## Full text: Gravesande, Willem Jacob: An essay on perspective

An ESSAY viſible Portions of the ſaid Baſes. Note, This
Method may be demonſtrated without Algebra,
but it would be very long.

53.

## 93. Problem IX.

68. To find the Accidental Point of ſeveral pa-
rallel Lines, which are inclin’d to the Geome-
trical Plane.

Let A B be the Direction of one of the Lines,
whoſe accidental Point is ſought; and ECP, the
Angle that the ſaid Lines make with the Geo-
metrical Plane.

Fig. 36.

## 94. Operation .

Draw a Line, O D, thro’ the Eye O, parallel
to A B, and thro’ the Point D, wherein it cuts
the Horizontal Line, and which is the acciden-
tal Point of the Directions of the given Lines,
draw D F perpendicular to the ſaid Horizontal
Line; in which aſſume D G, equal to DO. Fi-
nally, thro’ the Point G, draw the Line G F,
making an Angle with the Horizontal Line, equal
to E C P; and then the Point F, (the Interſection
of this Line) and the Perpendicular D F, is the
accidental Point ſought.

Note, When the Lines are inclin’d towards
the perſpective Plane, D F and G F muſt be
drawn below the Horizontal Line: And, contra-
riwiſe, when the ſaid Lines are inclin’d towards
the oppoſite Part of the perſpective Plane, the
aforeſaid Lines muſt be drawn above the ſaid
Horizontal Line, as is done here.

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