Gravesande, Willem Jacob 's, An essay on perspective

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          <p>
            <s xml:id="echoid-s2426" xml:space="preserve">
              <pb o="111" file="0187" n="216" rhead="on PERSPECTIVE."/>
            Angle with the Horizon, equal to half the
              <lb/>
            Inclination of the Picture leſs {1/4} of a right Angle.
              <lb/>
            </s>
            <s xml:id="echoid-s2427" xml:space="preserve">This Angle is found exactly enough for Practice,
              <lb/>
            by inclining the Mirrour L, until the Repreſenta-
              <lb/>
            tion of the Top of the Machine, after a double Re-
              <lb/>
            flection, appears in the other Mirrour under an
              <lb/>
            Angle with the Horizon, equal to the Inclination
              <lb/>
            one would give the Picture. </s>
            <s xml:id="echoid-s2428" xml:space="preserve">Note, If the Incli-
              <lb/>
            nation of the Picture be leſſer than {1/4} of 90 De-
              <lb/>
            grees, the Looking-Glaſs L muſt not be inclin’d
              <lb/>
            towards the other, as is directed, but
              <note symbol="*" position="right" xlink:label="note-0187-01" xlink:href="note-0187-01a" xml:space="preserve">23:
                <unsure/>
              </note>
            contrary Way, in making the Angle of the In-
              <lb/>
            clination of the Looking-Glaſs, equal to the
              <lb/>
            Difference of the Inclination of the Picture, and
              <lb/>
            {1/4} of 90 Degrees.</s>
            <s xml:id="echoid-s2429" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2430" xml:space="preserve">25. </s>
            <s xml:id="echoid-s2431" xml:space="preserve">When the Objects are to be repreſented
              <lb/>
            for a parallel Picture, the Looking-Glaſs L muſt
              <lb/>
            be placed in the Diſpoſition of Numb. </s>
            <s xml:id="echoid-s2432" xml:space="preserve">15. </s>
            <s xml:id="echoid-s2433" xml:space="preserve">and
              <lb/>
            the Looking-Glaſs H in that mentioned, Numb.
              <lb/>
            </s>
            <s xml:id="echoid-s2434" xml:space="preserve">13. </s>
            <s xml:id="echoid-s2435" xml:space="preserve">by inclining it towards the Horizon, under an
              <lb/>
            Angle of 45 Degrees; </s>
            <s xml:id="echoid-s2436" xml:space="preserve">the poliſhed Side thereof
              <lb/>
            facing downwards, when the Picture is ſuppoſed
              <lb/>
            underneath the Eye; </s>
            <s xml:id="echoid-s2437" xml:space="preserve">and upwards, when it is
              <lb/>
            ſuppoſed above the Eye.</s>
            <s xml:id="echoid-s2438" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2439" xml:space="preserve">26. </s>
            <s xml:id="echoid-s2440" xml:space="preserve">This Diſpoſition of the Machine may be
              <lb/>
            likewiſe uſeſul for inclin’d Pictures, making very
              <lb/>
            ſmall Angles with the Horizon; </s>
            <s xml:id="echoid-s2441" xml:space="preserve">in which Caſe,
              <lb/>
            the Inclination of one of the Looking-Glaſſes
              <lb/>
            muſt be diminiſh’d, by half of the Inclination
              <lb/>
            of the Picture.</s>
            <s xml:id="echoid-s2442" xml:space="preserve"/>
          </p>
          <p style="it">
            <s xml:id="echoid-s2443" xml:space="preserve">27. </s>
            <s xml:id="echoid-s2444" xml:space="preserve">A Demonſtration of the Inclination of the
              <lb/>
            Mirrours.</s>
            <s xml:id="echoid-s2445" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2446" xml:space="preserve">We have mentioned, that for a
              <note symbol="*" position="right" xlink:label="note-0187-02" xlink:href="note-0187-02a" xml:space="preserve">22.</note>
            lar Picture, one of the Mirrours muſt make an
              <lb/>
            Angle of 112 {1/2} Degrees with the Horizon; </s>
            <s xml:id="echoid-s2447" xml:space="preserve">
              <note symbol="*" position="right" xlink:label="note-0187-03" xlink:href="note-0187-03a" xml:space="preserve">11.</note>
            the other, L, muſt be inclin’d towards the </s>
          </p>
        </div>
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    </echo>
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