Gravesande, Willem Jacob 's, An essay on perspective

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        <div xml:id="echoid-div36" type="section" level="1" n="15">
          <p>
            <s xml:id="echoid-s359" xml:space="preserve">
              <pb o="9" file="0029" n="30" rhead="on PERSPECTIVE."/>
            right, is alſo perpendicular to the Geometrical
              <lb/>
            Plane, and the third is then in the Direction of the
              <lb/>
            firſt.</s>
            <s xml:id="echoid-s360" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div38" type="section" level="1" n="16">
          <head xml:id="echoid-head18" xml:space="preserve">
            <emph style="sc">Corollary</emph>
          II.</head>
          <p style="it">
            <s xml:id="echoid-s361" xml:space="preserve">11. </s>
            <s xml:id="echoid-s362" xml:space="preserve">If tworight Lines, equal between themſelves, and
              <lb/>
            parallel to the perſpective Planes, be equally diſtant
              <lb/>
            from the perſpective Plane, their Appearances will be
              <lb/>
            equal.</s>
            <s xml:id="echoid-s363" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s364" xml:space="preserve">For, becauſe they are in a Plane, parallel to
              <lb/>
            the perſpective Plane, they will have the ſame
              <lb/>
            Proportion to each other, as their Repreſentations.</s>
            <s xml:id="echoid-s365" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div39" type="section" level="1" n="17">
          <head xml:id="echoid-head19" xml:space="preserve">
            <emph style="sc">Theorem</emph>
          III.</head>
          <p style="it">
            <s xml:id="echoid-s366" xml:space="preserve">12. </s>
            <s xml:id="echoid-s367" xml:space="preserve">If a Line parallel to the Perſpective Plane, be
              <lb/>
            view’d by two Eyes, both being in a Plane, parallel
              <lb/>
            to the perſpective Plane, the Repreſentations of the
              <lb/>
            ſaid Line will be equal.</s>
            <s xml:id="echoid-s368" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s369" xml:space="preserve">If we ſuppoſe a Plane, parallel to the Per-
              <lb/>
            ſpective Plane, to paſs through the propoſed
              <lb/>
            Line, this Proportion will be had; </s>
            <s xml:id="echoid-s370" xml:space="preserve"> viz. </s>
            <s xml:id="echoid-s371" xml:space="preserve">As
              <note symbol="*" position="right" xlink:label="note-0029-01" xlink:href="note-0029-01a" xml:space="preserve">9.</note>
            Diſtance of the Eyes from this Plane, is to their
              <lb/>
            Diſtance from the Perſpective Plane, ſo is the
              <lb/>
            given Line to the Repreſentation thereof. </s>
            <s xml:id="echoid-s372" xml:space="preserve">But
              <lb/>
            the three firſt Terms of this Proportion are
              <lb/>
            the ſame for each of the Eyes, which are
              <lb/>
            in one and the ſame Plane parallel to the Per-
              <lb/>
            ſpective Plane: </s>
            <s xml:id="echoid-s373" xml:space="preserve">Therefore, the fourth Term of
              <lb/>
            the Proportion will likewiſe be the ſame in both
              <lb/>
            Caſes: </s>
            <s xml:id="echoid-s374" xml:space="preserve">Which was to be demonſtrated.</s>
            <s xml:id="echoid-s375" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div41" type="section" level="1" n="18">
          <head xml:id="echoid-head20" xml:space="preserve">
            <emph style="sc">Theorem</emph>
          IV.</head>
          <p style="it">
            <s xml:id="echoid-s376" xml:space="preserve">13. </s>
            <s xml:id="echoid-s377" xml:space="preserve">If a right Line, being continued, meets the per-
              <lb/>
            ſpective Piane in one Point, the Appearance thereof
              <lb/>
            will be a Part of the Line drawn from the ſaid Point
              <lb/>
            in the perſpective Plane, to another Point, </s>
          </p>
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