Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY, &c.
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">A Demonſtration of the Inclination of the Looking-
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Glaſs.</
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<
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<
s
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xml:space
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">Let A B be a Ray, proceeding from ſome
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">Fig, 75.</
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Point of an Object. </
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<
s
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">19.</
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if the Line D I hath the Inclination given to a
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Picture, and the Looking-Glaſs G H hath the In-
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clination we have preſcribed, that the Angle
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B a F, will be equal to the Angle B C D. </
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xml:space
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">Now to
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prove this, draw the Line F I parallel to the Ho-
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rizon, then the two Angles I D F and D F I, of
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the Triangle I D F, are together equal to the
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Angle D I E; </
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<
s
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xml:space
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">but the Angle D F I, which is the
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Inclination of the Looking-Glaſs, is equal
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half the Angle D I E, leſs half the Angle I F a;
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</
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<
s
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xml:space
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">and conſequently it is leſs than the Angle F D I,
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by the Quantity of the whole Angle I F a: </
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Therefore if the Angle I F a be added to the An-
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gle D F I, we ſhall have the Angle D F a, equal
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to the Angle F D I: </
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<
s
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xml:space
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">Therefore the Angle
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Fa B will be likewiſe equal to the Angle BCD.</
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<
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Which was to he demonſtrated.</
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<
s
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">In reaſoning nearly after the ſame Manner,
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we demonſtrated what is mentioned
<
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ning the Inclination of the Mirrour, when the
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Box is inclin’d a little backwards.</
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