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1238 A new Instrument for comparing Linear Measures. 
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either for the purposes of surveying, or for those of astronomy, 
cover spaces much too great to allow of a similar estimation, 
although they only undergo the magnifying power of the eye- 
glass. I received from M. Breguet, member of the Board of. 
Longitude, a platina wire made in England by an ingenious pro- 
cess. This wire had been passed through a hole whgn enveloped 
in a covering of silver; and when the compound of the two metals 
had been reduced to its greatest fineness, the silver was dissolved, 
and the platina wire left uncovered, The maker had written on 
the piece which contained the wire of which I speak the number 
GO00, to indicate that its diameter is =,',>dth of a fraction of 
an English foot, which M. Breguet thinks is a dine. If he has 
been rightly informed in this respect, there is an enormous mis- 
calculation in the evaluation of the maker; for his wire, when 
stretched and put in contact with the division of 100dths of a mil- 
limeter, covered the interval between two strokes, and the strokes 
themselves. Its magnitude is therefore more than 0-01 millimeter, 
whilst the English evaluation only makes it 0-00035 millimeter : 
and if, as I am inclined to believe, it is not 6000dths of a dine, 
but only 6,000dths of an inch, that he intended to indicate by the 
number 6000 written on the side of the wire, there is still an er- 
ror of 3-5ths; for the -,4,,dth part of an English inch=0-0042 
millimeter. 
This wire, which has probably the greatest degree of fineness 
that can be attained in the present state of the arts, does not 
therefore give us the most delicate line that can be rendered 
perceptible to the eye ; and my divisions of 1Odths of millimeters 
on glass serve as a proof of this. The thickness of each line of 
this division is only about the third of the length of the interval 
contained between two immediately adjoining strokes; so that 
this thickness is, according to what | have said above, less than 
the third of the diameter of the English platina wire. For these 
reasons I have determined not to put the wire in the focus of the 
microscope, but to put a piece of plain glass there, on which 
M. Richer has traced for me two lines at right angles, of such 
a degree of fineness and neatness, that when one of these lines 
projects itself between two strokes of the divisions on glass, the 
proportion between its distances from each adjoining stroke may 
easily be estimated. This expedient affords-also the advantage 
of great solidity, and that of rendering the application of a ver- 
nier to the apparatus quite easy by having on the glass in the 
focus ten parallel strokes, which should cover 9 or 11 of the di- 
visions of 10Qdths of a millimeter, 
It is unnecessary to trouble ourselves about the loss of light 
occasioned by this glass in the focus ; for notwithstanding its 
mterposition betweeu the eye and the object, an intensity of light 
that 
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