Gravesande, Willem Jacob 's, An essay on perspective

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        <div xml:id="echoid-div323" type="section" level="1" n="175">
          <p>
            <s xml:id="echoid-s2031" xml:space="preserve">
              <pb o="89" file="0157" n="182" rhead="PERSPECTIVE."/>
            the Diſtance of the two Points whereat the
              <lb/>
            Thread is faſten’d on the Ruler, deſcribe an
              <lb/>
            Arc cutting the Geometrical Line in F; </s>
            <s xml:id="echoid-s2032" xml:space="preserve">then
              <lb/>
            faſten the Extremity of one of the aforeſaid
              <lb/>
            Threads in the ſaid Point F, and the Extremity
              <lb/>
            of the other in the Point O: </s>
            <s xml:id="echoid-s2033" xml:space="preserve">Take moreover
              <lb/>
            another Thread, put through the Eye of a
              <lb/>
            Needle, as in the aforegoing Method; </s>
            <s xml:id="echoid-s2034" xml:space="preserve">one End
              <lb/>
            of which, faſten in F, and afterwards put it a-
              <lb/>
            bout a Pin placed in O. </s>
            <s xml:id="echoid-s2035" xml:space="preserve">Then the only Dif-
              <lb/>
            ference between this Way and the precedent one,
              <lb/>
            is, that the Ruler M N muſt be uſed, by always
              <lb/>
            keeping the Threads M F, and N O tight, in-
              <lb/>
            ſtead of one turning about a Point.</s>
            <s xml:id="echoid-s2036" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2037" xml:space="preserve">For the Demonſtration of this, vide n. </s>
            <s xml:id="echoid-s2038" xml:space="preserve">39.</s>
            <s xml:id="echoid-s2039" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div325" type="section" level="1" n="176">
          <head xml:id="echoid-head188" xml:space="preserve">
            <emph style="sc">Prob</emph>
          . II.</head>
          <p style="it">
            <s xml:id="echoid-s2040" xml:space="preserve">111. </s>
            <s xml:id="echoid-s2041" xml:space="preserve">To find the Perſpective of one or more Lines
              <lb/>
            perpendicular to the Geometrical Plane.</s>
            <s xml:id="echoid-s2042" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2043" xml:space="preserve">Take two Rulers L C and N Z, having the
              <lb/>
              <note position="right" xlink:label="note-0157-01" xlink:href="note-0157-01a" xml:space="preserve">Fig. 60.</note>
            four Ends of two Threads, or rather four Braſs
              <lb/>
            or Steel Wires of equal Length fixed on them,
              <lb/>
            at the Places L, I, N and M, ſo that L I be equal
              <lb/>
            to M N. </s>
            <s xml:id="echoid-s2044" xml:space="preserve">Then fix one oſ theſe Rulers upon the
              <lb/>
            Edge of the perſpective Plane, perpendicular to
              <lb/>
            the Baſe Line. </s>
            <s xml:id="echoid-s2045" xml:space="preserve">Now take a Thread, put thro’
              <lb/>
            the Eye of a Needle, hang a Plummet at one
              <lb/>
            End, for the ſame Uſe as in Problem I ,
              <note symbol="*" position="right" xlink:label="note-0157-02" xlink:href="note-0157-02a" xml:space="preserve">109.</note>
            faſten the other End to the Slider or Curſor D,
              <lb/>
            freely moveable on the Ruler L C; </s>
            <s xml:id="echoid-s2046" xml:space="preserve">put this
              <lb/>
            Thread about a Pin, ſet up againſt the Ruler C L
              <lb/>
            inc, ſo that C H be equal to the Height of the Eye.</s>
            <s xml:id="echoid-s2047" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div327" type="section" level="1" n="177">
          <head xml:id="echoid-head189" xml:space="preserve">
            <emph style="sc">Operation</emph>
          .</head>
          <p>
            <s xml:id="echoid-s2048" xml:space="preserve">Let T be the Repreſentation of the Foot of a
              <lb/>
            Perpendicular. </s>
            <s xml:id="echoid-s2049" xml:space="preserve">Move the Curſor D along C </s>
          </p>
        </div>
      </text>
    </echo>
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