DOI Page / Citation link:
https://doi.org/10.11588/diglit.18826#0034
28 ON PERSPECTIVE. [lECT. I«
true point of the drawing-board, fo will its vanifhing
point be removed on the horizontal line. But, pleafe
to obferve, that while it remains on the drawing-
board, it may wifh in vain for any other line on which
to vanifh ; the original plane has the abfolute power of
directing it in this refpect.
Thus, have we illuftrated the nature of planes, ot
the horizontal line, and of its center, of the ground
plane, and of vanifhing lines and points. What has
been faid, has related only to planes, in a horizontal
pofition,—What muft we do with vertical planes?
The fame principles anfwer this qucftion. Inftead of
fuppofing that, from the center beam, or ray, of a
fpectator's eye, a line is extended laterally, which, in
confequence, forms the horizon; we muft fuppofe a
iine to be extended perpendicularly, above, and be-
low, the center; then is its office with refpect to ver-
tical planes, the fame as that of the horizontal line
with refpect to horizontal planes. I move this draw-
ing-board (holding it vertically) along the edge of the
table ; when it arrives at the direct beam of the eye.
it becomes in unity with the faid centrical vertical line ;
confequently it appears a mere plane, or, it vanifhes.
And this portion of a card-paper plane, is, you fee.
directed by that to which it is affixed, fo that although
it will not vanifh in the fame point as the drawing-board
(which now reprefents a portion of a vertical plane at
the fame elevation as the fpectator's eye), yet it will
vanifh on the fame line, in a point correfpondent to
its obliquity in refpect to its original plane.
As to vertical planes parallel to the fpectator, I
fcarcely think them objects of explanation, after what
has
true point of the drawing-board, fo will its vanifhing
point be removed on the horizontal line. But, pleafe
to obferve, that while it remains on the drawing-
board, it may wifh in vain for any other line on which
to vanifh ; the original plane has the abfolute power of
directing it in this refpect.
Thus, have we illuftrated the nature of planes, ot
the horizontal line, and of its center, of the ground
plane, and of vanifhing lines and points. What has
been faid, has related only to planes, in a horizontal
pofition,—What muft we do with vertical planes?
The fame principles anfwer this qucftion. Inftead of
fuppofing that, from the center beam, or ray, of a
fpectator's eye, a line is extended laterally, which, in
confequence, forms the horizon; we muft fuppofe a
iine to be extended perpendicularly, above, and be-
low, the center; then is its office with refpect to ver-
tical planes, the fame as that of the horizontal line
with refpect to horizontal planes. I move this draw-
ing-board (holding it vertically) along the edge of the
table ; when it arrives at the direct beam of the eye.
it becomes in unity with the faid centrical vertical line ;
confequently it appears a mere plane, or, it vanifhes.
And this portion of a card-paper plane, is, you fee.
directed by that to which it is affixed, fo that although
it will not vanifh in the fame point as the drawing-board
(which now reprefents a portion of a vertical plane at
the fame elevation as the fpectator's eye), yet it will
vanifh on the fame line, in a point correfpondent to
its obliquity in refpect to its original plane.
As to vertical planes parallel to the fpectator, I
fcarcely think them objects of explanation, after what
has