Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY
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<
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VI.</
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">Let A C be a Line inclined to the Geometrical
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Plane, and O D another Line drawn parallel to
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A C, from the Eye to the perſpective Plane. </
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if B A be drawn in the Geometrical Plane, pa-
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rallel to the baſe Line, and likewiſe D E, in the
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perſpective Plane, parallel to the ſaid Line, ſo that
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B A be to A C, as E d to D O. </
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pearance of the Line B C, paſſing through the Point
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B, and the Extremity of the Line A C, being con-
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tinued, will meet the Point E.</
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s
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need but demonſtrate, that O E is parallel to
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B C: </
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Manner:</
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<
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</
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">whence the Angle (E D O) of the Triangle
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O E D, is equal to the Angle (B A C) of the
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Triangle A C B: </
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are ſimilar; </
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Proportional. </
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angles, have two of their Sides parallel, the
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third B C is alſo parallel to O E; </
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demonſtrated.</
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.</
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the Appearance of B C will paſs thro’ the Point E,</
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ſpective Plane, ſuppoſed to be perpendicu-
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lar, or upright.</
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have hitherto conſider’d the Geometrical Plane,
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as it were the Ground upon which the </
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