File:Astronomy for high schools and colleges (1881) (14763797665).jpg

Identifier: astronomyforhigh00newc (find matches)
Title: Astronomy for high schools and colleges
Year: 1881 (1880s)
Authors: Newcomb, Simon, 1835-1909 Holden, Edward Singleton, 1846-1914, joint author
Subjects: Astronomy
Publisher: New York, H. Holt and Company
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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each circle isviewed by four microscopes, two of which, i? B^ areshown in the cut. The microscopes are 90° apart. Thecut shows also the hanging level L Z, by which theerror of level of the axis ^ ^ is found. The instrument can be used as a transit to determineright ascensions, as before described. It can be also usedto measure declinations in the following way. If the tele-scope is pointed to the nadir, a certain division of the cir-cles, as iV, is under the first microscope. If it is pointedto the pole, the reading will change by the angular distancebetween the nadir and the pole, or by 90° + , 0 beingthe latitude of the place (supposed to be known). Thepolar reading P is thus known when the nadir readingiVis found. If the telescope is then pointed to variousstars of unknown polar distances, p^ p^p, etc., as theysuccessively cross the meridian, and if the circle readingsfor these stars are P, P, P , etc., it follows thatp = P-P ; p = p ^p.^ y// = p _ p^ etc. 84 A8TB0N0MT,
Text Appearing After Image:
Fig. 35.—^the meridian circle. THE MERIDIAN CIRCLE. 85 To determine the readings P, P, P, etc., we use the micro-Bcopes P, P, etc. The observer, after having set the telescope sothat one of tlie stars shall cross the field of view exactly at its cen-tre (which may be here marked by a single horizontal thread inthe reticle), goes to each of the microscopes in succession andplaces his eye at A (see Fig. 1, page 80). He sees in the field of themicroscope the image of the divisions of the graduated scale (Fig. 2)formed at D (Fig. 1), the common focus of the lenses A and C.Just at that focus is placed a notched scale (Fig. 2) and twocrossed spider lines. These lines are fixed to a sliding frame a a,which can be moved by turning the graduated head F. This headis divided usually into sixty parts, each of which is 1 of arc onthe circle, one whole revolution of the head serving to move thesliding frame a a, and its crossed wires through 60 or V on thegraduated circle. The notched scale is no

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