put the telescope upon the quadrant : make fast the nonius plate to the arch with two pair of hand vices, and take the telescope away. NOW with one Mint of a beam compass, iii the centre of the quadrant, and the other at the middle of the nonius plate, draw a faint arch from end to end : this arch cuts the faint line befo•e mentioned, make a fine point : From this point lay off ozz each side another, which may be at any distance in the arch; only care must be taken, that they be equally distant from the middle point: From the two last make a faint intersection, as near as possible to either of the chamfered edges of the nonius plate : Through this intersection the first division of the nonius must be cut.
Put the telescope again upon the centre of the quadrant, the steady-pins into the nonius plate as before ; unscrew the hand-vices, and bring the last-mentioned intersection to e or upon the limb, where fasten it again with the haud.vices, and take away the telescope. Now, from the point before divided, the nonius divisions must be cut ; by lodging the left hand point of the beam compass in the point upon the arch, and cutting with the right.
Here great care must be taken to cut the first division of the nonius through the point of intersection ; which may be clone by altering the distance of the points in the beam compass, if necessary : This will not sensibly affect the perpendicularity of the divisions, provided the intersection be placed very near to e, or 60°.
Flaying cut the nonius divisions, suppose of the 90 arc, take up the plate, polish off the bur, and fasten it to the limb, as before ; but here great care must be taken to make the first division of the nonius coincide with the 60th deg. so as to appear one line ; and the nouns of the 96 are may be cut in the same manner, making the first division coincide with 64 = 60° upon the limb.
_Vow, take up the plate, and draw a tangent at the point in the faint arc in the middle of the nonius plate ; and with a distance about a quarter of an inch longer than the no nius, lay off from the tangent point another in the tangent line ; also lay off this distance from the centre of the col lar at the object end of the telescope, and make a fine point : Then extend the beam-compass nearly the whole length of the nonius, or centre plate (which should reac;•, at least, half an inch beyond the telescope, on the contraiy side,) and lay off other points in the tangent line before mentioned, and in a line passing through the centre of the quadrant, at right angles to the telescope.
Now screw the two plates to the telescope, and draw lines by the edge of a steel ruler through the correspond ent points, to which lines the plates must be carefully filed. Then the ends of the plates will be in lines parallel to each other, and to the axis of the tube, which affords an excel lent mechanical method of finding the line of collimation of the telescope.
The apparatus used for this purpose by the late Mr Graham, was a box equal in length to the telescope, hav ing deep sides to prevent its bending by its own weight. The ends were Of hard wood. Instead of which, I use two flat pieces of brass, which I can move according to different lengths required ; and by the help of a small spirit-level, these pieces of brass may be fixed in one and the same plane. 'Ibis apparatus should stand firm upon the ground, where a distant and distinct object can be seen. Rest the ends of the nonius and centre plates upon the two pieces of brass, and observe what point of the object is cut at the intersection of the horizontal and vertical wires. Invert the telescope, and if the horizontal wire does riot cut the same point of the object, it must be altered by the screws for that purpose, the difference. By repeating this, you may approximate extremely near the truth.
In the middle, between the 90 and 96 arches of the mu ral quadrant, in the royal observatory, is an arch of points (96,) which are used with a silver wire, of about 600 in an inch, carried by a small frame, screwed to the end of the nonius plate. When the wire, in an observation, falls be tween two points, it must, by the micrometer screw, be made to bisect the nearest point to the left hand, the in strument sheaving the zenith distance ; and the minutes and seconds shcwn by the micrometer added. If the next point to the right hand be hisected,the minutes and seconds must be subtracted. This arch of points was divided in every respect like the other arch of 96.
Having gone through the whole process of dividing the mural arc, &c. it will be necessary to skew some reason why this kind of management bath succeeded better than any other, as far as 1 either know or have heard.