Gravesande, Willem Jacob 's, An essay on perspective

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        <div xml:id="echoid-div219" type="section" level="1" n="115">
          <pb o="62" file="0114" n="131" rhead="An ESSAY"/>
        </div>
        <div xml:id="echoid-div221" type="section" level="1" n="116">
          <head xml:id="echoid-head122" xml:space="preserve">
            <emph style="sc">Proe</emph>
          . III.</head>
          <p style="it">
            <s xml:id="echoid-s1472" xml:space="preserve">79. </s>
            <s xml:id="echoid-s1473" xml:space="preserve">To find the Repreſentation of a Figure in the
              <lb/>
            Geometrical Plane, when the Perſpective Plane is
              <lb/>
            placed above the Eye.</s>
            <s xml:id="echoid-s1474" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s1475" xml:space="preserve">When the perſpective Plane is ſituated above
              <lb/>
            the Eye, we ſuppoſe the Geometrical Plane to
              <lb/>
            paſs through the Top of the Perſpective Plane;
              <lb/>
            </s>
            <s xml:id="echoid-s1476" xml:space="preserve">upon which Geometrical Plane are drawn the
              <lb/>
            Figures of Objects meeting it; </s>
            <s xml:id="echoid-s1477" xml:space="preserve">as alſo the Seats
              <lb/>
            of thoſe Objects that are underneath it, by
              <lb/>
            Means of Perpendiculars; </s>
            <s xml:id="echoid-s1478" xml:space="preserve">and the Height of the
              <lb/>
            Eye is here meaſur’d by a Perpendicular drawn
              <lb/>
            from the Eye to the Geometrical Plane; </s>
            <s xml:id="echoid-s1479" xml:space="preserve">whence
              <lb/>
            the perſpective Plane, elevated in reſpect to the
              <lb/>
            Eye, is the ſame thing, as an Eye elevated in
              <lb/>
            regard to the perſpective Plane.</s>
            <s xml:id="echoid-s1480" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s1481" xml:space="preserve">Let I L be the Baſe Line, and H the Foot of
              <lb/>
              <note position="left" xlink:label="note-0114-01" xlink:href="note-0114-01a" xml:space="preserve">Fig. 42.</note>
            the vertical Line; </s>
            <s xml:id="echoid-s1482" xml:space="preserve">then in the Baſe Line aſ-
              <lb/>
            ſume the Points I and L at Pleaſure, towards
              <lb/>
            the Sides of the perſpective Plane. </s>
            <s xml:id="echoid-s1483" xml:space="preserve">Make I S
              <lb/>
            equal to {1/3} or {1/4} Part of I H, and raiſe the Per-
              <lb/>
            pendicular S X, in the Point S, to the Baſe Line,
              <lb/>
            equal to a correſpondent Part of the Height and
              <lb/>
            Diſtance of the Eye taken together; </s>
            <s xml:id="echoid-s1484" xml:space="preserve">draw the
              <lb/>
            Line X I G, and moreover Y L Q, in aſſuming
              <lb/>
            L T equal to {1/3} or {1/4} & </s>
            <s xml:id="echoid-s1485" xml:space="preserve">c. </s>
            <s xml:id="echoid-s1486" xml:space="preserve">of L H. </s>
            <s xml:id="echoid-s1487" xml:space="preserve">Again draw
              <lb/>
            the Line G Q in the Geometrical Plane, pa-
              <lb/>
            rallel to the Baſe Line, and diſtant therefrom
              <lb/>
            (for Example) a third Part of the Height of
              <lb/>
            the Eye; </s>
            <s xml:id="echoid-s1488" xml:space="preserve">draw alſo F P in the perſpective Plane,
              <lb/>
            parallel to the Baſe Line, and diſtant therefrom,
              <lb/>
            a fourth Part of the Eye’s Diſtance; </s>
            <s xml:id="echoid-s1489" xml:space="preserve">theſe two
              <lb/>
            Lines will cut X I in G and F, and Y L in Q
              <lb/>
            and P. </s>
            <s xml:id="echoid-s1490" xml:space="preserve">Note, if the Diſtance of G Q from the
              <lb/>
            Baſe Line, had been aſſumed equal to a fourth
              <lb/>
            Part of the Eye’s Diſtance; </s>
            <s xml:id="echoid-s1491" xml:space="preserve">then F P muſt </s>
          </p>
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