reversed. The cross hairs should still coincide with the object P. If they do not, but intersect some other point as P', Fig. 161, correct half the total deviation PP' by means of the diaphragm screws dd, Fig. 52, and half by means of the tangent screw ď of the horizontal circle. The cross hairs should now coincide with the same point P when the horizontal axis of the telescope is reversed. If not, the operation must be repeated until they do. In the transit theodolite, in order to reverse the ends of the horizontal axis of the telescope without lifting it out of its bearings, which is liable to shift the instrument, turn the telescope over vertically, so that the object glass points away from the point P, and then turn the upper part of the instrument through exactly 180° by means of the vernier won the upper plate t.

In the plain theodolite the adjustment of the line of collima

tion may also be effected by twisting the telescope half round in its y's about its own axis, and observing whether the cross hairs still continue to coincide with the distant point P. Should they not, half the deviation is to be adjusted by the diaphragm screws d, d and half by the tangent screw d' of the horizontal plate. By this means the line of collimation is made to coincide with the axis of the y's, and in this case the adjustment of the axis of the y's perpendicular to the horizontal axis of the telescope is supposed to be correctly made by the instrument maker.

P

Accurate Method of Adjusting the Collimation Line of Transit Theodolite.-An objection to the foregoing method is that in lifting the telescope out of its bearings and replacing it the instrument is liable to be moved, unless it is done with extreme care. Also when with a transit instrument to avoid this the telescope is revolved vertically and the instrument turned through exactly 180° in azimuth, any error due to reading the vernier or in the graduations of the horizontal plate, is introduced. The following is the best way to adjust the collimation line of a transit theodolite. Set up the instrument at A, Fig. 162, and direct the cross hairs to a well-defined mark at B. A fine point marked on a peg driven into the ground will do. Turn the telescope over vertically and note where the cross hairs cut a well-defined distant point P. Again direct the telescope on to B, this time with the ends of the horizontal axis reversed. This is effected by leaving the telescope still pointing on P, and turning the whole of the instrument through 180° in azimuth, then accurately bisecting в again by means of the tangent screw d'. Again turn the telescope over verti cally, and see whether the cross hairs still bisect P. If they do not, but intersect the object say in P', Fig. 162, then the point p" midway between P and P' is the correct point, i.e., p" is in line with A and B and BAP" is a true straight line. To adjust the collimation line, correct one quarter the total deviation PP' by the diaphragm screws d, d, i.e., P'p, and one quarter by the tangent screw d' of the horizontal limb, i.e., pp"; in other words, by means of the diaphragm screws d, d bring the cross hairs to bear on where P'p= P'P, Fig. 162, then by means of the tangent screw d' of the horizontal circle bring them to bear on p" midway between P and P'.

A

B

Fig. 162.
Adjustment of

Collimation
Line.

The cross hairs when the telescope is revolved vertically should now intersect both the point в and the point p" in both positions of the horizontal axis, and the correction must be persevered in until this is effected, when the adjustment will be

correct.

By using both faces of the instrument in this manner and taking the mean of the two points P and P' each time the telescope is

rotated vertically, a straight line may be ranged out correctly even when the collimation line is out of adjustment.

The points B and p should be selected as nearly on a level as possible. In any case the difference of altitude of B and P should be small, otherwise part of the error may be due to adjustment being out, as the position of the supports is reversed.

AP should be much greater than AB. Any error will then be increased, and a more accurate adjustment will be effected.

By this method it will be observed that although we may have moved the instrument when rotating it through 180° in azimuth, the cross hairs are again adjusted exactly on to the point в before the telescope is directed on to the distant point P for the second time. The instrument is not liable to be disturbed, therefore, as the telescope has simply to be turned over vertically.

If the ends of the horizontal axis of the telescope be reversed by lifting out the telescope and replacing it, there can be no error due to the supports, adjustment 1, introduced, as the position of the supports is not reversed. It is, however, not easy to lift out the telescope and replace it without shifting the instrument. In this case, however, it is better to reverse the horizontal axis by lifting out the telescope, as the cross hairs are always made to bisect the point B again by means of the tangent screw d before the telescope is turned over on to the point P. We may then be sure that none of the error is due to adjustment 1 of the supports being out.

3. Adjustment of the Level on the Telescope.This level should be parallel to the collimation line of the telescope. In the transit theodolite it is to be adjusted precisely in the same way as the level, adjustment 2, page 214. In the plain theodolite, level up the level by means of the tangent screw of the vertical arc. Take the telescope out of the Y's, and replace it with the ends reversed. If the bubble deviates from the centre of its run, correct half the deviation by the screws which attach the level to the telescope, and half the deviation. by the tangent screw of the vertical arc. Repeat the operation until the bubble remains in the centre of its run, when the telescope is reversed end for end in the Y's. This makes the level parallel to the Y's, with the axis of which the collimation line is supposed to coincide. It is, however, more accurate to adjust

in the same way as described for the level, adjustment 2, page 214. This method of adjustment should be used when possible.

4. Index Error of Vertical Circle.-Having adjusted the collimation line and made the level attached to the telescope parallel to it, the vernier of the vertical circle should read zero when the bubble of the telescope level is in the centre of its run. If it does not, then the difference of the vernier reading from zero is the index error of the vertical circle which is the correction for each vertical angle observed.

The collimation line may be made to coincide with the axis of the telescope in the same way as described for the level, adjustment 1, page 213. This adjustment, if made, should be done before adjustment 2 or 3.

Besides the foregoing adjustments, the following points. should be attended to before making any of the permanent adjustments.

Centring of the Object Glass and Eyepiece.—The centre of the object glass a and of the eyepiece b should coincide exactly with the axis and collimation line of the telescope. As a rule this adjustment must be left to the instrument maker, as no means is provided for making it. The centring of the object glass is to be effected by focussing the object glass on to a very near point, and then twisting the telescope half round on its axis. If the image of the point continues to coincide with the intersection of the cross hairs, the object glass is correctly centred; but if not, half the deviation is to be corrected by moving the object glass, and the operation repeated until the image of the same point continues to coincide with the intersection of the cross hairs when the telescope is twisted round on its axis. The same operation is then to be repeated for a very distant point and again for a near point if necessary until the adjustment is correct. Before centring the object glass the cross hairs should be put exactly in the axis of the telescope.

As usually no means are provided for altering the object glass, it is a good plan to make a continuous scratch on the ring of the glass and its slide so as to be able to see at any time that the glass is always in the same position relatively to its slide. Otherwise if the glass happens to be not quite truly ground and a little eccen

tric, the adjustments may be put out if it is taken out to be cleaned and not screwed up again to precisely the same position. By means of the continuous scratch the glass may be always screwed up again into the same position.

The glasses of the eyepiece should also be similarly centred to the axis of the telescope so as to see the intersection of the cross hairs precisely in the centre of the field of view.

Adjustment of the Diaphragm Ring so that the Cross Hairs are truly Vertical and Horizontal.-The cross hairs are placed on the diaphragm ring exactly vertical and horizontal by the instrument maker, but sometimes in adjusting the collimation line the ring may be turned a little. To ascertain if one cross hair is vertical (in which case the other being at right angles to it will be horizontal), after having made the temporary adjustments, sight on to a plumb line. If the vertical hair coincides with the plumb line, the cross hairs are truly vertical and horizontal. If not, slightly loosen two adjacent screws of the four diaphragm screws d, d, d, d, and with a knife, key, or other small instrument, tap gently against the screw heads so as to turn the ring slightly in the telescope, and persevere until the vertical hair coincides with the plumb line. When this is effected, tighten up the screws again. This adjustment must never be made after adjustment of the collimation line without again testing the latter, otherwise it may be put out.

When there is no vertical hair in the diaphragm adjust the horizontal hair on to a levelled straight-edge.

To Replace the Cross Hairs.-Pull out the tube of the eyepiece b and loosen all the four diaphragm screws, and let the diaphragm ring fall out of the telescope. The best way to put on new cross hairs is to take a thread from a spider's web on a forked stick, and having placed a little varnish, glue, or shellac on the diaphragm ring, lay the hair across the ring, and bring it down exactly on the marks on the ring. Then put on the other hairs in the same way. To now replace the diaphragm ring, cut a piece of stick long enough to reach into the telescope to the place where the diaphragm is to be fixed, and point one end so as to fit into one of the screw-holes of the diaphragm. By means of this stick as a handle insert now the ring edgeways into the telescope, and