Gravesande, Willem Jacob 's, An essay on perspective

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            <s xml:id="echoid-s2520" xml:space="preserve">
              <pb o="116" file="0192" n="221" rhead="An ESSAY on"/>
            ſemblance. </s>
            <s xml:id="echoid-s2521" xml:space="preserve">The Reaſon of which is, that the Li-
              <lb/>
            neaments appear lively and diſtinct within the
              <lb/>
            Machine, when the Re-union of the Rays pro-
              <lb/>
            ceeding from a given Point in the Face, happens
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            exactly upon the Paper in one Point only: </s>
            <s xml:id="echoid-s2522" xml:space="preserve">But
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            the leaſt Diſtance that one Point is more than
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            another from the Convex Glaſs, (when the Di-
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            ſtance of the Face from the Glaſs is ſo ſmall, as
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            it muſt be to repreſent it in its natural Bigneſs)
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            ſo alters the Place of the ſaid Re-union, that for
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            different Parts of the Face, thoſe Places of Re-
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            union will differ about two Inches and a half.
              <lb/>
            </s>
            <s xml:id="echoid-s2523" xml:space="preserve">Whence it is no wonder that all the Lineaments
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            be not repreſented as could be wiſhed; </s>
            <s xml:id="echoid-s2524" xml:space="preserve">ſince in
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            all Diſtances choſen, there will be always a great
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            many Rays, whoſe Re-union will fall above an
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            Inch beſides the Paper. </s>
            <s xml:id="echoid-s2525" xml:space="preserve">The Confuſion ariſing
              <lb/>
            from this Diverſity, though not being very di-
              <lb/>
            ſtinguiſhable by the Eye, yet is prejudicial, and
              <lb/>
            hinders our getting the exact Reſemblance of the
              <lb/>
            Face. </s>
            <s xml:id="echoid-s2526" xml:space="preserve">We have obſerved this, in order to give
              <lb/>
            an exact Idea of the Goodneſs of this Machine,
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            in equally ſhewing wherein it may be really
              <lb/>
            uſeful, and wherein its apparent Uſefulneſs is
              <lb/>
            ſubject to an Error rather diſcovered by Experi-
              <lb/>
            rience than Reaſon.</s>
            <s xml:id="echoid-s2527" xml:space="preserve"/>
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          <p>
            <s xml:id="echoid-s2528" xml:space="preserve">38. </s>
            <s xml:id="echoid-s2529" xml:space="preserve">We muſt not forget in all the precedent
              <lb/>
            Problems, to examine the Aperture the Convex
              <lb/>
            Glaſs ought to have; </s>
            <s xml:id="echoid-s2530" xml:space="preserve">for although we cannot re-
              <lb/>
            duce this Aperture to a fixed Meaſure, yet it is
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            proper to obſerve the following Remarks. </s>
            <s xml:id="echoid-s2531" xml:space="preserve">1. </s>
            <s xml:id="echoid-s2532" xml:space="preserve">The
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            Convex Glaſs may commonly have the ſame
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            Aperture, as we would give a Perſpective Glaſs,
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            having the ſaid Glaſs for its Object Glaſs. </s>
            <s xml:id="echoid-s2533" xml:space="preserve">2. </s>
            <s xml:id="echoid-s2534" xml:space="preserve">When
              <lb/>
            Objects are very much enlightned, the ſaid A-
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            perture muſt be leſſened; </s>
            <s xml:id="echoid-s2535" xml:space="preserve">and contrariwiſe, when
              <lb/>
            they are expoſed to a weaker Light, it muſt be
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            made greater; </s>
            <s xml:id="echoid-s2536" xml:space="preserve">and when any Repreſentation </s>
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