Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY
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<
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xml:space
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">The Reaſon of this is evident, from the Demon-
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ſtration of the upright Cone: </
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<
s
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xml:space
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">For, it is manifeſt, that
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X is the Seat of the Center of the Circle, which the
<
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Cone continued forms in the Horizontal Plane; </
s
>
<
s
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xml:space
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">and
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conſequently, the Point s, in regard to the Circle BED,
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is in the ſame Situation as the Eye is, in reſpect of
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the Interſection of the continued Cone, and the Hori-
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zontal Plane.</
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<
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<
s
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xml:space
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">Note, moreover, that a Cone can ſcarcely ever
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be thrown into Perſpective, by the common Me-
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thod, ſo exact as by this.</
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<
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VII.</
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">55. To find the Perſpective of a Line, perpendicular
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to the Geometrical Plane.</
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s
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xml:space
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equal to B C, and perpendicular to the Geome-
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trical Plane, in the Point A.</
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<
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.</
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<
s
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">Aſſume E D, any where in the Baſe Line,
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equal to B C; </
s
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<
s
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xml:space
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">and from the Points D and E,
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draw D F and E F to ſome Point F, taken at
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pleaſure in the Horizontal Line. </
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>
<
s
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xml:space
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">Then having
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found a, the Repreſentation of the Point A;</
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>
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<
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*
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xlink:label
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xml:space
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">22.</
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draw a H parallel to the Baſe Line, and aI per-
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pendicular thereto: </
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<
s
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xml:space
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">And if aI be made equal to
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G H, the ſaid a I will be the Perſpective
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ſought.</
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<
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.</
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xml:space
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">6.</
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pendicular to the Baſe Line, and equal to
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xlink:label
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Perſpective of the Line A L, drawn from </
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