The end of the eye-tube is represented in Fig. 11, where CD is the circular head, divided into 360°, and subdivided by the vernier V; L is the level, and AB the part of the eye-piece which contains the diaphragm with the fixed and moveable wires. The head CD, and the level L, are firmly fixed to the eye-tube '1', and from the head CD there rises an annular shoulder concentric with the tube, and containing the diaphragm across which the fixed wire is stretched. This diaphragm, which is re presented in Plate CCCLXXV1. Fig. 12. with the wire extended across, projects through the circle of brass EF. All these parts remain immoveable, while the outer tube AB, and the other half EF of the circu lar head which contains the vernier V, have a rotatory motion upon the shoulder which rises from CD. The tube AB is merely an outer case to protect a little tube within it, which contains the eye-glass, and the moveable diaphragm with its fibre extended across it. The in closed tube is screwed into the ring EF, and the outer tube is also screwed upon the same ring ; so that by mov ing AB, a motion of rotation is communicated to the vernier V, and to the diaphragm and wire belonging to the inner tube, while the rest of the eye-piece, contain ing the other diaphragm with its wire, remains stationary. By this means the moveable wire is made to form every possible angle with the fixed wire, and the angle is de termined by the method which we have already explained. The fixed wire is placed a little out of the centre of the diaphragm to which it belongs, and the diaphragm itself is placed in a cell, in which it can be turned round, so as to adjust the wire to a horizontal line, when the level is set. The moveable wire is likewise placed at a little distance from the centre of its diaphragm, as repi esented in Plate CCCLXXVI. Fig. 13 ; but by means &f screws which pass through the inner tube into the edge of this diaphragm, it can be moved in a plane at right angles to the axis of the eye-piece, so that the moveable wire may be placed either in the centre of the field, or at different distances from it. See Y'reatise on New Phil. Inst. p. 112.
On Lucid Disc, and Luminous Image Micrometers In measuring the distance of close double stars, Sir William Herschel found it difficult to apply the common wire micrometer, from the various sources of imperfec tion to which it is liable, but particularly from the neces sity of illuminating the wires. Ile, therefore, set him self to construct a micrometer for this particular purpose, and in this way he contrived the lamp micro meter.
This instrument is represented in Plate CCCLXXVI. Fig. 14. " where ABGCFE is a stand nine feet high, on which a semicircular board qhog /z is moveable up ward or downward, in the manner of some fire-screens, as occasion may require, and is held in its situation by a peg p put into any one of the holes of the upright piece AB. This board is a segment of a circle of 14 inches radius, and is about three inches broader than a semi circle, to give room for the handles r D, e P, to work. The use of this board is to carry an arm L, 30 inches long, made to move on a pivot at the centre of the circle, by means of a string, which passes in a groove on the edge of the semicircle ftgohg; the string is fastened to a hook o, (not expressed in the figure, being at the back of the arm L,) and, passing along the groove from o h to q, is turned over a pulley at q, and goes down to a small barrel e, within the plane of a circular board, where a double jointed handle e P commands its motion.
By this contrivance we see the arm L may be lifted up to any altitude from the horizontal position to the perpendicular, or be suffered to descend by its own weight below the horizontal to the reverse perpendicu lar situation. The weight of the handle P is sufficient to keep the arm in any given position ; but if the motion should be too easy, a friction spring applied to the bar rel will moderate it at pleasure.
In front of the arm L a small slider, about three inches long, is moveable in a rabbet from the end L to wards the centre, backward and forward. A string is fastened to the left side of the little slider, and goes to wards L, where it passes round a pulley at m, and returns under the arm from m n, towards the centre, where it is led in a groove on the edge of the arm, which is of a cir cular form, upward to a barrel (raised above the plane of the circular board) at r, to which the handle r D is fastened. A second string is fastened to the slider, at the right side, and goes towards the centre, where it passes over a pulley n, and the weight se, which is sus pended by the end of the string, returns the slider to wards the centre, when a contrary turn of the handle permits it to act.
Two small lamps, 2 inches high, 11 in breadth, by 11 in depth, are shown at a and b. The sides, back, and top, are made so as to permit no light to be seen, and the front consists of a thin brass sliding door. The flame in the lamp a is placed of an inch from the left side, from the front, and half an inch from the bottom. In the lamp b it is placed at the same height and distance, measuring from the right side. The wick of the flame consists only of a very thin lamp-cotton thread ; for the smallest flame being sufficient, it is easier to keep it burning in so confined a place. In the top of each lamp must be a little slit, lengthways, and also a small opening in one side near the upper Part, to permit air enough to circulate, to feed the flame. To prevent every reflection of light, the side opening of the lamp a should be to the right, and that of the lamp b to the left. In the sliding door of each lamp is made a small hole with the point of a very fine needle, just op posite the place where the wicks are burning, so that when the sliders are shut down, and every thing dark, nothing shall be seen but two fine lucid points of the size of two stars of the third or fourth magnitude. The lamp a is placed so, that its lucid point may be in the centre of the circular board, where it remains fixed. The lamp b is hung to the little slider which moves in the rabbet of the arm, so that its lucid point, in a horizontal position of the arm, may be on a level with the lucid point in the centre. The moveable lamp is suspended on a piece of brass fastened to the slider by a pin exact ly behind the flame on which it moves as a pivot. The lamp is balanced at the bottom by a leaden weight, so as always to remain upright, when the arm is either lifted above, or depressed below the horizontal position. The double-jointed handles r D, e P, consist of light deal rods, ten feet long, and the lowest of them may have di visions, marked on it near the end P, expressing exactly the distance from the central lucid point, in feet, inches, and tenths.