MICROMER. These ingenious contri vances for measuring small spaces or angles with great accuracy or convenience, present many varieties.
Wire Micrometer.—This frequently consists of two fine wires or spider's threads, of which one is fixed and the other moveable, in the eye-tube of a telescope at the place where the image is formed. The moveable wire is in a sliding plate which, by a screw, is moved parallel to the other, and perpendicularly to the axis of the telescope, till the object appears to be com prehended between thewires ; and a graduated scale is provided for the measurement of the angle. Each division is equal to the extent of space moved by the wire during one revo lution of the screw, and the head of the screw is graduated, in order to subdivide the scale. The instrument was invented by Mr. Gas coigne in the seventeenth century, but it ap pears to have been afterwards neglected, and was re-invented by M. Auzout. The micro meter microscope, for reading off the divi sions of graduated circles, is an instrument of this kind.
The Position Wire Micrometer has lately come very much into use for observations of double stars, and is the wire micrometer proper.for equatorials. In this construction there are two wires parallel to each other, each moveable by its own screw : the whole appa ratus can also be turned round in the plane of the wires, so as to place the wires in any direction, the angle round which it is turned being read off by two verniers upon a small circle called the position circle. The mode of measuring double stars by this instrument , is a very refined problem in practical as tronomy.
The Divided Object-Glass Micrometer. If an object-glass be cut across so as to form two semicircles, and the semilenses be sepa rated by sliding one beyond the other, each portion will form its proper image, and these will retreat from each other as the semilenses are moved. The semilenses are mounted on
slides, and the quantity of separation read off upon a scale.
The Divided Eye-Glass Micrometer. This consists of a convex lens bisected in the direc tion of a diameter, and placed either at the end of a telescope nearest the eye, or between some of the other lenses of the eye-piece; it acts as a micrometer by separating the two halves of the lens, as in the former case.
The Reticule Micrometer is employed where less accurate measurement is required. The reticule, or diaphragm, as it is sometimes called, is any fixed arrangement of wires or bars which can be applied to a telescope for the purpose of measurement. Suppose, for example, a cross like an X to be cut out of brass plate, and inserted in the principal focus of a telescope. A star, in passing through the field of the instrument, is occulted at its passage behind each of the bars, and the time noted. The interval will show, by an easy calculation, how far it passes from the vertex; and the mean of the times, the moment when it passes the axis of the diaphragm. This. reticule is very convenient for mapping, it placed in the meridian, or for cometary obser vation, if the telescope is mounted as an equa. torial, however rudely.
The Circular Micrometer. A. metal ring is set in the centre of a perforated glass plate, and the outer and inner edge of the ring is turned true. The plate is fixed in the focus of a telescope, arid the observer notes the time when a star disappears at the outer ring, re appears on the inner ring, disappears again, and finally reappears. These data afford the means for measuring small angles.
It need hardly be stated that manufacture of such apparatus as the above must take a high rank in philosophical instrument making.