Gravesande, Willem Jacob 's
,
An essay on perspective
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on PERSPECTIVE.
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<
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VIII.</
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both of them being Perpendicular to the Geometri-
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cal Plane.</
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<
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">Let G H I L M N be the Baſe of the Priſm
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in the Geometrical Plane, and the viſible Part
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thereof upon the perſpective Plane, let be n g h i;
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<
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">then to compleat the Repreſentation of the
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Priſm, draw Perpendiculars from the Points
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n g h and i to the Baſe Line, whoſe Length let
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be ſuch that they may repreſent
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lars to the Geometrical Plane, equal to the
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Height of the Priſm, and find the
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">50.</
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of the other Angular Points of the upper Sur-
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face of the Priſm, in confidering them as Points
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elevated above the Geometrical Plane: </
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<
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being done, if the Repreſentations of all the
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ſaid Angular Points be joyn’d, the whole
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Priſm will be thrown into Perſpective.</
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<
s
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">Now to throw a Cylinder into Perſpective,
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the Repreſentation of its Baſe and upper Sur-
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face muſt firſt be had, by finding the
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">50.</
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ance of ſeveral Points of the Periphery of its
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upper Surface, and then two Perpendiculars
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muſt be ſo drawn to the Baſe Line, that they
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may touch the Appearances of the two circular
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Euds of the Cylinder, and the Appearance of
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the Cylinder will be had. </
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">But to avoid uſeleſs
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Operations, the viſible Part of the Baſe of the
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Cylinder may be thus determin’d. </
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Line A S from the Point A to the Station Point S,
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then this Line muſt be biſected in the Point R,
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about which, as a Centre, and with the Radius
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R A, the Circular Arc B A C, muſt be deſcrib’d
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cutting the Baſe of the Cylinder in the </
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