MICROMETER, from the Greek words fcixfog, small, and I.GETVIV a measure, is the name given to an instrutnent for measuring small angular distances in the heavens, or small rectilineal spaces of any kind.
'rile micrometer was invented 'fly our countryman, Mr. Gascoigne, about the year 1640-1, at which time he drew up an account of it in a letter to Mr. Oughtred. It consisted of two pieces of brass, ground to a very fine edge, and their edges were made to approach to or recede from each other, by a mechanical contrivance. Mr. Gascoigne had made use of it for several years in measuring the diameters of the moon and planets, in ascertaining distances at land, and in many nice astronomical observations. According to Mr. R. Town Icy, into whose hands one of these micrometers fell, a foot could be divided into 40,000 parts. Mr. Gas -oigne had prepared a Treatise on optics for the press, but he was killed during the civil wars, in the service of Charles 1. The MS. of this treatise was never found.
It appears from the Ephemerides of the Marquis of Malvasia, published in 1662, that he employed a net of silver wire in the focus of the eye-glass of his telescopes, for measuring the diameters of the planets, the dis tances of the fixed stars, and for taking an accurate drawing of the lunar spots. He had also a contrivance for turning this net in the focus of the telescope, in order to cause the star to move along one of his wires, and he obtained the angular distance of the wires, by counting the number of seconds which a star required to pass over one of the intervals of his net.
In the year 1666, Messrs. Auzout and Picard, with out knowing any thing of the micrometer of Gascoigne, published the description of a micrometer, which con sisted of silver wires, or fibres of silk, which were open ed and shut by means of a screw.
The micrometer was soon pretty generally intro duced among the instruments of an observatory, and underwent great improvements : New forms of it were invented ; and by the successive labours of astronomers and opticians, it has been brought to a very high degree of perfection. These various improvements and new forms of the micrometer, will be described under the following nine chapters.
THE earliest micrometer of this description that ap pears to have been used by astronomers, was construct ed by the celebrated Huygens. In the focus of the eye-glass of his telescope, he fixed a brass plate with a circular aperture a little less than that of the eye glass. He measured the angle subtended by the dia
meter of this circle, by measuring the time of a star's passage over it, which he found to be 174 minutes. He then prepared two or three long and slender brass plates of various breadths, whose sides were very straight, and converged very gradually. In using these plates to measure the diameter of a planet, he slid one of them through two slits in the opposite sides of the tube, so that the plane of the long plates touched the plane of the circular aperture, or field ; and he then observed in what part of the plate the breadth of it just covered the whole planet. By taking this breadth between the points of a pair of fine compasses, and by comparing it with the diameter of the aperture, he oh tamed the apparent diameter of the planet. Sir Isaac Newton has remarked, that the diameters of the pla nets are always somewhat bigger, when measured in this way, than they ought to be. This error, however, might have been corrected by using long tapering slits in place of plates.
A fixed micrometer, or reticulum, for determining the relative places of stars, was invented and used by M. Cassini. It consists of four hairs a b, c d, e f, g h, (Plate CCCLXXV. Fig. L) crossing one another at right angles in the focus of the eye-glass at the same point i, so that the two first arc inclined 45° to the two last. The telescope is then directed, so that the pre ceding star may appear upon the hair a b, and is then turned about its axis till the star moves along a b. The time of the first star's arrival at the centre i, is t hen noted by a clock, and likewise the time of the subsequent star', arrival at the perpendicular hair e d. The interval be tween these times, converted into degrees and minutes, is the difference of the right ascensions of the two stars. In order to find the difference of their declinations, the times of the subsequent star's arrival at the points k and I of the oblique hairs e f, g h, is noted. The half of the interval between these times, is the time in which it describes l mn, or m k, which, converted into degrees and minutes, gives the angular distance / m ; and this being diminished, in the ratio of the radius to the co sine of the star's declination, gives the value of in i, the difference in declination of the two stars.