Gravesande, Willem Jacob 's - An essay on perspective

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3716An ESSAY for then, what is on the Right, will appear on
the Left.

Therefore, I lay my Perſpective Plane upon
the Geometrical Plane; ſo that it be between
the Horizontal Plane, and the Figures requir’d
to be thrown into Perſpective.

Problem I.

22. To find the Appearance of a Point, which is in
the Geometrical Plane.

Let Z be the Geometrical Plane, I E the Baſe
11Fig. 7. Line, D V the Horizontal Line, V the Point
of Sight, D one of the Points of Diſtance, and
A the given Point.

Operation.

From the Point A, let fall the Perpendicular
A B upon the Baſe Line; and from the Point of
Concurrence B, draw the Line B V to the Point
of Sight; then aſſume B E in the Baſe Line
equal to B A, and from the Point E draw the
Line E D to the Point of Diſtance D: And the
Point (a), the Interſection of B V and E D, is
the Repreſentation ſought.

Demonstration.

23. The Appearance of the Line A B, is a Part of the Line B V. Now, if we conceive a Line
2216. iſſuing from the Eye towards the Point D, and
another from the Point A towards the Point E;
theſe two Lines will be parallel, becauſe they
are in parallel Planes, aud each make half a
right Angle with the Perſpective Plane;

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Scan	Original
61	28
62	29
63	30
64
65
66
67
68
69
70	31
71	32
72	33
73	34
74	35
75	36
76
77
78
79
80
81
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85	37
86	38
87	39
88	40
89	41
90	42

Scan	Original
61	28
62	29
63	30
64
65
66
67
68
69
70	31
71	32
72	33
73	34
74	35
75	36
76
77
78
79
80
81
82
83
84
85	37
86	38
87	39
88	40
89	41
90	42

Scan	Original
61	28
62	29
63	30
64
65
66
67
68
69
70	31
71	32
72	33
73	34
74	35
75	36
76
77
78
79
80
81
82
83
84
85	37
86	38
87	39
88	40
89	41
90	42