Gravesande, Willem Jacob 's
,
An essay on perspective
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on PERSPECTIVE.
"/>
ved backwards or forwards, or elſe the Looking-
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glaſs raiſed or lower’d, until the Rays proceed-
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ing from the Statue may be reflected by the Mir-
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rour upon the Convex Glaſs. </
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<
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xml:space
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">When theſe Alte-
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rations of the Box, or Mirrour, are not ſufficient to
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throw the Rays upon the Convex Glafs, the whole
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Machine muſt be removed backwards or for-
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wards.</
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<
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.</
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<
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">Concerning the before-mention’d Inclination of the
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Mirrours.</
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">In order to demonſtrate, that the Mirrour
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L hath been conveniently inclin’d, we need on-
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ly prove, that the reflected Rays fall upon the
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Table A under the ſame Angle, as the direct
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Rays do upon a Plane, having the ſame Situation
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as one would give to the Picture.</
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<
s
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">Now let A B be a Ray falling from a Point of
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ſome Object upon the Mirrour G H, and from
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thence is reflected in the Point a upon the Table
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of the Machine: </
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<
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">We are to demonſtrate, that if
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the Line D I be drawn, making an Angle with
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FE equal to the Inclination of the Picture; </
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<
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is, if the Angle DIE be the double of the
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D F I; </
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">I ſay, we are to demonſtrate, that the
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Angle B a f is equal to the Angle BCD.</
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<
s
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">The Angle DIE, by Conſtruction, is the double
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of the Angle DFI; </
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<
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">and conſequently this laſt Angle
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is equal to the Angle I D F; </
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<
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xml:space
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">and ſince the Angle
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of Incidence C B D is equal to the Angle of Re-
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flection a B F, the Triangle BCD is ſimilar to
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the Triangle F a B: </
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">Whence it follows, that the
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Angle Ba F is equal to the Angle BCD. </
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<
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was to be demonſtrated.</
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