Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/259

Rh diameters of the principal planets. He inserted a slip of metal, of variable breadth, at the focus of the tele scope, 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. 1 In the Phil. Trans., 1667, No. 21, p. 373, 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 Mr Townley and Dr 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 image of a distant body; and the micrometers of Malvasia, Auzout, and Picard are the natural developments of this discovery. Gascoigne was killed at the battle of Marston Moor on the 2d July 1644, in the twenty-fourth year of his age, and his untimely death was doubtless the cause that delayed the publication of a discovery which anticipated, by twenty years, the combined work of Huygens, Malvaison, Auzout, and Picard in the same direction. As the powers of the telescope were gradually developed, it was found that the finest hairs or filaments of silk, or the thinnest silver wires that could be drawn, were much too thick for the refined purposes of the astronomer, as they entirely obliterated the image of a star in the more powerful telescopes. To obviate this difficulty Professor Felice Fontana of Florence (Saggio del real gabinetio dijisica e di storia naturale, 1755) first proposed the use of spider webs in micrometers, 2 but it was not till the attention of Troughton had been directed to the subject by Rittenhouse that the idea was carried into practice. 3 In 1813 Wollaston proposed fine platinum wires, prepared by surrounding a platinum wire with a cylinder of silver, and drawing out the cylinder with its platinum axis into a fine wire. 4 The surrounding silver was then dissolved by nitric acid, and a platinum wire of extreme fineness remained. But experience soon proved the superiority of the spider web ; its perfection of shape, its lightness and elasticity, have led to its universal adoption. Beyond the introduction of the spider line it is unneces sary to mention the various steps by which the Gascoigne micrometer assumed the modern forms now in use, or to describe in detail the suggestions of Hooke, 5 Wren, Smeaton, Cassini, Bradley, Maskelyne, Herschel, Arago, 1 Mem. Acad. des Sciences, 1717, p. 78 sq. 2 In 1782 (Phil. Trans., vol. Ixxii. p. 163) Sir W. Herschel writes: &quot; I have in vain attempted to find lines sufficiently thin to extend them across the centres of the stars, so that their thickness might be neglected.&quot; It is a matter of regret that Fontana s sugges tion was unknown to him. 3 Quekett in his Treatise cm (he Microscope ascribes to Ramsden the practical introduction of the spider web in micrometers. The evidence appears to be in favour of Troughton. 4 Phil. Trans., 1813, pp. 114-118. 5 Dr Hooke made the important improvement on Gascoigue s micrometer of substituting parallel hairs for the parallel edges of its original construction (Hooke s Posthumous Works, p. 497). 243 Pearson, Bessel, Struve, Dawes, &c., or the successive productions of the great artists Ramsden, Troughton, Fraunhofer, Ertel, Simms, Cooke, Grubb, Clarke, and Repsold. It will be sufficient to describe those forms with which the most important work has been done, or which have survived the tests of time and experience. Before astronomical telescopes were mounted parallactically, the Position measurement of position angles was seldom attempted. Indeed, angles, in those days, the difficulties attached to such measures, and to the measurement of distances with the filar micrometer, were exceed ingly great, and must have taxed to the utmost the skill and patience of the observer. For, on account of the diurnal motion, the direc tion of the axis of the telescope when directed to a star is always changing, so that, to follow a star with an altazimuth mounting, the observer requires to move continuously the two handles which give slow motion in altitude and azimuth. Sir &quot;William Herschel was the first astronomer who measured Herschel s position angles ; the instrument he employed is described in Phil, instru- Trans., 1781, vol. Ixxi. p. 500. It was used by him in his earliest ment. observations of double stars (1779-83); but, even in his matchless hands, the measurements were comparatively crude, because of the difficulties he had to encounter from the want of a parallactic mount ing. In the case of close double stars he estimated the distance in terms of the disk of the components. For the measurement of wider stars he invented his lamp-micrometer, in which the components of a double star observed with the right eye were made to coincide with two lucid points placed 10 feet from the left eye. The distance of the lucid points was the tangent of the magnified angles sub tended by the stars to a radius of 10 feet. This angle, therefore, divided by the magnifying power of the telescope gives the real angular distance of the centres of a double star. With a power of 460 the scale was a quarter of an inch for every second. The Modern Filar Micrometer. When equatorial mountings for telescopes became more general, no filar micrometer was considered complete which was not fitted with a position circle. 6 The use of the spider line or filar micrometer became universal ; the methods of illumination were improved ; and micrometers with screws of previously unheard-of fineness and accuracy were produced. These facilities, coupled with the wide and fascinating field of research opened up by Sir William Herschel s discovery of the binary character of double stars, gave an impulse to micrometric research which has continued unabated to the present time. A still further facility was given to the use of the filar micrometer by the introduction of clock work, which caused the telescope automatically to follow the diurnal motion of a star, and left the observer s hands entirely at liberty. 7 The modern filar micrometer has now assumed forms of five types. Classifica- Type A. Micrometers in which there are two webs, each mov- tion of able by a fine screw with a divided head. This is the usual English micro form of filar micrometer. meters. Type B. Micrometers in which one web is movable by means of a fine screw with a divided head, and the other by a screw without a divided head. The latter screw, in ordinary use, is only employed to change the coincidence-reading of the two webs, for eliminating the errors of the micrometer screw. This is the ordi nary German form of micrometer as originally made by Fraunhofer and since by Merz, and employed by the Struves and other principal Continental astronomers down to the present day. Type C. A similar form of micrometer to B, except that the coincidence-point cannot be changed, there being no second screw to alter the position of the fixed web. Type D. A micrometer somewhat similar in general construction to form B, except that, in addition to means of changing the zero point, there is a screw head by which a fine movement can be given to the whole micrometer box, in the direction of the axis of the micrometer screw. This is the modern form of micrometer as con structed by Repsold. Type E. Micrometers fitted with two eye-pieces for measuring angles larger than the field of view of an ordinary eye-piece. The micrometer of type A is due to Troughton ; it is represented Trough- in figs. 1, 2, 3. Fig. 1 is a horizontal section in the direction of ton s filar the axis of the telescope. The eye-piece ab consists of two piano- micro- convex lenses a, b, of nearly the same focal length, and with the two meter. 6 Herschel and South (Phil. Trans., 1824, part iii. p. 10) claim that the micrometer by Troughton, fitted to their 5-feet equatorial telescope, is the first position micrometer constructed capable of measuring position angles to 1 of arc. 7 So far as we can ascertain, the first telescope of large size driven by clockwork was the 9-inch equatorial made for Struve at Dorpat by Fraunhofer ; it was completed in 1825. The original idea appears to be due to Passement (Mem. Acad., Paris, 1746). In 1757 he pre sented a telescope to the king, so accurately driven by clockwork that it would follow a star all night long.