DOI Page / Citation link:
https://doi.org/10.11588/diglit.18826#0033
LECT. I.] ON PERSPECTIVE, 2/
any part of it, but its edge. Now, obferve, that if
the ground plane, were it produced, would vanith into
the horizontal line, all leffer planes lying, being, and
fituate, (as the lawyers fay) on this ground plane, or
rather forming parts of this ground plane, would vanifh
into the fame line. But, although not thus produced,
yet the ground plane (and confequently all planes upon
it) have a perpetual tendency, and inclination, toward
the faid horizontal line, till at length they unite with it,
and in appearance become a part of it.
If a plane lying on the ground follow the direction
of that ground, i. e. if it lie ftraight before the fpec-
tator, the point to which the fides of a portion of it
cut direct will tend, is, that which is ftruck by the
center beam, or ray, of the fpectator's eye ; it is, I fay,
the center of the horizontal line. But if this portion
of a plane lying on the ground, be fituate in an ob-
lique direction with refpect. to the fpectator, then that
point on the horizontal line to which it apparently
tends (i. e. its vanifhing point) will be removed on
that line, from the center, to one fide of it, according
to its obliquity, e. gr. If it is ten degrees oblique
from the eye, its point of tendency will be ten degrees
diftant from the center, and fo on.
As an example, I have, you fee, laid this portion
of a plane of card paper, obliquely on this plane of
mahogany, (which reprefents the ground plane imme-
diately before the fpectator) but as the directions of the
fides of thefe two portions or planes do not agree, it
is certain the lines they form, if prolonged, would
never arrive at the fame point in the horizon; but, ac-
cording to the variation of the card paper, from the
true
any part of it, but its edge. Now, obferve, that if
the ground plane, were it produced, would vanith into
the horizontal line, all leffer planes lying, being, and
fituate, (as the lawyers fay) on this ground plane, or
rather forming parts of this ground plane, would vanifh
into the fame line. But, although not thus produced,
yet the ground plane (and confequently all planes upon
it) have a perpetual tendency, and inclination, toward
the faid horizontal line, till at length they unite with it,
and in appearance become a part of it.
If a plane lying on the ground follow the direction
of that ground, i. e. if it lie ftraight before the fpec-
tator, the point to which the fides of a portion of it
cut direct will tend, is, that which is ftruck by the
center beam, or ray, of the fpectator's eye ; it is, I fay,
the center of the horizontal line. But if this portion
of a plane lying on the ground, be fituate in an ob-
lique direction with refpect. to the fpectator, then that
point on the horizontal line to which it apparently
tends (i. e. its vanifhing point) will be removed on
that line, from the center, to one fide of it, according
to its obliquity, e. gr. If it is ten degrees oblique
from the eye, its point of tendency will be ten degrees
diftant from the center, and fo on.
As an example, I have, you fee, laid this portion
of a plane of card paper, obliquely on this plane of
mahogany, (which reprefents the ground plane imme-
diately before the fpectator) but as the directions of the
fides of thefe two portions or planes do not agree, it
is certain the lines they form, if prolonged, would
never arrive at the fame point in the horizon; but, ac-
cording to the variation of the card paper, from the
true