. Astronomy for high schools and colleges . the instrument which carries the tele-scope at one end, and a counterpoise at the other. The whole in-strument can revolve 180° in azimuth about this pillar. The tele-scope has a micrometer at its eye-end, and it also carries a dividedcircle provided with a fine level. A second level is provided,whose use is to make the rotation axis horizontal. The peculiarfeatures of the zenith telescope are the divided circle and its at-tached level. The level is, as shown in the cut, in the plane ofmotion of the telescope (usually the plane of the meridian), and
. Astronomy for high schools and colleges . the instrument which carries the tele-scope at one end, and a counterpoise at the other. The whole in-strument can revolve 180° in azimuth about this pillar. The tele-scope has a micrometer at its eye-end, and it also carries a dividedcircle provided with a fine level. A second level is provided,whose use is to make the rotation axis horizontal. The peculiarfeatures of the zenith telescope are the divided circle and its at-tached level. The level is, as shown in the cut, in the plane ofmotion of the telescope (usually the plane of the meridian), and itcan be independently rotated on the axis of the divided circle, andset by means of it to any angle with the optical axis of the circle is divided from zero (0°) at its lowest point to 90° ineach direction, and is firmly attached to the telescope tube, andmoves with it. By setting the vernier or index-arm of the circle to any degreeand minute as a, and clamping it there (the level moving with it) THE ZENITH TELESCOPE. 91. Fig. 39.—the zenith telescope. 92 ASTRONOMY. and then rotating the telescope and the whole system about thehorizontal axis until the bubble of the level is in the centre of thelevel-tube, the axis of the telescopes will be directed to the zenithdistance a. The filar micrometer is so adjusted that a motion of itsscrew measures differences of zenith distance. The use of the ze-nith telescope is for determining the latitude by Talcottsmethod. The theory of this operation has been already given onpage 48. A description of the actual process of observation willillustrate the excellences of this method. Two stars, A and B, are selected beforehand (from Star Cata-logues), which culminate, A south of the zenith of the place of ob-servation, B north of it. They are chosen at nearly equal zenith dis-tances I* and 1^, and so that ^^—^^ is less than the breadth of thefield of view. Their right ascensions are also chosen so as to be aboutthe same. The
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