Gravesande, Willem Jacob 's
,
An essay on perspective
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the Circular Baſe of the Cone will be deter-
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min’d.</
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<
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">To prove this, draw the Lines B C and L F, cut-
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ting the Line A S in the Points N and M; </
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xml:space
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the Line G n equal to A N, and draw the Line
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n D m. </
s
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<
s
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">It is now manifeſt, that if the Cone be
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continued out above its Vertex, (that is, if the oppo-
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ſite Cone be form’d) it will cut the Horizontal Plane
<
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in a Circle equal to B E C, whoſe Seat will be BEC:
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</
s
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<
s
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xml:space
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">So that the Point S, in reſpect of B E C, is in the
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ſame Situation as the Eye hath, with reſpect to the
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Circle form’d in the Horizontal Plane, by the Conti-
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nuation of the Cone. </
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s
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">Whence it follows, that B C
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is the Seat of the viſible Portion of that Circle. </
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xml:space
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">For,
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by Conſtruction, B and C are the Points of Contact
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of the Tangents to the Circle B E C, which paſs
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thro’ the Point S; </
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<
s
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xml:space
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">becauſe the Angle ABS, which
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is in a Semicircle, is a right one.</
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s
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">Now, if a Plane be conceiv’d, as paſſing thro’ ſome
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Points in the Horizontal Plane, whoſe Seats are
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B and C, and which cuts the two oppoſite Cones
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thro’ their Vertex; </
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s
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">it is evident, that this Plane
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continued, will cut the Geometrical Plane in a Line
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parallel to B N C; </
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xml:space
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ſaid Plane, will determine the viſible Part of the
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Cone’s Baſe. </
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<
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xml:space
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">So, ſince G n was made equal to
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A N, we have only to prove, that P m is equal to
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A M: </
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">For, it follows from thence, that L M F is
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the Common Section of the Geometrical Plane, and
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the Plane which we have here imagin’d.</
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D G: </
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