Gravesande, Willem Jacob 's, An essay on perspective

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          <p>
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              <pb o="65" file="0119" n="137" rhead="on PERSPECTIVE."/>
            being produced, will meet each other in the
              <lb/>
            Horizontal Line.</s>
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        <div xml:id="echoid-div229" type="section" level="1" n="120">
          <head xml:id="echoid-head126" xml:space="preserve">CHAP. V.</head>
          <p style="it">
            <s xml:id="echoid-s1565" xml:space="preserve">Of throwing Figures into Perſpective, when
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            the Perſpective Plane is conſider’d as being
              <lb/>
            inclined.</s>
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        <div xml:id="echoid-div230" type="section" level="1" n="121">
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            <emph style="sc">Problem</emph>
          I.</head>
          <p style="it">
            <s xml:id="echoid-s1567" xml:space="preserve">81. </s>
            <s xml:id="echoid-s1568" xml:space="preserve">TO find the Perſpective of a Figure in the
              <lb/>
              <note position="right" xlink:label="note-0119-01" xlink:href="note-0119-01a" xml:space="preserve">Fig. 44.</note>
            Geometrical Plane.</s>
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          <p>
            <s xml:id="echoid-s1570" xml:space="preserve">Let X be the Vertical Plane; </s>
            <s xml:id="echoid-s1571" xml:space="preserve">S I the Station
              <lb/>
            Line, S the Station Point, and H the Interſecti-
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            on of the Station Line and Baſe Line. </s>
            <s xml:id="echoid-s1572" xml:space="preserve">Now
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            draw the Vertical Line H V through the Point H,
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            making an Angle with S I, equal to the Angle
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            of Inclination of the perſpective Plane; </s>
            <s xml:id="echoid-s1573" xml:space="preserve">then
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            raiſe the Perpendicular I O to S I, in the Sta-
              <lb/>
            tion Point S, equal to the Height of the Eye;
              <lb/>
            </s>
            <s xml:id="echoid-s1574" xml:space="preserve">and through the Extremity of the ſaid Perpen-
              <lb/>
            dicular, draw the principal Ray O V, paral-
              <lb/>
            lel to S I, and cutting H V in the Point of
              <lb/>
            Sight V.</s>
            <s xml:id="echoid-s1575" xml:space="preserve"/>
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            <s xml:id="echoid-s1576" xml:space="preserve">Now it is evident, that O V determines the
              <lb/>
            Length of the principal Ray, and H V the Di-
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            ſtance from the Baſe Line to the Horizontal
              <lb/>
            Line; </s>
            <s xml:id="echoid-s1577" xml:space="preserve">and ſince the Demonſtration of the
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            Problems in the aforegoing Chapters regarding
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            the Geometrical Plane, have alſo Relation to
              <lb/>
            the perſpective Plane being inclined, the ſaid
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            Problems may be here uſed; </s>
            <s xml:id="echoid-s1578" xml:space="preserve">and conſequently,
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            this inclined perſpective Plane is reduced to a
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            Perpendicular one, view’d by an Eye, whoſe
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            Height is H V, and Diſtance O V.</s>
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