Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY
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<
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<
s
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echoid-s1937
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">Let Z be the Geometrical Plane; </
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<
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echoid-s1938
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">A the Seat
<
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<
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">Fig. 57.</
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>
of the given Point; </
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<
s
xml:id
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echoid-s1939
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xml:space
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">and A B the Direction of
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the Sun’s Rays.</
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<
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">Operation</
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>
.</
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<
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<
s
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">Draw two right Lines, making a right Angle
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with each other; </
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<
s
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echoid-s1942
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">in one of which, aſſume P E,
<
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/>
equal to the Height of the given Point, above
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the Geometrical Plane: </
s
>
<
s
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echoid-s1943
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">Then draw the Line
<
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E C thro’ the Point E; </
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<
s
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echoid-s1944
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xml:space
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">making an Angle with
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C P, equal to the Sun’s Altitude; </
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<
s
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">and make A B
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equal to C P. </
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<
s
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echoid-s1946
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">Find the Appearance of the Point
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B; </
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>
<
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">and the Repreſentation ſought, will be
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had.</
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<
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</
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<
s
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">Note, This Operation, as likewiſe all the
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others in this Chapter, regard all the Situations
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of the perſpective Plane; </
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<
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">and is ſo evident, that
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there is no need of demonſtrating it.</
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<
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>
. II.</
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<
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">104. </
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">To find the Repreſentation of an elevated
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Point, whoſe Appearance, as alſo that of its Seat, is
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given, without uſing the Geometrical Plane.</
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</
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s
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">Find F, the accidental Point of the
<
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">Fig. 58.</
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Rays, and D, that of their Direction: </
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">Then
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<
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position
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">68, 88,
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98.</
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>
draw a Line from the Point D, through a, the
<
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Perſpective of the Seat of the given Point; </
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>
<
s
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echoid-s1957
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xml:space
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">and
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another from F, through I, the Perſpective of
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the given Point: </
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>
<
s
xml:id
="
echoid-s1958
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xml:space
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">And then b, the Interſection
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of the ſaid two Lines, will be the Point ſought;
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</
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>
<
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">as is manifeſt.</
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<
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.</
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order to find the accidental Point of any
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<
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ber of inclin’d Lines, we have </
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