Gravesande, Willem Jacob 's - An essay on perspective

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13765on PERSPECTIVE. being produced, will meet each other in the
Horizontal Line.

CHAP. V.

Of throwing Figures into Perſpective, when
the Perſpective Plane is conſider’d as being
inclined.

Problem I.

81. TO find the Perſpective of a Figure in the
11Fig. 44. Geometrical Plane.

Let X be the Vertical Plane; S I the Station
Line, S the Station Point, and H the Interſecti-
on of the Station Line and Baſe Line. Now
draw the Vertical Line H V through the Point H,
making an Angle with S I, equal to the Angle
of Inclination of the perſpective Plane; then
raiſe the Perpendicular I O to S I, in the Sta-
tion Point S, equal to the Height of the Eye;
and through the Extremity of the ſaid Perpen-
dicular, draw the principal Ray O V, paral-
lel to S I, and cutting H V in the Point of
Sight V.

Now it is evident, that O V determines the
Length of the principal Ray, and H V the Di-
ſtance from the Baſe Line to the Horizontal
Line; and ſince the Demonſtration of the
Problems in the aforegoing Chapters regarding
the Geometrical Plane, have alſo Relation to
the perſpective Plane being inclined, the ſaid
Problems may be here uſed; and conſequently,
this inclined perſpective Plane is reduced to a
Perpendicular one, view’d by an Eye, whoſe
Height is H V, and Diſtance O V.

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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
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

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