Gravesande, Willem Jacob 's, An essay on perspective

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          <p>
            <s xml:id="echoid-s186" xml:space="preserve">In order to underſtand well how Mathema-
              <lb/>
            ticks may be apply’d to Drawing; </s>
            <s xml:id="echoid-s187" xml:space="preserve">let us ſup-
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            poſe a Man A, viewing an Object; </s>
            <s xml:id="echoid-s188" xml:space="preserve">and be-
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              <note position="left" xlink:label="note-0020-01" xlink:href="note-0020-01a" xml:space="preserve">Fig. 1.</note>
            tween him and the Object he looks at, let us
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            imagine a tranſparent Plane C. </s>
            <s xml:id="echoid-s189" xml:space="preserve">Suppoſe more-
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            over, that Lines be drawn upon this Plane, as
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            in D, which cover the Bounds of the Object B
              <lb/>
            in reſpect of the Spectator A, and each Part that
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            he ſees thereof. </s>
            <s xml:id="echoid-s190" xml:space="preserve">Now, ſince all Objects are ſeen
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            by the Rays of Light coming from every of
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            their Points, and terminating at the Eye, and
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            not otherwiſe; </s>
            <s xml:id="echoid-s191" xml:space="preserve">and ſince that here all the Rays
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            proceeding from the Object B, likewiſe paſs
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            thro’ every Point of the Repreſentation D; </s>
            <s xml:id="echoid-s192" xml:space="preserve">it is
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            manifeſt, that this Repreſentation will have the
              <lb/>
            ſame Effect upon the Spectator’s Eye, as the
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            ſaid Object B hath. </s>
            <s xml:id="echoid-s193" xml:space="preserve">Now, by means of Geo-
              <lb/>
            metry, we can find the Points of the Figure D,
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            on the Plane C, placed in a given Situation,
              <lb/>
            thro’ which the Rays coming from the Object B
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            to the Eye of the Spectator A, do paſs; </s>
            <s xml:id="echoid-s194" xml:space="preserve">and
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            theſe Points are the Interſections of the Rays
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            and the Plane. </s>
            <s xml:id="echoid-s195" xml:space="preserve">Alſo, (as others have very well
              <lb/>
            obſerv’d) a Perſpective Plane, or Picture in Paint-
              <lb/>
            ing, may be conceiv’d as a Window, upon which
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            the Objects ſeen thro’ it are repreſented.</s>
            <s xml:id="echoid-s196" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s197" xml:space="preserve">Now, without Mathematicks, this Repreſen-
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            tation cannot be well found: </s>
            <s xml:id="echoid-s198" xml:space="preserve">For when Objects
              <lb/>
            are drawn by only viewing, or looking at them;
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            </s>
            <s xml:id="echoid-s199" xml:space="preserve">their true Repreſentations after this way, will
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            be very often miſs’d on; </s>
            <s xml:id="echoid-s200" xml:space="preserve">whereas, by Geome-
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            try, we can always obtain them.</s>
            <s xml:id="echoid-s201" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s202" xml:space="preserve">This Obſervation only, is ſufficient to eſta-
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            bliſh the Neceſſity of Perſpective: </s>
            <s xml:id="echoid-s203" xml:space="preserve">Tho’ there are
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            ſome Painters, who (according to the common
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            Maxim) affirm, That what they do not know
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            of this Art, is not worth the Pains of learn-
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            ing.</s>
            <s xml:id="echoid-s204" xml:space="preserve"/>
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