Gravesande, Willem Jacob 's, An essay on perspective

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        <div xml:id="echoid-div65" type="section" level="1" n="34">
          <p>
            <s xml:id="echoid-s557" xml:space="preserve">
              <pb o="19" file="0041" n="43" rhead="on PERSPECTIVE."/>
            Sight to the Horizontal Line, equal in Length
              <lb/>
            to the principal Ray; </s>
            <s xml:id="echoid-s558" xml:space="preserve">and the Extremity of this
              <lb/>
            Perpendicular will be the Point ſought.</s>
            <s xml:id="echoid-s559" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s560" xml:space="preserve">If nothing is determin’d, the Place of the Eye
              <lb/>
            may be taken at pleaſure in the Horizontal
              <lb/>
            Plane.</s>
            <s xml:id="echoid-s561" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div66" type="section" level="1" n="35">
          <head xml:id="echoid-head37" xml:space="preserve">
            <emph style="sc">Method</emph>
          III.</head>
          <p>
            <s xml:id="echoid-s562" xml:space="preserve">29. </s>
            <s xml:id="echoid-s563" xml:space="preserve">The ſame Things being given as in the
              <lb/>
            precedent Method, about the Eye O, as a Cen-
              <lb/>
            ter; </s>
            <s xml:id="echoid-s564" xml:space="preserve">deſcribe the Arc of a Circle I H, touching
              <lb/>
              <note position="right" xlink:label="note-0041-01" xlink:href="note-0041-01a" xml:space="preserve">Fig. 9.</note>
            the Horizontal Line.</s>
            <s xml:id="echoid-s565" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div68" type="section" level="1" n="36">
          <head xml:id="echoid-head38" xml:space="preserve">
            <emph style="sc">Operation</emph>
          .</head>
          <p>
            <s xml:id="echoid-s566" xml:space="preserve">About the given Point A, as a Center, de-
              <lb/>
            ſcribe the Arc of a Circle L C, touching the Baſe
              <lb/>
            Line: </s>
            <s xml:id="echoid-s567" xml:space="preserve">Then draw two Lines, C H and L I,
              <lb/>
            touching the two Circles L C and H I; </s>
            <s xml:id="echoid-s568" xml:space="preserve">and the
              <lb/>
            Point a, the Interſection of the ſaid two Lines,
              <lb/>
            will be the Appearance ſought.</s>
            <s xml:id="echoid-s569" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div69" type="section" level="1" n="37">
          <head xml:id="echoid-head39" xml:space="preserve">
            <emph style="sc">Demonstration</emph>
          .</head>
          <p>
            <s xml:id="echoid-s570" xml:space="preserve">30. </s>
            <s xml:id="echoid-s571" xml:space="preserve">To demonſtrate this, draw the Line A B
              <lb/>
            perpendicular to the Baſe Line; </s>
            <s xml:id="echoid-s572" xml:space="preserve">O V perpendi-
              <lb/>
            cular to the Horizontal Line; </s>
            <s xml:id="echoid-s573" xml:space="preserve">and A C, O H
              <lb/>
            perpendicular to the Tangent H C. </s>
            <s xml:id="echoid-s574" xml:space="preserve">All theſe
              <lb/>
            Perpendiculars will cut the Lines to which they
              <lb/>
            are perpendicular, in the Points wherein theſe
              <lb/>
            laſt touch the Circle L B C, or H V I. </s>
            <s xml:id="echoid-s575" xml:space="preserve">Like-
              <lb/>
            wiſe draw the Line A E from the given Point
              <lb/>
            A, to the Point E, wherein the Line H C cuts
              <lb/>
            the Baſe Line. </s>
            <s xml:id="echoid-s576" xml:space="preserve">Finally, draw O D, from the
              <lb/>
            Eye O to the Point D, wherein the ſaid Line
              <lb/>
            HC cuts the Horizontal Line.</s>
            <s xml:id="echoid-s577" xml:space="preserve"/>
          </p>
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