The fibres for micrometers have generally been silver wires drawn to a great degree of fineness. Muschen broek info' ms us, that an artist of Nuremberg drew gold wire so fine, that 500 inches of it only weighed one grain ; hut he does not state by what means it was made. In 1775, Felix Fontana recommended the spi der's web as a substitute for silver wire, and he is said (though we suspect an error in the statement) to have found them so small as the 8000th part of a line. The use of the spider's web was introduced by Mr. Trough ton, who found it to be so fine, opaque, and elastic, as to answer all the objects of practical astronomy. He has found, however, that it is only the stretcher, or the long line that supports the web, which possesses these valu able properties. The facility with which fine glass fibres can at all times be obtained induced Dr. Brewster to recommend them for micrometrical fibres, and some of those which he employed were about of au inch in diameter, bisected longitudinally with a fine trans parent line shout the 3000dth of an inch in diameter. Dr. Brewster sometimes employed threads of rnelted sealing wax. Mr. Wallace has more recently recom mended the fibres of asbestos, which can be obtained to any degree of fineness.
The formation of micrometrical fibres has, however, been brought to a high degree of perfection by Dr. Wollaston, who has discovered a method of making them of any degree of fineness. Along the axis of a cylindrical mould he placed a small platinum wire, and then filled the mould with melted silver. The silver was now drawn out till it was about the 300dth part of an inch, for example, in diameter, and it i manifest, that if the platinum wire was of the diameter of the silver wire, before the operation of drawing commenced, it must be of an inch in diameter when the sil ver wire is In this state the silver wire, with the platinum one inclosed, is bent into the form of the letter U, making a hook at each of its ends, and in this state it is suspended by a gold wire and dipped in hot nitric acid or aquafortis. The silver is now dissolved by the nitric acid, except at its extremities, and the platinum wire remains untouched by the acid. The hooks at the end of the wire retaining the silver served to make the platinum wire visible. By this ingenious method he easily obtained platinum wires, or gold wires of or Toisa an inch in diameter, and, with a little attention, he formed them so small as the part of an inch. The single lens micrometer by which Dr. Wollaston measured the diameter of these fibres, will be described in Chapter IX.
Micromet•ical fibres may be placed in delicate parallel grooves formed on the diaphragm of the first eye-glass, and fixed in their places, for temporary purposes, by a thin layer of bees-wax or a drop of varnish ; but when they are required to be kept at an invariable distance, it is safer to pinch them to the diaphragm by a small screw nail near the extremity of each wire. In order that the fibres may be placed exactly in the anterior focus of the eye-glass, the diaphragm should be made moveable along the axis of the eye-tube.
See Townley, Phil. Trans. No. 25. Hooke, Phil. Trans. Abr. vol. i. p. 2 17. Hooke's Posthumous Works, p. 497,498. Bevis's Account of Gaseoigne's Observations, in Phil. Trans. vol. xlviii. p. 190. Auzout and Picard, Men:. Acad. Par. See Rozier. Bradley in Smith's Optics, p. 345, 346. Fontana, Saggio del real gabinetto di .A'isica e di storia naturale de Firenze, Rom. 1775. Prony
in Lanz and Bettancourt's Essais sur la Comp. de Ma chines, p. 15. Brewster's Treatise on New. Phil. Instru ments, p. 74. Wollaston, Phil. Trans. 1813, Part I. Edinburgh Philosophical Journal, vol. i. p. 202.
On IVire Micrometers, in which the scale is varied, or the Wires opened and shut optically.
The idea of varying the magnitude of the meshes of a net of silver wire, permanently fixed in the focus of the eye-glass of a telescope, for the purpose of measur ing the digits of eclipses, seems to have been first sug gested by M. de La Hire. The same idea afterwards occurred to the late celebrated Mr. Watt, (as he himself informed us,) who constructed a sort of micrometer up on this pi inciple ; but he never published any account of it, and did not examine its optical properties. See Edinburgh Philosophical Journal, vol. ii. p. 124.
The idea of opening and shutting one or more pairs of wires optically, instead of mechanically, was first ap plied as a general principle for micrometers by Dr. Brewster. In his treatise on New Philosophical Instru ments, he has described various micrometers of this kind, which seem to possess properties worthy of the attention of practical astronomers ; and, if we are not misinformed, one of these tclescopical micrometers has been recently fitted up for use in the Observatory at Greenwich, at the desire of our celebrated astronomer royal, Nit.. Pond.
" The diameter of the sun, or any portion of space, may be comprehended between a pair of fixed wires placed in the eye-piece of a telescope, either by a me chanical or an optical contrivance ; in the one case, by varying the distance of the wires till they contain ex actly the solar disc ; and in the other, by expanding or contracting the image of the sun till it exactly fills the space between a pair of fixed wires. Thus let S's', Plate CCCLXXV. Fig. 7. be the sun in contact with the lower wire CD, the wire AB may be moved into the position ab, so as to touch the upper limb S' of the sun ; or if the wires AB, CD, are both fixed, we may, by increasing the magnifying power of the telescope, expand the image S's' into Ss, till its north and south limbs are in accurate contact with the fixed wires. In the first of these methods, which has been already ex plained in the description of the common wire micro meter, the angle subtended by the sun is measured by the revolutions ol the screw, w hich are necessary to bring the wire AB from a state of coincidence with CD in the position ab :—In the second method, which is the principle on which the new instrument is founded, the angle is measured by the change of magnifying power which is required to enlarge the solar image, till its diameter is exactly equal to the cjistance between the wires. Though the wires are in this case absolutely fixed, yet the angle which they subtend at the observ er's eye continually changes with the magnifying power of the telescope ; for if the sun S's' fills half the space between the wires AB, CD, before the magnifying power is increased, the angle subtended by these wires must be equal to twice the diameter of the sun, or about 62 minutes ; and when the solar image has been ex panded to Ss, the wires AB, CD, only subtend an angle equal to the sun's diameter, or about 31 minutes ; so that if this expansion of the sun's image has been produced by a gradual change in the magnifying power of the telescope, the wires must have subtended every possible angle between 31 and 62 minutes.