The arc is of 4 inches radius and is divided on silver to read 20″, is complete with shades and telescope and packs into a case 7 × 6 × 2½ inches.
Fig. 307.—Optical square.
Fig. 308.—Double optical square.
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686.—Optical Square.—This extremely handy little instrument is invaluable for taking offsets in chaining for any irregularity or obliquity to the right line in the boundaries of fields, hedgerows, fences, streams, etc., giving as it does instantly at sight a right angle to any object that may be sighted on either hand. The instrument is optically constructed exactly as a box sextant; but the glasses are fixed with their faces permanently at the angle of 45° to each other, by which means the reflection of 90° is truly given on principles fully discussed at the commencement of this chapter. This instrument being made very small, that is, 2 inches or less in diameter, it is found most convenient for manipulation to place the adjustments to the larger glass, that is, the index glass. The horizon glass, Fig. 307, h is therefore fixed firmly, like the index glass of the box sextant, by two screws to the sole plate. The index glass i is held and adjusted in exactly the same manner as the horizon glass of the box sextant, as shown in detail, [Figs. 299, 300], the only difference being that the frame which holds the glass is made of the entire height. The rim of the case of the optical square is formed of a short length, 3/8 inch to 5/8 inch, of a pair of telescope tubes which slide easily together. One of these is attached to the sole plate and the other to the cover, so that at first they close together as a box and lid. All the openings required for sight, as Fig. 307 at Q for horizon sight, o for index sight, and e for pin-hole or eye sight, are cut through the two tubes.
Fig. 309.—Optical square.
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687.—The inner case is cut in the plane of some part of the circumference of the instrument from a pin-hole into a bayonet notch, made with a horizontal slot for the two cases to revolve upon each other upon a pin, sufficiently to close and open the sight holes. This plan secures the instrument from any intrusion of dust when it is closed and out of use. An adjusting key is placed in the case, held by a tube or stud at the position k. The weight of the entire instrument is about 4 oz. if of ordinary make; but smaller ones are made in German-silver or silver, 1¼ inches diameter, 3/8 inch thick, weighing under 2 oz. These latter are very convenient for the waistcoat pocket, and are equally as exact as the larger instruments. Fig. 309 shows the general outward appearance of the optical square.