Gravesande, Willem Jacob 's, An essay on perspective
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          <pb o="110" file="0186" n="215" rhead="An ESSAY"/>
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            <s xml:id="echoid-s2407" xml:space="preserve">20. </s>
            <s xml:id="echoid-s2408" xml:space="preserve">Concerning what hath been ſaid of the
              <lb/>
            Picture being parallel, it muſt be obſerv’d, that
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            in the precedent Demonſtration, the Angle of
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            Inclination of the Picture in this Demonſtration
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            is meaſur’d next to the Objects; </s>
            <s xml:id="echoid-s2409" xml:space="preserve">and if this An-
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            gle be diminiſh’d till it become nothing, we
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            ſhall have a Picture parallel to the Horizon un-
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            derneath the Eye. </s>
            <s xml:id="echoid-s2410" xml:space="preserve">But, by the Demonſtration,
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            the Inclination of the Mirrour being half the In-
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            clination of the Picture, it follows, that the In-
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            clination of the Mirrour is alſo equal to nothing,
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            and conſequently it ought to be likewiſe parallel
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            to the Horizon. </s>
            <s xml:id="echoid-s2411" xml:space="preserve">In the ſame manner we de-
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            monſtrate, that the Looking-glaſs muſt be verti-
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            cally ſituated, when we conſider the Picture pa-
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            rallel above the Eye: </s>
            <s xml:id="echoid-s2412" xml:space="preserve">For to give this Situation
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            to the Picture, the Angle of the Inclination of
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            the Picture meaſur’d next to the Objects, muſt
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            be augmented till it be 180 Degrees, whoſe half
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            90 Degrees is conſequently the Inclination of the
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            Mirrour.</s>
            <s xml:id="echoid-s2413" xml:space="preserve"/>
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        <div xml:id="echoid-div375" type="section" level="1" n="204">
          <head xml:id="echoid-head224" xml:space="preserve">
            <emph style="sc">Prob</emph>
          . II.</head>
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            <s xml:id="echoid-s2414" xml:space="preserve">22. </s>
            <s xml:id="echoid-s2415" xml:space="preserve">To repreſent Objects, ſo that what appears on
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            the right Hand, ought to be on the left.</s>
            <s xml:id="echoid-s2416" xml:space="preserve"/>
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          <p>
            <s xml:id="echoid-s2417" xml:space="preserve">23. </s>
            <s xml:id="echoid-s2418" xml:space="preserve">Having placed the Box X, in the Situation
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              <note position="left" xlink:label="note-0186-01" xlink:href="note-0186-01a" xml:space="preserve">Fig. 70.</note>
            as per Figure, the Door B muſt be opened, and
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            the Aperture N ſhut; </s>
            <s xml:id="echoid-s2419" xml:space="preserve">then putting the Mirrour H
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            in the Diſpoſition mentioned in Numb. </s>
            <s xml:id="echoid-s2420" xml:space="preserve">11. </s>
            <s xml:id="echoid-s2421" xml:space="preserve">raiſe
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            up the Mirrour L towards the Top of the Box;
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            </s>
            <s xml:id="echoid-s2422" xml:space="preserve">and incline it towards the firſt Mirrour, in ſuch
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            manner that it makes an Angle with the Hori-
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            zon of 22 {1/2} Degrees; </s>
            <s xml:id="echoid-s2423" xml:space="preserve">that is, that the Top of the
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            Machine, after a double Reflection, appears ver-
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            cal in the Mirrour H.</s>
            <s xml:id="echoid-s2424" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2425" xml:space="preserve">24. </s>
            <s xml:id="echoid-s2426" xml:space="preserve">Now if Objects are to be repreſented for
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            the Picture inclin’d, the Mirrour L muſt make </s>
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