Such is Mr. Iladley's own description of the origi nal sea octant, as it was called, for measuring the dis tances of the sun and moon from any of the fixed stars, for which purpose he proposed that it should be plac ed upon a stand. He has described, however, a modification of it, by which it may be held in the hand without any other support, and in which the telescope is to magnify four or five times. The object of this was to measure the altitude of the sun, moon, and stars, from the visible horizon at sea. It differed from the foregoing instrument, chiefly in placing the spe cula and telescope with regard to the sector and tubes. In the new form the line drawn along the middle of the index falls on the anterior surface of the larger speculum at an angle of about 4 or 5 degrees. The axis of the telescope or the line of sight falls on the surface of the second speculum at an angle of about 70 or 71 degrees. It has also a third speculum, to be used when the angle is greater than 90 degrees, for observing the sun's altitude, by means of the op posite part of the horizon. On this the line of direc tion of the sight falls at an angle of about 32 or 33 degrees.
Various improvements on this admirable instru ment have been made since the time of its invention. Grant, Ewing, Dollond, Magellan, Ramsdell, Mayer, Borda, and some of our living artists, have also im proved it. It has been modified in various ways; but whether it is in the form of a quadrant, a sextant, or a circle, the principle is always the same, viz. the re flexion of mirrors placed at different distances on the plane of the instrument. In our article CIRCLY, the reader will find the fullest details of all the most im portant improvements which the reflecting circle has undergone. Hadley's quadrant, as now fitted up, and the method of using it, is shown in Fig. 7, where FG is the divided limb, EH the moveable index, F. the index-glass or mirror, D the horizon-glass, and b the sight or eye-hole, where the eye of the observer is ap plied. When a telescope is used, it is placed in the position b13. The horizon-glass consists of a trans parent half, and a reflecting half, as described by Hadley. In taking the altitude of the sun, let BA be the visible horizon, and S the sun. The sun's rays SCE fall on the mirror at E., which is turned round, by moving the index Ell till the ray ED, which it re flects, reaches the eye at 13, after a second reflexion from the reflecting half of the horizon-glass D. The observer then keeps moving the index, till the lower or upper limb of the sun thus seen by reflexion is coin cident with or 'touches the visible horizon. The ver
nier II on the index will thus point out the altitude of the limb of the sun observed.* If S is the moon, and A a fixed star, the limb of the moon is brought to touch the star in the same manner, and their distance is thus obtained. When it is required to measure an angle between 90 and degrees, a second fixed spe culum is placed at K, so as to be at right angles to the moveable one E in its remotest situation. It will then produce a deviation of two right angles in one of the objects. Thus if s is a star, and a another star nearly opposite to it, the ray N E will be reflected in the di rection Eli, and again in the direction K e to the eye at e, which carries it in the direction e a. This is called the back observation, which is of great use when coasting along shores which intercept the hori zon of the sea, on the side in which the sun is; but the difficulties attending the rectification of the mirrors is so great that they arc rarely used, and are even suppressed upon most sextants.t When tl.e sun is obscured, or when the light of one of the objects is stronger than that of the other, the light is reduced by coloured glasses placed between E and D, as shown at C iu Fig. 14, or between B and K.
II. Description of Professor ,Imici's Prismatic Sextant This very ingenious instrument is represented in Plate CCCCLXXXV1I, Figs. 8, 9, 10. In Fig. S. ABC is a prism placed before the object glass E, so that its base AB is in a line with the axis of the teles cope directed to the distant object Q. The parallel rays from the object falling upon the face BC, will be refracted towards the base BA, where they will be to tally reflected, and will emerge from the face AC in lines parallel to their first direction. These rays fall ing upon the object glass E of a telescope, will form an image of the object Q, which will coincide with the direct image of the same object formed by the rays which pass below the prism. This coincidence will give the zero of the scale which measures the angu lar separation of these two images. If the prism is now turned round its edge A in the direction BCA, it will show new objects in succession coincident with the object Q, until the side AC shall be parallel to the object glass E. We shall now have the super-position of all those points that are 90° distant from the point Q, and therefore it is evident that we can thus mea sure all angular distances as far as 90° and a little more, as far indeed as 102° with common glass.