SEXTANT, an instrument for measur ing the angular distance of objects by means of reflection. The principle of its construction depends on the theorem that if a ray of light suffer double reflection the angle between the original ray and its direction after the second reflection is double the angle made by the reflecting surfaces. The instrument of which this theorem is the principle is a brass sector of a circle in outline, the sector being the sixth part of a complete circle, for which reason the instrument is called a sextant. It consists of a graduated limb, forming about the sixth part of a circle. Two mirrors, the index mirror (1) and the lower part of the horizon glass at 5, have for object the reflection of the rays of light coming from the object under observation. The telescope (2) collects and transmits to the eye the rays of light emanating from the horizon glasses. An index and a vernier (13) permit the read ing, on the graduated limb, of the quan tity of light which the mirror has turned. When observing altitudes, the instru ment is held perpendicularly to the hori zon in the plane of the line joining the two objects. In taking noon observations
at sea, to determine the latitude, the ob server takes his place shortly before meridian, and turning down one or sev eral of the shades, to prevent his eye being injured by the glare, directs the telescope or site tube to the sun, moving the index so as to bring its reflected image to coincide with the sea horizon; as the sun rises, he gradually advances the limb, clamping it and using the regu lating screw for this purpose, as the sun's path becomes more nearly horizontal, and slightly rocking the instrument from side to side to insure that it is in a vertical plane at the moment when the sun at tains its greatest height. The reading of the limb at the moment when the sun begins to dip is noted, and a very simple calculation., adding his declination de rived from the "Nautical Almanac" to the true zenith distance obtained by ob servation, gives the latitude.