Gravesande, Willem Jacob 's
,
An essay on perspective
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the Back-ſide thereof hath in it a Square ope-
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ning N, each Side being about four Inches in
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Length, which may be ſhut by the little Board I
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ſliding between two Rulers.</
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<
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">11. </
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<
s
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xml:space
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">Overthe ſaid Square opening, there is a Slit
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parallel to the Horizon, going along the whole
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Breadth of the Back-ſide of the Box; </
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<
s
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xml:space
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">through
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which Slit, a little Mirrour or Looking-Glaſs is to
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be put into the Box, whoſe two Sides may ſlide be-
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tween two Rulers ſo placed, that the poliſhed
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Side of the Glaſs being turn’d towards the Door
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B, may make an Angle of 112 {1/2} Degrees, with
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the Horizon. </
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<
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xml:space
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">Note, This Diſpoſition of the
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Looking-glaſs could not well be repreſented in
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the Figure.</
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<
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<
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xml:space
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<
s
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">The before-mention’d Mirrour hath a ſmall
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Iron Plate, on the Middle of that Side which is
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without the Box, (when the ſaid Looking-glaſs
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is in the Diſpoſition mention’d in N. </
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<
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">11.) </
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<
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xml:space
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">being the
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Baſe of a Screw faſten’d to the Middle thereof, that
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ſothe Looking-glaſs may be fix’d upright (as appears
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per Figure) in any Place H upon the Top of the
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Machine, and vertically turn all ways; </
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<
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xml:space
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">and this
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is done by putting the Screw through a Hole
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made in the plain Board of N. </
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<
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<
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">and through a
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Slit made for this End in the plain Board of N. </
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</
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<
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">and then fixing it with the Nut R. </
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<
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xml:space
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">Now when
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the Mirrour is taken from this Situation, the ſaid
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Slit is ſhut by a ſmall Board ſliding between two
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little Rulers within the Machine. </
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<
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">As to the Slit
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mention’d N. </
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<
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<
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">it is partly ſhut by the little
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Board I, when the Aperture N is open’d, and the
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two Ends thereof remaining open’d, are ſhut by
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two ſinall Rulers.</
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<
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<
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">There are two Iron Staples on one Side of
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the Box, like thoſe which are on the
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xlink:label
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of the Machine; </
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">in which a Ruler going ſeveral
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Inches out behind the Box X may ſlide, </
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