Gravesande, Willem Jacob 's - An essay on perspective

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16884An ESSAY

Let Z be the Geometrical Plane; A the Seat
11Fig. 57. of the given Point; and A B the Direction of
the Sun’s Rays.

Operation.

Draw two right Lines, making a right Angle
with each other; in one of which, aſſume P E,
equal to the Height of the given Point, above
the Geometrical Plane: Then draw the Line
E C thro’ the Point E; making an Angle with
C P, equal to the Sun’s Altitude; and make A B
equal to C P. Find the Appearance of the Point
B; and the Repreſentation ſought, will be
had.

Note, This Operation, as likewiſe all the
others in this Chapter, regard all the Situations
of the perſpective Plane; and is ſo evident, that
there is no need of demonſtrating it.

Proe. II.

104. To find the Repreſentation of an elevated
Point, whoſe Appearance, as alſo that of its Seat, is
given, without uſing the Geometrical Plane.

Find F, the accidental Point of the 22Fig. 58. Rays, and D, that of their Direction: Then
3368, 88,
98. draw a Line from the Point D, through a, the
Perſpective of the Seat of the given Point; and
another from F, through I, the Perſpective of
the given Point: And then b, the Interſection
of the ſaid two Lines, will be the Point ſought;
as is manifeſt.

Remarks.

order to find the accidental Point of any
ber of inclin’d Lines, we have

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Scan	Original
81
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86	38
87	39
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97	46
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Scan	Original
81
82
83
84
85	37
86	38
87	39
88	40
89	41
90	42
91
92
93
94	43
95	44
96	45
97	46
98
99
100
101	47
102	48
103
104
105
106	49
107	50
108	51
109	52
110

Scan	Original
81
82
83
84
85	37
86	38
87	39
88	40
89	41
90	42
91
92
93
94	43
95	44
96	45
97	46
98
99
100
101	47
102	48
103
104
105
106	49
107	50
108	51
109	52
110