Gravesande, Willem Jacob 's, An essay on perspective

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        <div xml:id="echoid-div45" type="section" level="1" n="20">
          <pb o="11" file="0031" n="32" rhead="on PERSPECTIVE."/>
          <p>
            <s xml:id="echoid-s398" xml:space="preserve">For the Horizontal Plane, is parallel to the
              <lb/>
            Geometrical Plane.</s>
            <s xml:id="echoid-s399" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div46" type="section" level="1" n="21">
          <head xml:id="echoid-head23" xml:space="preserve">
            <emph style="sc">Corollary</emph>
          III.</head>
          <p style="it">
            <s xml:id="echoid-s400" xml:space="preserve">16. </s>
            <s xml:id="echoid-s401" xml:space="preserve">The Repreſentations of all Lines parallel to
              <lb/>
            the ſtation Line, concur in the Point of Sight.</s>
            <s xml:id="echoid-s402" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s403" xml:space="preserve">This follows, becauſe the principal Ray is
              <lb/>
            parallel to the ſaid Lines.</s>
            <s xml:id="echoid-s404" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div47" type="section" level="1" n="22">
          <head xml:id="echoid-head24" xml:space="preserve">
            <emph style="sc">Corollary</emph>
          IV.</head>
          <p style="it">
            <s xml:id="echoid-s405" xml:space="preserve">17. </s>
            <s xml:id="echoid-s406" xml:space="preserve">Two or more equal Lines being perpendicular,
              <lb/>
            or equally inclined the ſame Way, to the ſame Line pa-
              <lb/>
            rallel to the Station Line, have their Repreſentations
              <lb/>
            concurring in the principal Point.</s>
            <s xml:id="echoid-s407" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s408" xml:space="preserve">Becauſe all theſe Lines are parallel and equal,
              <lb/>
            the Line paſſing through their Vertices, is pa-
              <lb/>
            rallel to that paſſing through their Baſes, and
              <lb/>
            this being parallel to the Station Line, it fol-
              <lb/>
            lows, that the Appearances of the ſaid
              <note symbol="*" position="right" xlink:label="note-0031-01" xlink:href="note-0031-01a" xml:space="preserve">16.</note>
            and parallel Lines concur in the principal
              <lb/>
            Point.</s>
            <s xml:id="echoid-s409" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div49" type="section" level="1" n="23">
          <head xml:id="echoid-head25" xml:space="preserve">
            <emph style="sc">Theorem</emph>
          V.</head>
          <p style="it">
            <s xml:id="echoid-s410" xml:space="preserve">18. </s>
            <s xml:id="echoid-s411" xml:space="preserve">The Appearance of an indefinite Line does not
              <lb/>
            alter, when the Eye moves in a Line parallel to a
              <lb/>
            propoſed Line.</s>
            <s xml:id="echoid-s412" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s413" xml:space="preserve">The Repreſentation of this Line, is the In-
              <lb/>
            terſection of the perſpective Plane, and a Plane
              <lb/>
            paſſing through the Eye and the ſaid Line. </s>
            <s xml:id="echoid-s414" xml:space="preserve">Now
              <lb/>
            the Eye remains in the ſame Plane, when it
              <lb/>
            moves in a Line parallel to the propoſed Line;
              <lb/>
            </s>
            <s xml:id="echoid-s415" xml:space="preserve">and conſequently the Appearance of this laſt
              <lb/>
            Line, will not be changed by that Motion.</s>
            <s xml:id="echoid-s416" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s417" xml:space="preserve">Note, This Demonſtration doth not extend
              <lb/>
            to any particular Part of the given Line, but on-
              <lb/>
            ly to the Line in general.</s>
            <s xml:id="echoid-s418" xml:space="preserve"/>
          </p>
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