Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY
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<
s
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xml:space
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">Now we have proved, that F G is the half of
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F B, therefore G N is likewiſe equal to the half
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of B L, and conſequently equal to the Height
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of the ſuppoſed Perpendicular.</
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<
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">Again, the ſimilar Triangles F G N and F a I
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give</
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<
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<
s
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s
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">becauſe the Tri-
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angles F G D and F a H are ſimilar.</
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">a I : </
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<
s
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">Now becauſe G N has been proved to be e-
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qual to the Perpendicular, whoſe Perſpective is
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requir’d and D G is ſuppoſed equal to that Per-
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pendicular; </
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<
s
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">it follows, that G N and G D are
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equal; </
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<
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">and therefore a I and a H are alſo equal.
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">I might have aſſumed C P equal to the Perpen-
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dicular, and uſed the Points C and P inſtead of
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B and L. </
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<
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">But uſing the ſaid Points B and L is
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better: </
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<
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">For when the Points C and P are uſed,
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the Horizontal Line muſt almoſt always be con-
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tinued, that ſo a Line drawn through the Points
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c and a may cut it; </
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<
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">moreover this Interſection
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will ſometimes be at an infinite Diſtance; </
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as in uſing the Point B, M N can never be
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greater than thrice the Breadth of the Deſign to
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be drawn.</
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<
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.</
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<
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<
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Plane, may be conceived as the Extremity of a
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Perpendicular to the Geometrical Plane.</
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