Gravesande, Willem Jacob 's, An essay on perspective

Table of figures

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        <div xml:id="echoid-div388" type="section" level="1" n="209">
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            <s xml:id="echoid-s2536" xml:space="preserve">
              <pb o="117" file="0193" n="222" rhead="PERSPECTIVE."/>
            to be copyed, the Convex Glaſs muſt have the
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            leaſt Aperture poſſible; </s>
            <s xml:id="echoid-s2537" xml:space="preserve">but yet with this Cau-
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            tion, that the Light coming into the Machine,
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            muſt not be too much extenuated. </s>
            <s xml:id="echoid-s2538" xml:space="preserve">From theſe
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            Obſervations it is manifeſt, that we ought to be
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            provided with ſeveral round Pieces of Tin or
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            thin Braſs, having round Holes of different big-
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            neſſes therein, in order to give a neceſſary A-
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            perture to the Glaſs; </s>
            <s xml:id="echoid-s2539" xml:space="preserve">or Holes of different big-
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            neſſes may be made in a long thin Piece of Braſs
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            which may ſlide upon the Convex Glaſs: </s>
            <s xml:id="echoid-s2540" xml:space="preserve">Or
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            elſe, we may uſe a round Plate, having Holes
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            of different bigneſſes therein, which turning a-
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            bout its Centre, may bring any deſired Hole for
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            the Glaſſes Aperture.</s>
            <s xml:id="echoid-s2541" xml:space="preserve"/>
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        <div xml:id="echoid-div390" type="section" level="1" n="210">
          <head xml:id="echoid-head230" style="it" xml:space="preserve">A Deſcription of the Second Machine.</head>
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            <s xml:id="echoid-s2542" xml:space="preserve">39. </s>
            <s xml:id="echoid-s2543" xml:space="preserve">This Machine is a kind of Box, the Side
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              <note position="right" xlink:label="note-0193-01" xlink:href="note-0193-01a" xml:space="preserve">Fig. 78.</note>
            A C G B being open, whoſe breadth B D, and
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            height AB, are equal, each being about 18 Inches:
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            </s>
            <s xml:id="echoid-s2544" xml:space="preserve">Its greateſt width F B, is 10 Inches, and the Side
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            E F is ſloping, ſo that A E is but about 6 Inches.</s>
            <s xml:id="echoid-s2545" xml:space="preserve"/>
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            <s xml:id="echoid-s2546" xml:space="preserve">40. </s>
            <s xml:id="echoid-s2547" xml:space="preserve">The Frame G ſlides at the Bottom of the
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            ſaid Box, in which the Paper is faſtned.</s>
            <s xml:id="echoid-s2548" xml:space="preserve"/>
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          <note symbol="*" position="right" xml:space="preserve">15.</note>
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            <s xml:id="echoid-s2549" xml:space="preserve">41. </s>
            <s xml:id="echoid-s2550" xml:space="preserve">There is a round Hole in the Middle of
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            the Top of the Box, in which the Cylinder
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            carrying the Convex Glaſs ſcrews.</s>
            <s xml:id="echoid-s2551" xml:space="preserve"/>
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          <note symbol="*" position="right" xml:space="preserve">9.</note>
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            <s xml:id="echoid-s2552" xml:space="preserve">42. </s>
            <s xml:id="echoid-s2553" xml:space="preserve">The two Sticks H I and L M, ſlide in four
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            little Iron Staples fixed to the Inſide of the Top
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            of the Box, like thoſe mentioned in Numb. </s>
            <s xml:id="echoid-s2554" xml:space="preserve">7.
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            </s>
            <s xml:id="echoid-s2555" xml:space="preserve">Theſe Sticks come about two Feet without the
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            Box, and the Diſtance of their Extremities I and
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            M, is equal, or ſomething greater than the
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            Length of the Box. </s>
            <s xml:id="echoid-s2556" xml:space="preserve">Their Uſe is to hang a black
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            Cloth upon, which is faſtned to the Three Sides,
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            B A, A C and C D, of the opening of the Box,
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            that ſo the Box may be darkned, when </s>
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