242 M I C M I C MICKLE, WILLIAM JULIUS (1734-1788), son of the minister of Langholm, Dumfriesshire, holds a respectable place among the imitative minor poets of the 1 8th century. He wrote a poem on Knowledge carefully versified, pointing a moral on the vanity of intellectual pride at the age of eighteen, entered into business as a brewer at his father s request and against his own inclinations, soon became bankrupt, went to London on outlook for work as a man of letters, solicited patronage in vain, earned a living hardly by writing for magazines, made some impression in 1765 by "a poem in the manner of Spenser " called the Concubine (afterwards Syr Martyn), was appointed corrector to the Clarendon Press, and finally took a place among the leading poets of that very barren time by a translation of the Lusiad of Camoens into heroic couplets (specimen published 1771, whole work 1775). So great was the repute of the work that when Mickle appointed secretary to Commodore Johnstone visited Lisbon in 1779 the king of Portugal gave him a public reception. As a translator of Camoens Mickle has been superseded, but he aimed, not at close rendering of the original, but at making a poem which should be worthy of a permanent place in English literature. This ambition he was not capable of fulfilling, though he had great fluency and vigour. It may be doubted whether the fashionable forms which he imitated were the best suited to his natural gifts. He shows delight in lively action, a sense of dramatic effect, and, in the Concubine, the sub stance of which might have been conceived by Crabbe, considerable fulness of detail in coarse realistic painting. Certainly, if the Scottish poem There s nae luck aboot the hoose was Mickle s, he mistook his medium. Scott read and admired Mickle s poems in his youth, and, besides founding Kenilworth on the ballad of Cumnor Hall, was a good deal influenced by him in style. Mickle s prose is lively and vigorous. MICKOMETEE, an instrument generally applied to telescopes and microscopes for measuring small angular distances with the former or the dimensions of small objects with the latter. Before the invention of the telescope the accuracy of astronomical observations was necessarily limited by the angle that could be distinguished by the naked eye. The angle between two objects, such as stars or the opposite limbs of the sun, was measured by directing an arm furnished with fine " sights " (in the sense of the " sights " of a rifle) first upon one of the objects and then upon the other, or by employing an instrument having two arms each furnished with a pair of sights, and directing one pair of sights upon one object and the second pair upon the other. The angle through which the arm was moved, or, in the latter case, the angle between the two arms, was read off upon a finely graduated arc. With such means no very high accuracy was possible. Archimedes concluded from his measurements that the sun s diameter was greater than 27 and less than 32 ; and even Tycho Brahe was so misled by his measures of the apparent diameters of the sun and moon as to conclude that a total eclipse of the sun was impossible. 1 Maestlin in 1579 determined the relative positions of eleven stars in the Pleiades (Historia Coelestis Lucii Baretti, Augsburg, 1666), and Winnecke has shown (Monthly Notices R. A. S., vol. xxxix. p. 146) that the probable error of these measures amounted to about 1 Grant, History of Physical Astronomy, p. 449. 2 This is an astonishing accuracy when the difficulty of the objects is considered. Few persons can see with the naked eye much less measure more than six stars of the Pleiades, although all the stars measured by Maestlin have been seen with the naked eye by a few individuals of exceptional powers of eye-sight. The invention of the telescope at once extended the possibilities of accuracy in astronomical measurements. The planets were shown to have visible disks, and to be attended by satellites whose distance and position angle relative to the planet it was desirable to measure. It became, in fact, essential to invent a "micrometer" for measuring the small angles which were thus for the first time rendered sensible. There is now no doubt that William Gascoigne, a young gentleman of Yorkshire, was the first inventor of the micrometer. Crabtree, a friend of his, taking a journey to Yorkshire in 1639 to see Gascoigne, writes thus to his friend Horrocks. " The first thing Mr Gascoigne showed me was a large telescope amplified and adorned with inventions of his own, whereby he can take the diameters of the sun and moon, or any small angle in the heavens or upon the earth, most exactly through the glass, to a second." The micrometer so mentioned fell into the possession of Mr Richard Townley of Lancashire, who exhibited it at the meeting of the Iloyal Society held on the 25th July 1667. The principle of Gascoigne s micrometer is that two Gas- pointers, having parallel edges at right angles to the coigne i measuring screw, are moved in opposite directions sym- !1 metrically with and at right angles to the axis of the ni telescope. The micrometer is at zero when the two edges are brought exactly together. The edges are then separated till they are tangent to the opposite limbs of the disk of the planet to be measured, or till they respectively bisect two stars, the angle between which is to be determined. The symmetrical separation of the edges is produced and measured by a single screw ; the fractions of a revolution of the screw are obtained by an index attached to one end of the screw, reading on a dial divided into 100 equal parts. The whole arrangement is elegant and ingenious. A steel cylinder (about the thickness of a goose-quill), which forms the micrometer screw, has two threads cut upon it, one-half being cut with a thread double the pitch of the other. This screw is mounted on an oblong box which carries one of the measuring edges ; the other edge is moved by the coarser part of the screw relatively to the edge attached to the box, whilst the box itself is moved relatively to the axis of the telescope by the finer screw. This produces an opening and closing of the edges symmetrically with respect to the telescope axis. Flam- steed, in the first volume of the Historia Coelestis, has inserted a series of measurements made by Gascoigne extending from 1638 to 1643. These include the mutual distances of some of the stars in the Pleiades, a few observa tions of the apparent diameter of the sun, others of the distance of the moon from neighbouring stars, and a great number of measurements of the diameter of the moon. Dr Bevis (Phil. Trans., 1773, p. 190) also gives results of measurements by Gascoigne of the diameters of the moon, Jupiter, Mars, and Venus with his micrometer. Delambre gives 3 the following comparison between the re sults of Gascoigne s measurements of the sun s semi-diameter and the computed results from modern determinations : Gascoigne. Conn. d. Temps. October 25 (o.s.) 16 11" or 10" 16 10" 31 16 11" 16 11" 4 December 2 , 16 24" 16 16" 8 Gascoigne, from his observations, deduces the greatest variation of the apparent diameter of the sun to be 35"; according to the Connaissance des Temps it amounts to 32" 3. 3 These results prove the enormous advance attained in accuracy by Gascoigne, and his indisputable title to the credit of inventing the micrometer. Huygens, in his Systema Saturnium (1659), describes a micrometer with which he determined the apparent
3 Delambre, Hist. Ast. Moderne, vol. ii. p. 590.