Gravesande, Willem Jacob 's, An essay on perspective

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            <s xml:id="echoid-s2329" xml:space="preserve">
              <pb o="106" file="0182" n="211" rhead="An ESSAY"/>
            the Back-ſide thereof hath in it a Square ope-
              <lb/>
            ning N, each Side being about four Inches in
              <lb/>
            Length, which may be ſhut by the little Board I
              <lb/>
            ſliding between two Rulers.</s>
            <s xml:id="echoid-s2330" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2331" xml:space="preserve">11. </s>
            <s xml:id="echoid-s2332" xml:space="preserve">Overthe ſaid Square opening, there is a Slit
              <lb/>
            parallel to the Horizon, going along the whole
              <lb/>
            Breadth of the Back-ſide of the Box; </s>
            <s xml:id="echoid-s2333" xml:space="preserve">through
              <lb/>
            which Slit, a little Mirrour or Looking-Glaſs is to
              <lb/>
            be put into the Box, whoſe two Sides may ſlide be-
              <lb/>
            tween two Rulers ſo placed, that the poliſhed
              <lb/>
            Side of the Glaſs being turn’d towards the Door
              <lb/>
            B, may make an Angle of 112 {1/2} Degrees, with
              <lb/>
            the Horizon. </s>
            <s xml:id="echoid-s2334" xml:space="preserve">Note, This Diſpoſition of the
              <lb/>
            Looking-glaſs could not well be repreſented in
              <lb/>
            the Figure.</s>
            <s xml:id="echoid-s2335" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2336" xml:space="preserve">12. </s>
            <s xml:id="echoid-s2337" xml:space="preserve">The before-mention’d Mirrour hath a ſmall
              <lb/>
            Iron Plate, on the Middle of that Side which is
              <lb/>
            without the Box, (when the ſaid Looking-glaſs
              <lb/>
            is in the Diſpoſition mention’d in N. </s>
            <s xml:id="echoid-s2338" xml:space="preserve">11.) </s>
            <s xml:id="echoid-s2339" xml:space="preserve">being the
              <lb/>
            Baſe of a Screw faſten’d to the Middle thereof, that
              <lb/>
            ſothe Looking-glaſs may be fix’d upright (as appears
              <lb/>
            per Figure) in any Place H upon the Top of the
              <lb/>
            Machine, and vertically turn all ways; </s>
            <s xml:id="echoid-s2340" xml:space="preserve">and this
              <lb/>
            is done by putting the Screw through a Hole
              <lb/>
            made in the plain Board of N. </s>
            <s xml:id="echoid-s2341" xml:space="preserve">9. </s>
            <s xml:id="echoid-s2342" xml:space="preserve">and through a
              <lb/>
            Slit made for this End in the plain Board of N. </s>
            <s xml:id="echoid-s2343" xml:space="preserve">8.
              <lb/>
            </s>
            <s xml:id="echoid-s2344" xml:space="preserve">and then fixing it with the Nut R. </s>
            <s xml:id="echoid-s2345" xml:space="preserve">Now when
              <lb/>
            the Mirrour is taken from this Situation, the ſaid
              <lb/>
            Slit is ſhut by a ſmall Board ſliding between two
              <lb/>
            little Rulers within the Machine. </s>
            <s xml:id="echoid-s2346" xml:space="preserve">As to the Slit
              <lb/>
            mention’d N. </s>
            <s xml:id="echoid-s2347" xml:space="preserve">11. </s>
            <s xml:id="echoid-s2348" xml:space="preserve">it is partly ſhut by the little
              <lb/>
            Board I, when the Aperture N is open’d, and the
              <lb/>
            two Ends thereof remaining open’d, are ſhut by
              <lb/>
            two ſinall Rulers.</s>
            <s xml:id="echoid-s2349" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2350" xml:space="preserve">13. </s>
            <s xml:id="echoid-s2351" xml:space="preserve">There are two Iron Staples on one Side of
              <lb/>
            the Box, like thoſe which are on the
              <note symbol="*" position="left" xlink:label="note-0182-01" xlink:href="note-0182-01a" xml:space="preserve">7.</note>
            of the Machine; </s>
            <s xml:id="echoid-s2352" xml:space="preserve">in which a Ruler going ſeveral
              <lb/>
            Inches out behind the Box X may ſlide, </s>
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