joining the third foot screw and the centre of the line joining the previous two foot screws.] Repeat step 2 and step 3 in the same quadrant till both the bubble remain central. By rotating the upper part of the instrument through 180°, the level tube is brought parallel to first two-foot screws in reverse order. The bubble will remain in the centre if the instrument is in permanent adjustment.Otherwise, repeat the whole process starting from step 1 to step 5. The same principle applies for a bulls-eye level: Bring the level parallel to any two of the foot screws, by rotating the upper part of the instrument. The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward. Rotate the upper part of the instrument through 180°, so the level is over the remaining foot screw. The bubble will remain in the centre if the instrument is in permanent adjustment. If not adjust this screw to halve the error. Then rotate back through 180° and check the error. Adjust those screws to halve the residual error. Continue until the bubble is always central on the ring. Focusing To obtain an accurate clear sighting, the cross hairs should be in focus; adjust the eyepiece to do this. Focusing of eyepiece lens For focusing of the eye piece, point the telescope to the sky or hold a piece of white paper in front of telescope. Move the eye-piece in and out until a distinct sharp black image of the cross-hairs is seen. This confirms proper focusing. To clearly view the object being sighted focus the objective lens. Focusing of objective lens It is done for each independent observation to bring the image of the object in the plane of cross hairs. It includes following steps of operation: First, direct the telescope towards the object for observation. Next, turn the focusing screw until the image of the object appears clear and sharp as the observer looks through properly focused eye-piece. If focusing has been done properly, there will be no parallax i.e., there will be no apparent movement of the image relative to the cross hairs if the observer moves his eye from one side to the other or from top to bottom. See also Permanent adjustments of theodolite Tape (surveying) Ranging rods Survey camp Local attraction References Surveying Civil engineering Engineering