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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

hold it there until two opposite screws are put in place and screwed into the ring. Now pull the stick out of the screw-hole of the diaphragm, and with it turn the diaphragm about the two screws already screwed into the ring until the other two screw-holes are in their proper position. Screw in the other two screws, and adjust the diaphragm for verticality of the cross hairs and for collimation. Glass diaphragms will save any of this trouble with cross hairs.

To Replace a Spirit Level or Bubble Glass.-Remove the level from the instrument, pull off its sliding ends, and take out the broken glass. Put in the new one with the graduated side up. Roll some paper round its ends if it fits loosely. Putty or melted beeswax round the ends of the glass will hold it firmly in its tube.

Adjustment of the Sextant-For adjustments of the sextant, see Chapter X.

Adjustment of the Box Sextant.-When the box sextant is in adjustment both the mirrors are at right angles to the plane of the instrument, i.e., the top of the box, and when the vernier is at zero are parallel to each other. The index glass is permanently fixed by the maker. The horizon glass is adjusted as follows:Bring the zero of the vernier to the zero of the graduated arc, and look through the eyehole and the unsilvered half of the horizon glass at some distant object. If the instrument is in adjustment this object and its reflection will appear to coincide exactly. If not, the two will appear to be separated either horizontally or vertically, or both, as **. In this case, apply the key furnished with the instrument to the square-headed screw in the top of the box, and bring the object and its image into a horizontal position, thus * *. Then apply the key to the other square-headed screw in the side of the box, and bring the object and its image to coincide exactly. The instrument will then be in adjustment. See also adjustment of sextant, Chapter X.

Adjustment of the Compass.-The adjustments of the compass are as follows:

To make the Needle truly Horizontal.-Level the compass, and then see if the needle is level. If not, make it level by moving the wire which is fastened round it towards the high end.

To make the Sights Perpendicular to the Compass Plate. Hang up a plumb line, and having levelled up the compass sight on to it, see if the slits coincide with it. If the slits should not be in adjustment, they may be rectified by unscrewing the sights, and filing off a portion of the feet on the high side, otherwise they may be wedged up on the low side.

To Straighten the Needle.-Having levelled the compass, note if its two ends continue to point to exactly opposite degrees, while the compass is revolved completely. If they do, the needle is straight, and the pivot is in the centre of the graduated circle; if not, one or both of these are wrong. Level up the compass, then turn it until some graduation (say 90°) comes exactly to the north end of the needle. If the south end does not then point exactly to the opposite 270° division, lift off the needle, and bend the pivot pin to make it do so, noticing that every time the point is bent the compass must be turned slightly so as to put the north end of the needle at its 90° mark. Then turn the compass through 180°, or until the 270° mark comes exactly to the north end of the needle. Make a mark where the south end of the needle now is. Then remove the needle and bend it until its south end points midway between the 90° and the mark, while its north end is kept at 270° by moving the compass slightly.

Adjustment of the Pivot Pin.-After having straightened the needle, revolve the compass until a place is found where the two ends of the needle coincide with opposite degrees. Then rotate the compass through 90°. If the needle then coincides with opposite degrees the pivot pin is correct; but if not, the pivot pin is to be bent until it does. Repeat the operation until the needle coincides with opposite degrees while the compass is turned through a whole circle.

Pocket Magnifier.-In using a pocket magnifier for the compass readings, take care that it is held with its centre precisely over the point to be read and parallel to the graduated circle. Otherwise errors of several minutes may be made in a single reading.

Magnetic Attraction about the Person.-No part of the magnifier should be made of iron, as this will attract the

needle. Care should be taken that nothing is carried about the person, as knives, keys, &c., that may attract the needle. Trouble is sometimes experienced from felt hats, which have sometimes an iron wire round the inside of the brim to stiffen them.

Sight Slits. The sight slits are fixed by the instrument maker in line with the 360° and 180° divisions of the graduated circle. They may be tested by passing a fine thread through them, and observing whether it stands exactly over the 360° and 180° marks.

Remagnetising the Needle.-The needle sometimes loses part of its magnetism and becomes sluggish. It may be magnetised by drawing the north pole of a magnet several times from the centre to the south end of the needle, and the south pole in the same way from the centre to the north end of the needle, rubbing the magnet gently upon the needle. Take the magnet away from the needle while bringing it back to the centre. The needle should be held flat on a smooth hard surface while being magnetised. Bad action of the needle is more often due to defect in the point of the pivot pin. Remagnetising throws the needle off the level, and this is to be adjusted by moving the sliding wire.

Adjustment of the Level.

1. To make the Collimation Line coincide with the Axis of the Telescope.-Drive in three pegs as at A, B, C, Fig. 163, an equal distance apart, say 3 or 4 chains. Set up the level at a exactly half way between A and B, and read the staff held on the pegs at A and B. Then set up at b midway between B and c, and read the staff held on the pegs B and C. Then whether the level is in adjustment or not, because it has been set up midway between A and B, any errors of adjustment will affect the staff readings at A and B equally and similarly; the difference of the readings will therefore give the correct difference of level of A and B. Similarly the difference of the staff readings at B and C will give the correct difference of level of B and c. Now set up at c, Fig. 164, as near to A as it is possible to read the staff held at A, and read the staff at A, B, C. From these staff readings compute the differences of level of A, B, C. Compare these with the correct differences of level previously found and


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