The box sextant is an instrument about 3in. in diameter, to be held in the hand, for ascertaining approximate angles between any given stations. It is made with or without a telescope, and is in general appearance like Fig. 1. An enlarged diagrammatic plan is shown in Fig. 2, where A is the sight hole of the telescope; B is a fixed glass, the lower half silvered and the upper half plain; c is a mirror attached to the same pivot as the vernier arm D. The side of the case is open at E and 1' to admit the rays of light from the observed objects. The required angles may be between station poles, church spires, or any other definite lines or points. Suppose a single pole be looked at, the angle indicated should be 0° or zero; whether it will actually be so or not depends upon circumstances which the following remarks will explain. Suppose a pole to be fixed at G, which, bearing in mind the scale, would be abnormally close, it can be seen through the clear part of the glass at B on applying the eye to the sight hole at A. At the same time the rays of light from the pole (G will be streaming in all directions, and some of them will pass along the dotted line direct to the mirror C, and. when the vernier arm is placed in the position shown by the dotted line, the rays of light will be reflected to the silvered part of the glass B, and from thence to the eye at A, the appearance being as of one continuous pole down the two parts of the glass.
If the vernier be now examined, it will be seen that the broad arrow falls short of the zero of the scale owing to what may be called the width of base line of the instrument. If tip-pole be placed farther off as at H, the rays of light following the stroke-and-dot line will require the vernier arm to be shifted rather nearer the zero of the scale; but until the pole is at a distance of two chains from the observer there will be a similar error of less and less amount. Between two chains distance and an infinite distance the rays of light from the pole to B and C are now so nearly parallel that the error is under one minute of arc, so that the instrument can be used without difficulty under those conditions, it is usually adjusted by sighting it to the sun, which should appear through the smoked glass as a perfect sphere in whatever way the sextant may be held when the vernier is at zero. When an angle is to be taken at one station and between two others, the nearer station should be viewed through the plain glass, so that the sextaut may need to be held upside down. "When the angle to be read exceeds 900, an intermediate pole should be set up and the angles taken in two portions, as in viewing large angles the mirror c is moved so far round that its reflection, and that of the image it carries, is viewed almost edgeways in the mirror at B. The vernier arm is moved by means of a milled head screw on the top of the case.
It should be noted that the box sextant only gives angles in the plane of the instrument, so that if the stations observed are cot on the same Level, the angle given will be the direct angle between them, and not the horizontal angle such as would be given by a theodolite.
How To Use the Box Sextant.