Gravesande, Willem Jacob 's
,
An essay on perspective
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Table of Notes
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An ESSAY
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<
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<
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xml:space
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">Concerning what hath been ſaid of the
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Picture being parallel, it muſt be obſerv’d, that
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in the precedent Demonſtration, the Angle of
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Inclination of the Picture in this Demonſtration
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is meaſur’d next to the Objects; </
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<
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xml:space
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">and if this An-
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gle be diminiſh’d till it become nothing, we
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ſhall have a Picture parallel to the Horizon un-
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derneath the Eye. </
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<
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xml:space
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">But, by the Demonſtration,
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the Inclination of the Mirrour being half the In-
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clination of the Picture, it follows, that the In-
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clination of the Mirrour is alſo equal to nothing,
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and conſequently it ought to be likewiſe parallel
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to the Horizon. </
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<
s
xml:id
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xml:space
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">In the ſame manner we de-
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monſtrate, that the Looking-glaſs muſt be verti-
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cally ſituated, when we conſider the Picture pa-
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rallel above the Eye: </
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<
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">For to give this Situation
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to the Picture, the Angle of the Inclination of
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the Picture meaſur’d next to the Objects, muſt
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be augmented till it be 180 Degrees, whoſe half
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90 Degrees is conſequently the Inclination of the
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Mirrour.</
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<
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. II.</
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">To repreſent Objects, ſo that what appears on
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the right Hand, ought to be on the left.</
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<
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<
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">Having placed the Box X, in the Situation
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">Fig. 70.</
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as per Figure, the Door B muſt be opened, and
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the Aperture N ſhut; </
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<
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">then putting the Mirrour H
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in the Diſpoſition mentioned in Numb. </
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<
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up the Mirrour L towards the Top of the Box;
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</
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<
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xml:space
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">and incline it towards the firſt Mirrour, in ſuch
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manner that it makes an Angle with the Hori-
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zon of 22 {1/2} Degrees; </
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<
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xml:space
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">that is, that the Top of the
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Machine, after a double Reflection, appears ver-
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cal in the Mirrour H.</
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<
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<
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">Now if Objects are to be repreſented for
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the Picture inclin’d, the Mirrour L muſt make </
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