Gravesande, Willem Jacob 's - An essay on perspective

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5525on PERSPECTIVE. rence with the Geometrical Line; which would
come to the ſame thing.

Demonstration.

Firſt continue the Line E a, until it meets
the Horizontal Line in D, and draw a Line
from D to the Eye, and another through the Eye
parallel to the Baſe Line.

Then the Parallels O M and F C are at the
ſame Diſtance from each other, as L D is from
E B; whence it follows, that F O is equal to E D,
and therefore O D is parallel to A F. Whence 1113: the Appearance of E A, is a Part of E D. And
after the ſame Manner we prove, that the Re-
preſentation of B A is a Part of B a.

Remarks.

40. When there are no Lines drawn, and we
would uſe this Method, the Horizontal Line
may be laid aſide; and then having firſt drawn
the Geometrical Line, whoſe Diſtance from
the Baſe Line is equal to the Length of the Ray,
we aſſume the Diſtance from the Eye to the
Geometrical Line, equal to the Height of the
Eye.

Although this Method appears uſeleſs, as being
more difficult than the precedent ones, yet in
the Eighth Chapter we have ſhewn the Uſe that
may be drawn from it.

Corollary.

41. It follows from this Demonſtration, that
the Appearances of Lines paſſing through the
Station Point, are all perpendicular to the Baſe
Line; for if the Perpendicular O S be let

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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