Gravesande, Willem Jacob 's - An essay on perspective

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16682An ESSAY ſection with F a, will determine a X the Repre-
ſentation ſought.

Remarks.

Although we ſuppoſe the Eye in all the
Problems in this Chapter to be above the per-
ſpective Plane, yet it may likewiſe be under
the ſaid Plane; in which Cafe, the Geometrical
Plane is ſuppoſed above the Objects, as we have
already done on another Occaſion.

1179.

CHAP. VII.

Of Shadows.

FIRST we muſt obſerve, with thoſe who
have already treated on this Subject, that
when a luminous Body is equal to an opaque
Body it enlightens, the Shadow of the ſaid Body
is contain’d between parallel Lines, and con-
ſequently, it is equal upon all parallel Lines
placed at any Diſtance whatſoever beyond the
opaque Body. And when the luminous Body
is leſſer than the opaque Body, the Shadow
thereof, increaſes, and is infinitely augmented.
And on the contrary, when an opaque Body
is leſs than the luminous Body, the Shadow there-
of decreaſes and terminates in a Point.

Now becauſe the Sun is vaſtly greater than
any of the Bodies on the Earth’s Surface it en-
lightens, and is at ſo great a Diſtance therefrom,
therefore its Rays may be conſider’d as being
parallel; and conſequently, the Bodies it ſhines
upon as encloſed between parallels: And this
is the firſt Kind of Shadows I ſhall here explain;

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121	58
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126	60
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130	61
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132	63
133	64
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137	65
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146	71
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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