Gravesande, Willem Jacob 's, An essay on perspective

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        <div xml:id="echoid-div127" type="section" level="1" n="70">
          <pb o="36" file="0068" n="75" rhead="An ESSAY"/>
          <p style="it">
            <s xml:id="echoid-s926" xml:space="preserve">The Reaſon of this is evident, from the Demon-
              <lb/>
            ſtration of the upright Cone: </s>
            <s xml:id="echoid-s927" xml:space="preserve">For, it is manifeſt, that
              <lb/>
            X is the Seat of the Center of the Circle, which the
              <lb/>
            Cone continued forms in the Horizontal Plane; </s>
            <s xml:id="echoid-s928" xml:space="preserve">and
              <lb/>
            conſequently, the Point s, in regard to the Circle BED,
              <lb/>
            is in the ſame Situation as the Eye is, in reſpect of
              <lb/>
            the Interſection of the continued Cone, and the Hori-
              <lb/>
            zontal Plane.</s>
            <s xml:id="echoid-s929" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s930" xml:space="preserve">Note, moreover, that a Cone can ſcarcely ever
              <lb/>
            be thrown into Perſpective, by the common Me-
              <lb/>
            thod, ſo exact as by this.</s>
            <s xml:id="echoid-s931" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div128" type="section" level="1" n="71">
          <head xml:id="echoid-head76" xml:space="preserve">
            <emph style="sc">Problem</emph>
          VII.</head>
          <head xml:id="echoid-head77" style="it" xml:space="preserve">55. To find the Perſpective of a Line, perpendicular
            <lb/>
          to the Geometrical Plane.</head>
          <p>
            <s xml:id="echoid-s932" xml:space="preserve">It is requir’d to find the Appearance of a Line
              <lb/>
              <note position="left" xlink:label="note-0068-01" xlink:href="note-0068-01a" xml:space="preserve">Fig. 22.</note>
            equal to B C, and perpendicular to the Geome-
              <lb/>
            trical Plane, in the Point A.</s>
            <s xml:id="echoid-s933" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div130" type="section" level="1" n="72">
          <head xml:id="echoid-head78" xml:space="preserve">
            <emph style="sc">Operation</emph>
          .</head>
          <p>
            <s xml:id="echoid-s934" xml:space="preserve">Aſſume E D, any where in the Baſe Line,
              <lb/>
            equal to B C; </s>
            <s xml:id="echoid-s935" xml:space="preserve">and from the Points D and E,
              <lb/>
            draw D F and E F to ſome Point F, taken at
              <lb/>
            pleaſure in the Horizontal Line. </s>
            <s xml:id="echoid-s936" xml:space="preserve">Then having
              <lb/>
            found a, the Repreſentation of the Point A;</s>
            <s xml:id="echoid-s937" xml:space="preserve">
              <note symbol="*" position="left" xlink:label="note-0068-02" xlink:href="note-0068-02a" xml:space="preserve">22.</note>
            draw a H parallel to the Baſe Line, and aI per-
              <lb/>
            pendicular thereto: </s>
            <s xml:id="echoid-s938" xml:space="preserve">And if aI be made equal to
              <lb/>
            G H, the ſaid a I will be the Perſpective
              <lb/>
            ſought.</s>
            <s xml:id="echoid-s939" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div132" type="section" level="1" n="73">
          <head xml:id="echoid-head79" xml:space="preserve">
            <emph style="sc">Demonstration</emph>
          .</head>
          <p>
            <s xml:id="echoid-s940" xml:space="preserve">56. </s>
            <s xml:id="echoid-s941" xml:space="preserve">The Appearance of the ſaid Line, is
              <note symbol="*" position="left" xlink:label="note-0068-03" xlink:href="note-0068-03a" xml:space="preserve">6.</note>
            pendicular to the Baſe Line, and equal to
              <note symbol="*" position="left" xlink:label="note-0068-04" xlink:href="note-0068-04a" xml:space="preserve">10.</note>
            Perſpective of the Line A L, drawn from </s>
          </p>
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