Fig. 285.—Nautical or astronomical sextant.
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627.—The Limb G, which is made only about 1/12 inch in thickness, has generally a face of about ¾ inch in width, which is inlaid with silver or platinum, as [Fig. 127], p. 186, to take the graduation to about 140°. The limb is stiffened by a deep, thin rib about ½ inch wide, supported by a corner hollow. The exterior radial arms and interspace framing, Fig. 285 MM, which vary very much in design according to the taste of the maker, is made generally of about 1/14 of an inch in width upon the face of the bars, with a depth of 3/8 inch. This arrangement of the bars placed edgewise gives great stiffness to the surface of the arc with little weight. A handle L, made generally of ebony, is supported on two standards or brace-pieces N, which are carried off to about 2 inches from the back of the frame to hold the handle parallel with the face. The handle has sometimes a hole bushed through it with metal, to support the sextant upon a corresponding pin forming part of a stand or tripod when the instrument is used for taking observations on land. Three feet are placed at the corners of the frame of the sextant, one shown at Q, to support it conveniently on a table to take the reading of an observation just made.
628.—At the centre of the arc a female axis of about 1½ inches in depth E is attached by three screws to the frame perpendicular to the plane of graduation. This carries the male axis, which centres the vernier on the vernier arm M. The axis is covered by a protecting tube which forms one of the three feet upon which the instrument rests when laid down. The vernier arm is made of gun-metal of about 1/16 inch in thickness and from 1 inch diminishing to ¾ inch in width. This is stiffened by a light rib on its upper side.
629.—The Vernier V reads upon an 8-inch sextant, that is, one of eight inches radius, to 10″, the graduations being to 20′ and the vernier taking 120 divisions. A description of the vernier reading was given, [art. 318]. The vernier falls upon the arc on the plan shown [Fig. 127], p. 186. It is clamped near to position by the milled-headed screw H, and is adjusted by the tangent I. A magnifier J is placed on a jointed sling-piece K which traverses the vernier. This is sometimes provided with a ground glass shade to dull the silver for reading. The sling-piece moves the magnifier opposite to any division of the vernier.
630.—Over the axis of the vernier arm a large, oblong mirror, termed the index glass, A, is fixed with its face in a plane cutting the centre of the axis. The index glass is placed with its longest sides approximately in line with the vernier arm. This mirror is placed in a metal tray and is sometimes made adjustable by three screws; but it is better fixed by the maker by screwing the flange-piece, which forms one end of the tray, hard down. The index glass moves with the index arm and gives the first reflection of sun, moon, or star which falls thence upon the horizon glass B.
631.—The Horizon Glass B is placed upon a spur-piece formed in the same casting as the frame. This glass, which is worked perfectly parallel, has the lower half of its surface next the frame silvered. The silver is cut to a sharp line against the plain part. The horizon glass placed in its metal tray has adjustments given to it by means of capstan-headed screws in a manner that will be presently described.
632.—The Telescope screws into a ring fitted at R, which stands upon a bar erect from near the edge of the frame. The female screw by which the telescope is held is formed of two rings which adjust for the amount and direction of separation, so that the telescope may be directed coincident with the horizon glass. The bar or standard supports the ring fitting and is made of either square or triangular section, fitted accurately in a deep socket fitting, in which it slides to raise or lower the ring by means of a milled-headed screw placed on the end of the bar. This permits adjustment only sufficient to bring the axis of the telescope opposite the line of division between the plain and silvered parts of the horizon glass.
633.—Four Circular Shades, carried in square frames fitted with dark bluish-grey glasses, are jointed to the frame at C. These have nib-pieces at the upper corners, so that one or more of the shades may be turned up at a time by the finger-nail to intercept any surplus amount of light from a luminous body reflected from the index-glass; or the whole of the shades may be turned up when observation is made of the mid-day sun. Three other similar shades, but placed in circular frames are fixed at D, which hinge over and back, to be thrown in or out of interception, and are used to subdue the light from the horizon if required.