ADJUSTMENT AND USE OF INSTRUMENTS. IX. GENERAL REMARKS ON ADJUSTMENTS. 1. Care should be taken in all instrumental adjustments, where screws work in pairs, to loosen one before tightening its opposite. 2. Remember that the eye-piece inverts the image of the cross-hairs, and that consequently any movement of it, by means of the small capstan head screws on the outside of the telescope-barrel, should be in the direction which would seem to increase the error requiring correction. 3. Before beginning the adjustments, screw the object-glass close home, and make a pin-scratch across its rim and the end of the tube, by which to mark its proper place; draw out the eye-piece until the cross-hairs are exactly in focus; that is to say, until no movement of the eye shall appear to displace them, and bring the object to be observed clearly into view. 4. Never permit the glasses to be rubbed with a gritty fabric. To remove the dust from them, use a soft, clean handkerchief, and change often the part applied. 23 X. THE LEVEL. TO BRING THE INTERSECTION OF THE CROSS-HAIRS INTO THE OPTICAL AXIS OF THE TELESCOPE. 1. Set the instrument firmly, cast loose the wyes, and, by levelling and tangent screws, bring either of the cross-hairs to coincide with a well-defined object, distant from 400 to 600 feet, or as much farther as distinct vision can be had free from heat ripple. Gently rotate the telescope half-way around in the wyes. If the cross-hair selected for treatment then fails to coincide with the object, reduce the error one-half by means of the small capstan head screws at right angles to it on the telescope-barrel. Bring the spider-line again to coincide with the object by means of the levelling and tangent screws, and, if necessary, repeat the operation. Proceed in the same manner with the other cross-hair. If the error is large, bring both nearly right before undertaking their final adjustment. 2. Having thus adjusted the line of collimation upon a distant point, requiring the object-tube to be drawn well in, select a point close by, which shall require it to be thrust out almost to its limit. If any error appears, correct half of it with the small screws provided for the purpose, a little forward of the diaphragm, and usually protected by a movable sleeve on the outside; correct the other half with the levelling-screws. After completing this adjustment, test the former one on a distant object, and, if necessary, repeat the operations. 3. In the transit, the small guide-ring screws used for this adjustment are covered by the bulb of the cross-bar in which the telescope is fixed, and are therefore inaccessible. The adjustment, however, is one not liable to become deranged in either instrument, and, in the transit, is of comparatively small importance. 4. The young practitioner should bear in mind that the intersection of the cross-hairs may coincide with the optical axis of the telescope, and yet be out of centre as regards the field of view. Such eccentricity does not affect the working |