Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY
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Plane; </
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<
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xml:space
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">and at the Points R and S, raiſe the in-
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definite perpendiculars R G and S M; </
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xml:space
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">and aſſume
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the Point M at Pleaſure on S M; </
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<
s
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xml:space
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">from which
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raiſe the Perpendicular M N, equal to the given
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Line, and draw the Lines M O and N O, cutting
<
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<
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">Fig. 41.</
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the Line R G in the Points E and G. </
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s
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">Then
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having drawn a Line at Pleaſure in the per-
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ſpective Plane through the Point T, which is
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that wherein a Perpendicular falling from the
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Eye on the perſpective Plane meets it, aſſume
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T H in the ſaid Line, equal to R E, and T I e-
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qual to R G; </
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<
s
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">draw the Lines Ta, Ha, through
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the Point a, the Perſpective of the Foot of the
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given Perpendicular, and through the Point I,
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the Line I X, parallel to Ha, and cutting Ta
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in X, then a X will be the Appearance ſought.</
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<
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.</
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<
s
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xml:space
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">It is manifeſt , that the Point T, is the
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">13, 14.</
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dental Point of Lines perpendicular to the Geo-
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metrical Plane; </
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s
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">and conſequently the Per-
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ſpective ſought is a Part of T a. </
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<
s
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xml:space
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">Moreover, it
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is manifeſt , that if the Feet and
<
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">4.</
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of two equal right Lines, perpendicular to the
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Geometrical Plane be joyn’d by Lines, theſe
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Lines of Junction will have parallel Repreſen-
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tations; </
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<
s
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xml:space
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">becauſe they are parallel to each other,
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as likewiſe to the perſpective Plane. </
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<
s
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xml:space
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">And con-
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ſequently, ſince H I, by Conſtruction, is the
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Perſpective of a Line perpendicular to the
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Geometrical Plane, and equal to the given Line,
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and H a paſſes through the Appearances of the
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Foot of the ſaid Perpendicular, and the given
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Perpendicular; </
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<
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">I ſay, that X I, which is paral-
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lel to Ha, and paſſes through the Extremity of
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the Appeaarance H I, likewiſe paſſes through
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the Extremity of the given Line; </
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