Gravesande, Willem Jacob 's
,
An essay on perspective
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on PERSPECTIVE.
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in the Perpendicular R S, continued equal to R S,
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and draw f i parallel to the Baſe Line, in which
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take the Point f in E I, the ſame as F in F I ; </
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">110.</
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if the Threads which before were faſten’d in F,
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being placed in f, and by uſing them thus faſten’d,
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as likewiſe S Q for a Baſe Line, the Repreſen-
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tations of the Feet of the Perpendiculars be
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found , you will have the Repreſentations
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their Extremities.</
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">The Demonſtration of the two laſt Ways.</
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">If a Plane be imagined to paſs through
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">Fig. 60,
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and 61.</
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the Extremities of the equal Perpendiculars, it
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will be parallel to the Geometrical Plane, and
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will meet the perſpective Plane in S Q; </
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<
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R S is equal to the ſaid Perpendiculars: </
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the Extremities of theſe Perpendiculars form a
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Figure in this ſuppoſed Plane, ſimilar to that
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which their Feet form in the Geometrical Plane;
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">and the ſaid Figure hath the ſame Situation with
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regard to the Line Q S, as that on the Geome-
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trical Plane hath in reſpect of H R: </
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quently, if the Figure in the Geometrical Plane
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be ſo raiſed up, that it hath the ſame Reſpect to
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Q S, as it had to H R, and if the Appearances
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of the Feet of the propoſed Perpendiculars be
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found, the Repreſentations of their Extremties
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will be had. </
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<
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">But the before ſuppoſed Tranſpo-
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ſition of the Figure in the Geometrical Plane,
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gives it the requiſite Situation with regard to
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Q S, and the Repreſentation of the Figure
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conſider’d in this new Geometrical Plane is
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found ; </
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<
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">becauſe S Q, is taken for the Baſe
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O f (Fig. </
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above this Plane, and fi (Fig. </
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metrical Line in the ſaid Plane.</
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