Gravesande, Willem Jacob 's
,
An essay on perspective
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on PERSPECTIVE.
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<
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. V.</
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">50. To find the Repreſentation of a Point, elevated
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above the Geometrical Planc.</
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<
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s
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xml:space
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">Let G S be the Geometrical Line, and S the
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">Fig. 18.</
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Station Point: </
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s
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">Make S F, in the Geometrical
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Line, equal to the Height of the Eye; </
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<
s
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A be the Seat of the given Line.</
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<
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.</
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<
s
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">Aſſume F C in the Geometrical Line, equal to
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the Height of the Eye, above the Geometrical
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Plane: </
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s
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">Then draw Lines from the Point A to
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the Points S and C, and on the Point B, the In-
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terſection of the Line AS and the Baſe Line,
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raiſe the Perpendicular BI to the Baſe Line,
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equal to E B, plus FC; </
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>
<
s
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">and the Point I will be
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the Perſpective ſought.</
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.</
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">Let us ſuppoſe a Plane to paſs thro’ the
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given Point, and the Eye perpendicular to the
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Geometrical Plane; </
s
>
<
s
xml:id
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xml:space
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">then it is manifeſt, that the
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Interſection of theſe two Planes is the Line
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A B S, and the Interſection of the ſaid ſuppos’d
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Plane and the perſpective Plane, is B I. </
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s
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let X be this ſuppos’d Plane; </
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">Fig. 19.</
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mark’d with the ſame Letters in the precedent
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Figure, bi the Interſection of this Plane and
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the perſpective Plane; </
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">O the Eye, and D the
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propos’d Point: </
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be drawn, the Line B I of the precedent Figure
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will be equal to b i in this Figure.</
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