MIC 198 M I C 



MicUe. da Hill," an epistle from Lisbon. Having acquired some His poems were published in 179*, in one Yolume, 

 ' property, he returned to England, and settled at 4to. 

 Whitby, near Oxford, where he died in the year J 769- 



MICROMETER. 



Mi* OHM- i HE word MICROMETER, from the Greek words w 

 * small, and fu-r^i, a measure, is the name given to an 

 instrument for measuring small angular distances in 

 the heavens, or small rectilineal spaces of any kind. 

 Micrometer The micrometer was invented by our countryman, 

 invented by Mr. Gascoigne, about the year 1640-1, at which time 

 Mr. Gas- he drew up an account of it in a letter to Mr. Oughtred. 

 coijtne. r t cons i s ted of two pieces of brass, ground to a very 

 J 6*0 1. ^ ne e $g e> all d 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- 

 ley, into whose hands one of these micrometers fell, a 

 foot could be divided into 40,000 parts. Mr. Gas- 

 eoigne had prepared a Treatise on optics for the press, 

 but he was killed during the civil wars, in the service 

 of Charles I. 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 

 opened 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 

 i nvented ; and by the successive labours of astronomers 

 and opticians, it has been brought to a very high de- 

 gree of perfection. These various improvements and 

 new forms of the micrometer, will be described under 

 the following nine chapters. 



I. On fixed micrometers with an invariable scale. 



II. On Wire-micrometers, in which the wires are 



opened and shut mechanically. 



III. On Wire micrometers, in which the wires are 



-opened and shut optically. 



JV. On Double image micrometers, in which the 

 lenses or mirrors are opened and shut mecha- 

 nically. 



V. On Double image micrometers, m which the 



lenses, or mirrors, or prisms, are opened and 

 shut optically. 



VI. On Angular or Posirioli micrometers. 



VII. On Lucid disc micrometers. 



VIII. On Doubly refracting micrometers. 



IX. On Micrometers for microscope*. 



Microme- 

 ter. 



1. On Fixed Micrometers tvith an Invariable 

 Scale. 



THE earliest micrometer of this description that ap- On 

 pears to have been used by astronomers, was construct- microme- 

 ed by the celebrated Huygens. In the focus of the ters with 

 eye-glass of his telescope, he fixed a brass plate with an invari- 

 a circular aperture a little less than that of the eye- able " ale - 

 glass. He measured the angle subtended by the dia- Huygens's 

 meter of this circle, by measuring the time of a star's microme- 

 passage over it, which he found to be 17^ minutes. ter - 

 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 ob- 

 tained 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 Cassini'i 

 the relative places of stars, was invented and used rcticulum, 

 by M. Cassini. It consists of four hairs a b, c d, ef, PI-ATE 

 g h, crossing one another at right angles in the to- 

 cus of the eye-glass at the same point i, so that the 

 two first are inclined 45 to the two last. The te- 

 lescope is then directed, so that the preceding 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 then noted by a clock, 

 and likewise the time of the subsequent star's arrival at 

 the perpendicular hair e d. The interval between 

 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 h and 

 / of the oblique hairs ef, gh, is noted. The half of 

 the interval between these times, is the time in which 

 it describes Im, or m k, which, converted into degrees 

 and minutes, gives the angular distance 1m ; and this 

 being diminished in the ratio of the radius, to the co- 

 sine of the star's declination, gives the value of m i, 

 the difference in declination of the two stars. 



This reticulum was much improved by pur eminent 

 countryman, Dr. Bradley. In order to avoid the incon- impoed 

 venience of turning the telescope about its axis, he retioulu, 

 placed the ring a be, Fig. 2. of the reticulum, in a r 'S- * 

 groove cut in the fixed ring ABC, and having confined 

 it laterally by three small plates of brass at A, B, C, he 

 gave it a motion round the axis of the tube by the end- 

 less screw DE F, working in a toothed arch de fixed 

 to the moreable ring a lie. The hairs g h, ik erow one 



