Page:EB1911 - Volume 18.djvu/403

 extinction. It is the characteristic feldspar of volcanic rocks which are rich in soda, and is typically developed in the lavas of the island of Pantelleria near Sicily and those of Kilimanjaro and Mount Kenya in East Africa: the rhomb-shaped porphyritic feldspars of the “rhomb-porphyry” of southern Norway also belong here.

 MICROCOSM, a term often applied in philosophical and in general literature to man regarded as a “little world” (Gr.  ) in opposition to the “macrocosm,” great world, in which he lives. From the dawn of speculative thought in Greece the analogy between man and the world has been a common-place, and may be traced from Heraclitus and Empedocles, through Plato, Aristotle, the Stoics, the Schoolmen and the thinkers of the Renaissance down to the present day. Thus Lotze’s comprehensive survey of mental and moral science is termed Microcosmus. The most systematic expression of the tendency indicated by the term is the monadology of Leibnitz, in which the monad is regarded as containing within its own closed sphere an expression of the universe, the typical created monad being the human soul.

 MICROCOSMIC SALT, or ammonium sodium hydrogen or tho phosphate, NH4NaHPO4.4H2O, so named by the alchemists because it is contained in the decomposing urine of man (the “microcosm”). It is interesting historically as being the raw material from which Brand prepared phosphorus, whence it is also called “salt of phosphorus.” It may be obtained in large transparent crystals from a mixture of solutions of sal-ammoniac and disodium phosphate, or by saturating a solution of monosodium phosphate with ammonia. When heated to redness, it leaves a transparent glass of sodium metaphosphate, NaPO3, which like borax dissolves most metallic oxides, with formation of glasses that often exhibit characteristic colours, and which may be used in the qualitative analysis of substances. (See, § Analytical.)

 MICROMETER (from Gr. , small,  , a measure), 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 “” of a rifle) first upon one of the objects and then upon the other (q.v.), 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. Michael Maestlin in 1579 determined the relative positions of eleven stars in the Pleiades (Historia coelestis Lucii Baretti, Augsburg, 1666), and A. Winnecke has shown (Monthly Notices R.A.S., xxxix. 146) that the probable error of these measures amounted to about ±2′.

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. William Crabtree, a friend of his, taking a journey to Yorkshire in 1639 to see Gascoigne, writes thus to his friend Jeremiah 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 Richard Townley of Lancashire, who exhibited it at the meeting of the Royal Society held on the 25th of July 1667.

The principle of Gascoignes micrometer is that two pointers having parallel edges at right angles to the measuring screw, are moved in opposite directions symmetrically with and at right angles to the axis of the 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. Flamsteed, 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 observations 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 John 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 the following comparison between the results of Gascoigne’s measurements of the sun’s semi-diameter and the computed results from modern determinations:— 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. 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 diameters of the principal planets. He inserted a slip of metal, of variable breadth, at the focus of the telescope, and observed at what part it exactly covered the object under examination; knowing the focal length of the telescope and the width of the slip at the point observed, he thence deduced the apparent angular breadth of the object. The Marquis Malvasia in his Ephemerides (Bologna, 1662) describes a micrometer of his own invention. At the focus of his telescope he placed fine silver wires at right angles to each other, which, by their intersection, formed a network of small squares. The mutual distances of the intersecting wires he determined by counting, with the aid of a pendulum clock, the number of seconds required by an equatorial star to pass from web to web, while the telescope was adjusted so that the star ran parallel to the wires at right angles to those under investigation. In the ''Phil. Trans.'' (1667), No. 21, p. 373, Adrien Auzout gives the results of some measures of the diameter of the sun and moon made by himself, and this communication led to the letters of Townley and Bevis above referred to. The micrometer of Auzout and Picard was provided with silk fibres or silver wires instead of the edges of Gascoigne, but one of the silk fibres remained fixed while the other was moved by a screw. It is beyond doubt that Huygens independently discovered that an object placed in the common focus of the two lenses of a Kepler telescope appears as distinct and well-defined as the