Gravesande, Willem Jacob 's, An essay on perspective

List of thumbnails

< >
211
211 (106) 		212
212 (107)
213
213 (108) 		214
214 (109)
215
215 (110) 		216
216 (111)
217
217 (112) 		218
218 (113)
219
219 (114) 		220
220 (115)
< >
page |< < (113) of 237 > >|
    <echo version="1.0RC">
      <text xml:lang="en" type="free">
        <div xml:id="echoid-div375" type="section" level="1" n="204">
          <p>
            <s xml:id="echoid-s2470" xml:space="preserve">
              <pb o="113" file="0189" n="218" rhead="on PERSPECTIVE."/>
            Angle B C P, and its equal R C Q, is 22 {1/2} plus
              <lb/>
            {1/2} of D I A. </s>
            <s xml:id="echoid-s2471" xml:space="preserve">Now if the Angle R Q C be added
              <lb/>
            to this Angle, their Sum will be equal to the An-
              <lb/>
            gle D I A; </s>
            <s xml:id="echoid-s2472" xml:space="preserve">whence it follows, that the Angle
              <lb/>
            C R Q is equal to D I R. </s>
            <s xml:id="echoid-s2473" xml:space="preserve">Which was to be demon-
              <lb/>
            ſtrated.</s>
            <s xml:id="echoid-s2474" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2475" xml:space="preserve">30. </s>
            <s xml:id="echoid-s2476" xml:space="preserve">If the Angle R B N be alter’d, and it be
              <lb/>
            called a, the Angle D I A, b, and the right An-
              <lb/>
            gle d; </s>
            <s xml:id="echoid-s2477" xml:space="preserve">then the Angle C Q R = d + {1/2} b — a.</s>
            <s xml:id="echoid-s2478" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2479" xml:space="preserve">31. </s>
            <s xml:id="echoid-s2480" xml:space="preserve">When a Picture is parallel, it appears ma-
              <lb/>
              <note position="right" xlink:label="note-0189-01" xlink:href="note-0189-01a" xml:space="preserve">Fig. 74.</note>
            nifeſt, the Mirrours GH and M N being each in-
              <lb/>
            clin’d under an Angle of 45 Degrees, that a Ray,
              <lb/>
            which is perpendicular to the Horizon, likewiſe
              <lb/>
            falls, after a double Reflection, perpendicularly
              <lb/>
            upon the Table A.</s>
            <s xml:id="echoid-s2481" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div381" type="section" level="1" n="205">
          <head xml:id="echoid-head225" xml:space="preserve">
            <emph style="sc">Prop</emph>
          . III.</head>
          <p style="it">
            <s xml:id="echoid-s2482" xml:space="preserve">32. </s>
            <s xml:id="echoid-s2483" xml:space="preserve">To repreſent Objects which are roun’d about
              <lb/>
            the Machine, and make them appear erect to the Per-
              <lb/>
            ſon ſeated within the ſame.</s>
            <s xml:id="echoid-s2484" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2485" xml:space="preserve">The Back-ſide of the Machine muſt be turned
              <lb/>
            towards the Sun, and the Objects behind the
              <lb/>
            ſame repreſented by one Reflection only; </s>
            <s xml:id="echoid-s2486" xml:space="preserve">
              <note symbol="*" position="right" xlink:label="note-0189-02" xlink:href="note-0189-02a" xml:space="preserve">15.</note>
            their Appearance will always be clearer, altho
              <lb/>
            they be in the Shade, than the Appearance of the
              <lb/>
            Objects on the other Sides of the Machine, which
              <lb/>
            cannot be perceiv’d unleſs by a double Refle-
              <lb/>
            ction.</s>
            <s xml:id="echoid-s2487" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2488" xml:space="preserve">33. </s>
            <s xml:id="echoid-s2489" xml:space="preserve">The Objects that are on the right and left
              <lb/>
              <note position="right" xlink:label="note-0189-03" xlink:href="note-0189-03a" xml:space="preserve">Fig. 70.</note>
            of the Machine, may be repreſented by means
              <lb/>
            of the Mirrour H, ſituated as per Figure; </s>
            <s xml:id="echoid-s2490" xml:space="preserve">but
              <note symbol="*" position="right" xlink:label="note-0189-04" xlink:href="note-0189-04a" xml:space="preserve">12.</note>
            ſaid Mirrour muſt be cover’d with a Paſtboard Caſe,
              <lb/>
            having two Apertures therein; </s>
            <s xml:id="echoid-s2491" xml:space="preserve">the one next to
              <lb/>
            the Objects, and the other next to the Aperture
              <lb/>
            N, of the Box X. </s>
            <s xml:id="echoid-s2492" xml:space="preserve">The Reaſon of our uſing this
              <lb/>
            @recaution is, becauſe when the Mirrour is not co-
              <lb/>
            ver’d at all, it reflects the Rays of Light </s>
          </p>
        </div>
      </text>
    </echo>
Impressum