SEXTANT. 28 SEXTANT. the form of a sector embracing somewhat more than one-sixth (usually about one-fourth) of the whole circle; two mirrors (one wholly silvered and one silvered over one-half its surface) ; a movable arm pivoted at the centre of the sector and carrying the fully silvered mirror and a vernier; an arc along the circumference of the sector graduated into degrees, minutes, and sec- onds; and an eye-piece. The common form of the instrument is shown in Fig. 1. The frame is of brass. AA is the limb in which is inlaid a strip of silver on which are the gradu- ations of circular measure; the smallest divi- sions are usually 10' to 30', and the vernier en- ables angles to be read to at least 1' and usually to 10". The handle H by which the instrument is held in the hand is of wood. The mirrors M and in are of plate glass. The former has all its surface, while the latter has but the lower half silvered. Both are fitted with small screws for adjusting them in perpendicularity to the plane of the front face of the frame and in paral- lelism to each other when the inde.x arm is set at 0°. E is the eye-piece of the telescope, which is
Besides the ordinary telescope the instrument is usually provided with an inverting telescope, 1, and a tube without glasses, 1'" ; also colored eye- pieces to use in place of the colored shade glasses, P and Q, and an adjusting wrench or screw- driver. 'The theory of the instrument is shown in Fig. 2. AOC is the frame of the instrument in tlie form of a circular sector. VO is the in- dex arm carrying the index glass, I, and the ver- nier, V, and is pivoted at on the frame. H is the horizon glass, which is set in a clasp securely attached to the fxame in a position parallel to OC (the position of the index arm when set at 0° of the arc), but is susceptible of adjustment if thrown out of position. LO is parallel to MHT. To determine the angle at the eye (STil) between two distant objects, S and M, the pro- cedure is as follows: Turn the instrument until one object (M) can be seen through the telescope and the unsilvered half (which is tiie half farthest away from the plane of the instrument) of the horizon glass (H). Then turn the instru- ment until its plane coincides with that passing through both M and S. New move the imle.x arm until the reflection of S appears in the sil- vered half of H. By sliglitly turning the instru- ment both objects will be brought togetlier — one just on and one just clear of the edge of the silvered surface of H. Perfect the coincidence of the two objects and the reading of the vernier at V will give the angle. For purposes of navi- gation the angle commonly measured is that be- S / held in position by the adjustable clasp K. The mirror M is secured to the index arm S, which is pivoted beneath the centre of il and carries a vernier on its other end. R is a small magnifying glass for reading the vernier. C is the clamp for holding the index arm to the limb. B is the tangent screw for moving the arm slight- ly to perfect the angle; it only acts when the clamp screw C is set up. P and Q are colored shade glasses for use when observing the sun. Flu. 3. tween the sea horizon and the sun. moon, star, or planet. The angle is called the altitude of the heavenly body ; in the case of a star it can only be taken at twilight or when the moon is up," because the stars are not plainly seen by daylight and the horizon is not clearly visible at night. From an inspection of the sketch ( Fig. 3) it is readily seen that the angle through which the index arm moves is one-half that of the angle measured. For Ji = angle of incidence and n' =: the angle of reflection at the surface of the mirror 1 and X and X' the same at the mirror H; let LI be drawn parallel to HT. Then the angle measured is SIL = n + a; n' — a = » — a = x +x' = 2a;; II = x + 1/ ,• 2n. — n — a + 2y; n, -f a = 2y ; y = »■ .•. ». -f n = 2 IV. The arc, or limb, of the sextant has a gradu- ated scale cut in an inlaid silver strip. The fine- ness of the graduation varies; in high-grade instruments the smallest division of the scale is 10 minutes; in some cheaper instruments the
