149.—Prismatic Compasses are made from 2½ to 6 inches in diameter. The compass needle is sometimes made to carry a card dial for the 2½-inch size; for larger sizes the ring is now made uniformly of aluminium. The reading of the compass ring is effected by means of a glass prism, Fig. 36, P, which is cut to 45° upon one face and 90° for the two others, one 90° face being worked convex, so as to give magnifying power simultaneously with reflection of the ring at right angles, so that the reading of the compass appears to stand erect before the user of the instrument, and to be considerably magnified. As the reading is made on the side of the ring nearest the observer, the figures on the ring are engraved right to left. The prism is placed in a box, with a vertical sight slit SS over it, which cuts a line with the centre of the top of the prism. The box with its prism moves upwards or downwards in a sliding fitting SL by means of a thumb nail stud, which adjusts the prism until it is in exact focus with the divisions on the ring. The back of the prism-box has a hinge H, so that this box may be closed down to the level of the compass-box to render it portable when out of use. On the opposite side of the compass-box to that upon which the prism is placed a long vertical window SV is attached, having a central hair placed so as to cut a direct line from the slit SS in the prism-box across the axis of the needle. This window-piece is jointed to turn down upon the face of the compass-box and simultaneously to lift the compass needle off its centre by a part of it pressing the outer end of the lifting lever L. To prevent too great a continuity of the oscillation of the compass needle and the ring, through unsteadiness of the hand in holding it, a pin is placed at S, through the compass-box under the window, which carries a light spring B that just touches the ring lightly when the pin is pressed in, and thereby brings the compass ring to rest, or fixes it for reading with some degree of certainty. An open ring under the prism-box is sometimes used for placing a piece of ribbon through it, to attach it to some part of the person to save dropping the compass accidentally when it is used in the hand. When the instrument is out of use a metal cover is provided to protect the glass. The instrument is uniformly carried in a leather case with strap to pass over the shoulder. As these instruments are often carried by military surveyors, they are better made of a stiff aluminium alloy, which makes the instrument less than half its ordinary weight.
150.—Additional Parts commonly provided with the prismatic compass are a mirror and sunshades, shown only in section Fig. 36. The mirror M is carried in a frame attached with a sliding piece to the window, upon which it can be placed either upwards or downwards. It is jointed with a hinge so as to be set at any angle. By reflection from the mirror, bearings in azimuth are taken much above or below the horizontal plane. Sun-glasses are also provided in front of the prism, which are used for taking the sun's place either with or without the mirror, a single sun-glass being also used very comfortably for working towards the sun at all times. The sun-glasses, which are simply small, dark-coloured glass circles in frames, are not shown in the engraving.
151.—To Prepare to take Observations with the Prismatic Compass. After the window and prism are opened out, the prism is adjusted to read the divided ring sharply when the compass is about level, by raising or lowering the prism P by pressure of the thumb and forefinger of the right hand upon the stud placed upon the prism slide fitting, shown below SL, until the divisions appear clear.
152.—In Using the Prismatic Compass, the compass-box is held with the thumb of the right hand under the prism at SL and the forefinger upon the stud S. The object which it is desired to observe is sighted through the slit SS, cutting the left-hand side of the hair in the window SV, while the division which comes opposite the reading point at its edge by the reflection from the prism is noted. The ring when free oscillates for a time, but is easily brought to rest for reading by gently pressing the pin S upon which the forefinger is placed.
153.—Where objects are observed for taking their bearings above the horizontal plane, the length of the window will be sufficient to take in a vertical angle of 20° to 30°; but for such altitudes it is necessary to take very great care that the compass is held level, to get magnetic angles even approximately true. Below the horizon, angles can be obtained with somewhat greater certainty by means of reflections from the mirror. Altogether, except for taking nearly horizontal angles, or for very close work in filling in after the theodolite, it is much better to have the prismatic compass mounted upon a tripod stand. With a stand, where the angle in azimuth is much above or below the horizontal plane, it is better to have a small glass level, described further on, art. 181, to place across the compass when setting it up. If the compass ring is very carefully balanced across 90° to 270° two bright wire points may be placed inside the compass-box, level with the compass ring, which will answer for the cross levelling.
154.—Stands.—The author has made a very simple and inexpensive tripod stand for the prismatic compass, the head of which consists of a ball and socket only, clamped by a large milled-head screw. An axis through the ball permits horizontal adjustment, shown in section, Fig. 37.
Fig. 37.—Improved prismatic compass stand.
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