Gravesande, Willem Jacob 's, An essay on perspective

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            and make an Angle of 22 {1/2} with the Horizon.
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            <s xml:id="echoid-s2448" xml:space="preserve">Let MN and GH be two Mirrours in the before-
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              <note position="left" xlink:label="note-0188-01" xlink:href="note-0188-01a" xml:space="preserve">Fig. 72.</note>
            mentioned Situation; </s>
            <s xml:id="echoid-s2449" xml:space="preserve">we are to demonſtrate,
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            that if the Ray A B is parallel to the Horizon,
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            after being reflected in B and C, it ought to fall
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            perpendicularly upon the Machine. </s>
            <s xml:id="echoid-s2450" xml:space="preserve">The Angle
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            A B N is 112 {1/2} Degrees; </s>
            <s xml:id="echoid-s2451" xml:space="preserve">and conſequently
              <note symbol="*" position="left" xlink:label="note-0188-02" xlink:href="note-0188-02a" xml:space="preserve">11.</note>
            Angle A B M, and its equal, the Angle of Re-
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            flection C B G, are each 67 {1/2} Degrees. </s>
            <s xml:id="echoid-s2452" xml:space="preserve">The An-
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            gle B P Q, is the Complement of the Angle
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            N B A, plus the Angle P Q B, which is 22 {1/2}
              <note symbol="*" position="left" xlink:label="note-0188-03" xlink:href="note-0188-03a" xml:space="preserve">23.</note>
            grees; </s>
            <s xml:id="echoid-s2453" xml:space="preserve">whence the Angle B P Q is 45 Degrees.
              <lb/>
            </s>
            <s xml:id="echoid-s2454" xml:space="preserve">Again, the Angle PC B is the Complement of
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            the two Angles C B P and B PC to 180 Degrees; </s>
            <s xml:id="echoid-s2455" xml:space="preserve">
              <lb/>
            and conſequently it is 67 {1/2} Degrees, which is the
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            ſame as its equal, the Angle Q C a of Reflection. </s>
            <s xml:id="echoid-s2456" xml:space="preserve">
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            And reaſoning after the ſame Manner, the An-
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            gle C R Q of the Triangle R C Q, is a right one. </s>
            <s xml:id="echoid-s2457" xml:space="preserve">
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            Which was to be demonſtrated.</s>
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            <s xml:id="echoid-s2459" xml:space="preserve">28. </s>
            <s xml:id="echoid-s2460" xml:space="preserve">It is not abſolutely neceſſary to give the
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            Mirrours the aforeſaid Inclinations; </s>
            <s xml:id="echoid-s2461" xml:space="preserve">for the An-
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            gle A B N may be aſſumed at Pleaſure, from
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            which muſt be taken an Angle of 135 Degrees,
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            to have the Inclination of the Miror G H. </s>
            <s xml:id="echoid-s2462" xml:space="preserve">Ne-
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            vertheleſs, the Angles we have determin’d, are
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            the moſt advantagious for a perpendicular Pi-
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            cture.</s>
            <s xml:id="echoid-s2463" xml:space="preserve"/>
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          <p>
            <s xml:id="echoid-s2464" xml:space="preserve">29. </s>
            <s xml:id="echoid-s2465" xml:space="preserve">When a Picture is inclin’d, and makes
              <lb/>
              <note position="left" xlink:label="note-0188-04" xlink:href="note-0188-04a" xml:space="preserve">Fig. 73.</note>
            the Angle D I A with the Horizon, the Mirrour
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            M N muſt keep its Situation, and the
              <note symbol="*" position="left" xlink:label="note-0188-05" xlink:href="note-0188-05a" xml:space="preserve">24.</note>
            C Q R is equal to half the Angle D I A, leſs
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            {1/4} of a right Angle: </s>
            <s xml:id="echoid-s2466" xml:space="preserve">Then I ſay, the Angle
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            FaC, or its equal C R Q, will be equal to the
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            Angle B I D. </s>
            <s xml:id="echoid-s2467" xml:space="preserve">Now the Angle P B Q, is 112 {1/2}.</s>
            <s xml:id="echoid-s2468" xml:space="preserve">
              <note symbol="*" position="left" xlink:label="note-0188-06" xlink:href="note-0188-06a" xml:space="preserve">11.</note>
            Degrees; </s>
            <s xml:id="echoid-s2469" xml:space="preserve">whence the Angle B P Q, which is the
              <lb/>
            Complement of P B Q, and P Q B to two right
              <lb/>
            Angles, is 90 Degrees, leſs the half of the
              <note symbol="*" position="left" xlink:label="note-0188-07" xlink:href="note-0188-07a" xml:space="preserve">27.</note>
            gle D I A: </s>
            <s xml:id="echoid-s2470" xml:space="preserve">Wherefore becauſe N B C is 67 {1/2} </s>
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