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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">To demonſtrate which, draw the Line D L M
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thro’ the Point D, parallel to a b s. </
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<
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xml:space
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cauſe the Triangles D M O and D L i are ſimi-
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lar, we have,</
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</
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<
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<
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xml:space
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">D M = as: </
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<
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">D L = ab:</
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<
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xml:space
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<
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">L i. </
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<
s
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xml:space
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">Again,
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in the precedent Figure, the Triangles A S C and
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A B E are ſimilar: </
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A S: </
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">A B:</
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<
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">: C S: </
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<
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">E B.</
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<
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</
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<
s
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">The three firſt Terms of theſe two Progreſſions
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are the ſame: </
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<
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xml:space
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">For CS is equal to M O, ſince
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they are each the Difference of the Height of the
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Eye, and that of the given Point; </
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<
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xml:space
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quently, E B is equal to L i: </
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<
s
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xml:space
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">But B I was made
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equal to B E, pl{us} FC the Height of the given
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Point above the Geometrical Plane; </
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<
s
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xml:space
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">and b i is
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equal to Li, pl{us} b L; </
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<
s
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xml:space
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">which being equal to aD,
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is likewiſe the Height of the given Point above
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the Geometrical Plane; </
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<
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xml:space
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">whence the Lines B I
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and b i are equal. </
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<
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xml:space
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">Which was to be demon-
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ſtrated.</
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<
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</
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<
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<
s
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xml:space
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">Note, When the Height of the given Point is
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greater than the Height of the Eye, E B muſt
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be taken from that firſt Height, to have the
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Magnitude of B I.</
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<
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. VI.</
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">52. To throm a Pyramid, or Cone, into Perſpective.</
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<
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">Now, to throw a Pyramid into perſpective,
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<
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">Fig. 20.</
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the Appearance of its Baſe and Center muſt
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found : </
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>
<
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">After which, Lines muſt be drawn
<
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">50.</
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the Repreſentation of the Vertex, to the Ap-
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pearance of thoſe Angles of the Baſe that are
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viſible; </
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>
<
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xml:space
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">and then the Perſpective ſought will be
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had.</
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</
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<
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<
s
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xml:space
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">And to throw a Cone into perſpective, the
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<
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xlink:label
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">Fig. 21.</
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>
Repreſentation of its Baſe and Vertex muſt
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