MICROMETER. 



wire toss)*/ 

 degree of 



t ' .. 



Met hod of 



i* :.. 



difference. It is found difficult in practice to make the 

 two screws ab, ef, so equal that they experience no re- 

 sistance in the parts C, D ; but one of the two may be 

 suppressed if supplied by a simple axis, which is in 

 fact reducing it exactly to Mr. Hunter's screw. 



The fibres for micrometers have generally been silver 

 wires drawn to a great degree of fineness. Muschen- 

 Lroek informs us, that an artist of Nuremberg drew 

 gold wire so fine, that 5OO inches of it only weighed 

 one grain ; but 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 SOOOdth 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 an inch 

 in diameter, bisected longitudinally with a fine trans- 

 parent line about the SOOOdth of an inch in diameter, 

 i'.rewater sometimes employed threads of melted 

 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 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 wasabout the SOOdth part of 

 an inch, for example, in diameter, and it is manifest, that 

 if the platinum wire was T \.th of the diameter of the 

 Over wire, before the operation of drawing commenced, 

 it roust be yJ^dth of an inch in diameter when the sil- 

 ver wire is T J,th. 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 

 bv 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 



thort he easily obtained platinum wires, or gold 

 w . lre ? * rsW * rV n inch in diameter, and with 

 a little attention, he formed them so small as the , T i- c th 

 part of an inch. The single lens micrometer by v 

 Dr. Wollaston measured the diameter of these fibres 

 will be described in Chapter IX. 



Micrometrical fibres may be placed in delicate parallel 



POOVCT formed on the diaphragm of t.'u- fir^t eye-glass, 



and fixed in their places, for temporary purposes, by a 



hm layer of bees-wax or a drop of varnish ; but when 



they are required to be kept at an invariable distance, 



tiaaafer to pinch them to the diaphragm by a small 



crew nail near the extremity of each wire. In order 



U5& m "V ta P***? 1 ""riy in the 



201 



Fisica e di tloria naturale de Firenze, Rom. 1 775. Prony Wire 

 in Lanz and Bettancourt's Essaii sur la Camp, de Ma- Micromt- 

 chines, p. 15. Brew ster's Treatise on New Phil. Instru- 

 ment t, p. 74. Wollaston, Phil. Trans. 1813, Part I. 

 Edinburgh Philosophical Journal, voL i. p. 202. 



*>.-. 



CHAP. III. 



On Wire Micrometers, in tchich the scale is varied, or the 



II ires opened and shut optically. 



The idea of varying the magnitude of the meshes of On wire ml- 

 a net of silver wire, permanently fixed in the focus of crometert, 

 the eye-glass of a telescope, for the purpose of measur- ln whicl > 



tbe 



f _ 

 ens of the eye-glass, the diaphragm should be made 



moveable along the axis of the eye-tube. 



See Townley, Pkil. Trams. No. 25. Hooke, PkU. 



1 rant. Akr. vol. i. p. *1 7. Honke's Posthumous Works, 



i. Bevis's^coo.** ofGtucoi K ne's Observations 



Trans, vol. xlvni. p. 190. Auzout and Picard, 



\em.Acad. Par. See Rosier. Bradley in Smith's Optits, 



11. p. S45, 36. Fontana, Saggio del real gabinello di 



VOL. XIV. 



ing the digits of eclipses, seems to have been first sug- 

 gested by M. de La Hire. The same idea afterwards e^i"^ 

 occurred to the late celebrated Mr. Watt, (as he himself opened and 

 informed us, ) who constructed a sort of micrometer up- tout opti- 

 on this principle ; but he never published any account cJly. 

 of it, and ilul nt>t 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 applied 

 M a general principle for micrometers by Dr. Brewster. 

 In his treatise on AV- Philosophical Instruments, he has 

 described various micrometers of this kind, which 

 seem to possess properties worthy of the attention of 

 practical astronomers ; and, if we arc not misinformed, 

 one of these telescopical micrometers has been recently 

 fitted up for use i a the Observatory at Greenwich, at 

 the denre of our celebrated astronomer royal, Mr. Pond. 



he diameter of the sun, or any portion of space, Dr. Ilrewi- 

 may be comprehended between a pair of fixed wires ter*. wire 

 placed in the eye-piece of a telescope, either by a me- microme- 

 chanical or an optical contrivance ; in the one case, by ur - 

 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 SY, 

 Plat, \ XV. Fig. 7. be the sun in contact with p LATE 



the lower wire CD, the wire AB may be moved into CCCLXXV. 

 the position ab, to as to touch the upper limb S' of the Figs. 1, 8. 

 sun ; or if the wires AB, CD, are both fixed, we may, 9, 10, I). 

 by increasing the magnifying power of the telescope, 

 expand the image SY into ivt, till its north and south 

 limbs are in accurate contact with the fixed wires. In 

 the first of these methods, which has already been ex- 

 plained in the description of the common wire micro. 

 meter, the angle subtended by the sun is measured by 

 the revolutions of the screw, which are necessary to 

 bring the wire AB from a state of coincidence with CD 

 into the position ah : 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 distance betwon tin- 

 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 SY 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 

 6* minutes ; aivl when the <w>lar image has been ex- 

 panded to S*, 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. 



The gradual variation of the magnifying power, 

 which is thui essential to the construction of the irv>. 



