Gravesande, Willem Jacob 's - An essay on perspective

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16581on PERSPECTIVE. the given Lines; and conſequently L, or 1113, 14. will be the Accidental Point ſought.

Prob. IV.

99. To find the Repreſentation of one or more
Lines inclined to the Geometrical Plane.

Let a b be the Perſpective of the Direction of
22Fig. 56. the given Line: Now the Length of its Directi-
on may be determin’d, by means of the Tri-
angle ECP, according to the Directions of n. 69.
This being done, the Line b X muſt be drawn
through the Point b, equal to E P, and this re-
preſents a Perpendicular to the Geometrical
Plane; then a X being drawn, will be the Re-
preſentation ſought.

Method II.

100. To ſolve the ſame Problem by means of the
accidental Points F and T, the one being that of the
Lines propoſed; and the other, that of the Perpen-
33Fig. 56. diculars to the Geometrical Plane.

Operation.

Draw a Line from the Point F through the
Point a, which interſect in the Point X, by ano-
ther Line drawn from the Point T through b:
and then a X will be the Perſpective ſought.

Method III.

101. The ſame Things being given as in the
44Fig. 101. aforegoing Method, draw a I through the Point
a, equal to E P, repreſenting a Line perpendicu-
lar to the Geometrical Plane. Then draw a pa-
rallel to F T through the Point I, whoſe

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Scan	Original
121	58
122
123
124
125	59
126	60
127
128
129
130	61
131	62
132	63
133	64
134
135
136
137	65
138	66
139	67
140	68
141
142
143
144	69
145	70
146	71
147	72
148
149
150

Scan	Original
121	58
122
123
124
125	59
126	60
127
128
129
130	61
131	62
132	63
133	64
134
135
136
137	65
138	66
139	67
140	68
141
142
143
144	69
145	70
146	71
147	72
148
149
150

Scan	Original
121	58
122
123
124
125	59
126	60
127
128
129
130	61
131	62
132	63
133	64
134
135
136
137	65
138	66
139	67
140	68
141
142
143
144	69
145	70
146	71
147	72
148
149
150