Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY
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<
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I.</
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">The Repreſentation of a Line Parallel to the
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perſpective Plane, is parallel to the Line whereof it is
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the Repreſentation.</
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<
s
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">Let A B be a Line Parallel to the perſpective
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Plane; </
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<
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">we are to prove that a b its Repreſentati-
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on is Parallel thereto.</
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<
s
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">Theſe two Lines A B and a b, will never
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meet each other, becauſe a b is in the perſpective
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Plane, and A B is ſuppoſed parallel to the ſaid
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Plane. </
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<
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">But they are alſo in one and the ſame
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Plane, becauſe a b is the Interſection of the per-
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ſpective Plane, and the Plane O A B, paſſing
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through the Eye and the Line A B; </
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<
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fore they are parallel between themſelves: </
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<
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was to be demonſtrated.</
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<
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">Corollary</
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I.</
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">The Appearance of a Line, parallel to the baſe
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Line, is alſo parallel to the ſaid baſe Line.</
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<
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">For the baſe Line, and the Repreſentation
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being parallel to the ſame Line, are parallel to
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one another.</
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<
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II.</
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">The Repreſentation of a Line parallel to the
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vertical Line, is parallel to the ſaid vertical Line, and
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conſequently perpendicular to the baſe Line. </
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is demonſtrated as in the laſt Corollary.</
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<
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III.</
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ſpective Plane, and equally inclin’d the ſame Way </
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