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Knight's Mechanical Encyclopedia (ed. Knight) 2 0 Browse Search
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Knight's Mechanical Encyclopedia (ed. Knight), H. (search)
. 1. (Optics.) A double objectglass micrometer (see micrometer), called a heliometer from its being originally used for measuring the angle subtended by the sun. The telescope, in whose field it is placed, is usually of large size and mounted equatorially; that is, having two axes of motion at right angles to each other; one parallel to the axis of the earth and the other to the equator. The instrument was suggested by Roemer about 1678, and brought into use by Bouguer about 1748. G. Chambers credits its application or improvement to De Charmeres, a French naval officer of the last century. The object-glass is divided diametrically, and the parts are capable of motion in their own planes, and through considerable intervals by means of screws, and thus their optical centers can be separated. Each half-glass forming a separate image of any object, the two images will be at an angular distance, dependent on the amount of separation of the centers of the two half-glasses. Dol