SEXTANT. The Sextant is a valuable instrument for measuring angles in navigation and surveying. There were various earlier instruments by Davis, Hooke, Newton, Godfrey and others ; but Hadley's Sextant, invented in 1730, superseded all previous trivances.
The Sextant has a plane glass, called the Index Glass, silvered behind, and perpen dicular to the face of the instrument. • It is fixed on a centre perpendicular to the instru• ment, and moves with the Index-Bar, the end of which slides over a graduated arc. There is another plane glass, the lower half of which, next the instrument, is silvered, and the upper half left clear. It is called the Horizon Glass and should be parallel to the index glass when the index points to at the beginning of the arc. The Sextant also has a telescope for viewing the objects observed; whereas in the common quadrants there is merely a plate with a small hole for directing the sight. Suppose a ray of light to proceed from the eye, it will proceed in the direction of the telescope, and if it falls on the upper or unsilvered part of the horizon glass, it will pass forward in a continued straight line until it falls upon some exterior object. But if the ray falls upon the silvered part of the horizon glass, it will be reflected to the index glass (the horizon glass is so placed as to make equal angles with lines from the eye, and index glass), and again reflected from the index glass outwards (i.e. from the observer),
until it meets some external object. Now instead of supposing the rays to pass from the eye, suppose them to come from external objects to the eye ; then there will be two images presented at the same time, one formed by the rays which pass through the unsilvered part of the horizon glass, and another formed by the rays which have been previously reflected by the two glasses ; and the graduated arc measures the angle between the glasses, and also the angle between the objects observed. The Sextant furnishes the means of measuring the angle between any two well defined objects, in whatever direction they may be placed (so that the angle does not exceed and without requiring more steadiness than is necessary for seeing the objects distinctly. There are sets of dark glasses of varying intensity, which may be turned before either the index or horizon glass when the sun's light is too intense.