Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY
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Sides a d and d c of the Figure a b c d. </
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ſame may be demonſtrated of the other Sides;
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</
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<
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xml:space
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">and therefore the Figures are ſimilar.</
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<
s
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">Now to prove the other Part of the Theorem:
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</
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<
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">If a perpendicular be ſuppoſed to be let fall from
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the Eye upon the Plane of the Figure, and con-
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tinued as is neceſſary; </
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<
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">it is evident, that O D,
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will be to O d, as this Perpendicular, which
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meaſures the Diſtance from the Eye to the Plane
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of the Figure, is to the Diſtance of the Eye
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from the Perſpective Plane, which is meaſur’d
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by the Part of the perpendicular, contain’d be-
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tween the Eye and the perſpective Plane. </
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<
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this before was manifeſt; </
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O D : </
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<
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<
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">Whence there is the ſame Proportion between
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A d one of the Sides of the Figure, and A D its
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Appearance, as the Theorem expreſſes. </
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ſame may be demonſtrated of the other Sides of
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the Figure. </
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<
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I.</
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right Lines proceed, which are equal between them-
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ſelves, and parallel to the perſpective Plane; </
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of which is in the Geometrical Plane, the ſecond ele-
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vated Perpendicular to the firſt, and the third in-
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clined to it; </
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Lines are equal.</
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<
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the Lines as a Figure parallel to the perſpective
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Plane; </
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ſame Proportion as their Appearances.</
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<
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parallel to the baſe Line; </
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the perſpective Plane is perpendicular or </
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