Gravesande, Willem Jacob 's, An essay on perspective

Table of Notes

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              <pb o="114" file="0190" n="219" rhead="An ESSAY"/>
            Side-ways upon the Mirrour L, which being again
              <lb/>
            reflected by the ſaid Mirrour L, and going through
              <lb/>
            the Convex Glaſs, extremely weakens the Repre-
              <lb/>
            ſentation.</s>
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          <p>
            <s xml:id="echoid-s2494" xml:space="preserve">34. </s>
            <s xml:id="echoid-s2495" xml:space="preserve">The Objects before the Machine are re-
              <lb/>
            preſented according to N. </s>
            <s xml:id="echoid-s2496" xml:space="preserve">22, and 28.</s>
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        <div xml:id="echoid-div384" type="section" level="1" n="206">
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            <emph style="sc">Problem</emph>
          IV.</head>
          <p style="it">
            <s xml:id="echoid-s2498" xml:space="preserve">35. </s>
            <s xml:id="echoid-s2499" xml:space="preserve">To repreſent Pictures or Prints.</s>
            <s xml:id="echoid-s2500" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2501" xml:space="preserve">If we have a mind to repreſent Pictures and
              <lb/>
              <note position="left" xlink:label="note-0190-01" xlink:href="note-0190-01a" xml:space="preserve">Fig. 70.</note>
            Prints, they muſt be faſten’d againſt the Board
              <lb/>
            F on that Side, regarding the Back of the Ma-
              <lb/>
            chine, which muſt be ſo turned, that the Pictures
              <lb/>
            be expoſed to the Sun. </s>
            <s xml:id="echoid-s2502" xml:space="preserve">Then they are repreſent-
              <lb/>
            ed in this Situation as the other Objects,
              <note symbol="*" position="left" xlink:label="note-0190-02" xlink:href="note-0190-02a" xml:space="preserve">15.</note>
            with this Difference, that the Convex Glaſs in the
              <lb/>
            Cylinder C muſt be changed: </s>
            <s xml:id="echoid-s2503" xml:space="preserve">For if Prints are
              <lb/>
            requir’d to have their true Bigneſs, the focal Di-
              <lb/>
            ſtance of the Convex Glaſs muſt be equal to half
              <lb/>
            the Height of the Machine above the Table;
              <lb/>
            </s>
            <s xml:id="echoid-s2504" xml:space="preserve">that is, equal to half A C. </s>
            <s xml:id="echoid-s2505" xml:space="preserve">Again, if the ſaid
              <lb/>
            Pictures or Prints are requir’d to be repreſented
              <lb/>
            greater than they really are, the focal Diſtance
              <lb/>
            of the Convex Glaſs muſt ſtill be leſſer. </s>
            <s xml:id="echoid-s2506" xml:space="preserve">And
              <lb/>
            if, on the contrary, they are to be repreſented
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            leſſer than they really are, the focal Length of
              <lb/>
            the Glaſs muſt be greater than the Length A C. </s>
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              <lb/>
            Moreover, the proper Diſtance whereat the Pi-
              <lb/>
            ctures or Prints muſt be placed, may be found
              <lb/>
            in ſliding the Board F backwards or forwards,
              <lb/>
            until they diſtinctly appear within the Machine. </s>
            <s xml:id="echoid-s2508" xml:space="preserve">
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            This Diſtance alſo may be determin’d by the
              <lb/>
            following Proportion:</s>
            <s xml:id="echoid-s2509" xml:space="preserve"/>
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        <div xml:id="echoid-div386" type="section" level="1" n="207">
          <head xml:id="echoid-head227" style="it" xml:space="preserve">As the Machine’s Height above the Table, leſs the
            <lb/>
          Glaſs’s focal Length,
            <lb/>
          is to</head>
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