Gravesande, Willem Jacob 's - An essay on perspective

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13061on PERSPECTIVE. continued, it would cut the Horizontal Line in
the Point of Sight.

This is done in aſſuming C H equal to {1/3}, or
{1/4} Part, & c. of the Diſtance from the Point C,
to the Foot of the vertical Line; and in raiſing
the Perpendicular H E, in the Point H, equal
to {1/3} or {1/4} Part, & c. of the Height of the Eye.
Now A is a given Point, whoſe Appearance is
ſought.

Operation.

Draw a Parallel A B, through the Point A,
to the Baſe Line, meeting the Line C D in the
Point B, and let a ſecond Eye be ſuppoſed at
the ſame Height and Diſtance as the firſt; then
find F G the Repreſentation of A B for 1142. ſecond Eye, which continue until it meets the
Line C E in b, and in this Continuation aſſume
b a equal to F G; then a will be the Perſpective
ſought.

Demonstration.

Becauſe the Height and Diſtance of the ſecond
Eye, is equal to the Height and Diſtance of the
firſt; the ſaid two Eyes are both in one parallel
Line A B; and conſequently , the 2218. of A B muſt be a Part of F G continued, and
3312. alſo equal to F G: And therefore becauſe the Perſpective of B is in the Line C E, a b is the
4416. Perſpective of A B; and a , that of A. Which was to be demonſtrated.

78. Note, as to Lines perpendicular, and inclined
to the Geometrical Plane, ſee n. 76. This is
ſcarcely uſeful, unleſs for the Decorations of a
Theatre.

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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