DOI Page / Citation link:
https://doi.org/10.11588/diglit.18826#0077
LECT. II.] ON PERSPECTIVE. 57
the picture, the two perpendicular fides of the board
apparently tend to that point: i.e. the center; the
neareft part of the plane to the eye: the other two
fides of the fquare being parallel to the picture (the
glafs in this experiment) fuffer no change of form from
any perfpeclivity connected with them, except an
apparent diminution of magnitude, as they recede:
the mod diftant being the fmalleft. By the bye, this
want of perfpeclivity in the parallel fides of a fquare,
obliges us to feek for fome line which may have a
determinate, and exact, relation to a fquare, and alfo
to the horizon ; this we obtain, by means of the dia-
gonals, whofe angular declination from the fides being
45 degrees, gives 45 degrees from the center, on the
horizontal line, for their vanifning point; as is tlhrf-
trated in the examples.
But, my chief defign in this experiment is to fhevv,
that the effects we have been noticing: arife from the
paraUelifni of the vifual rays and the ground plane.
Now, in defign, we cannot caufe a variety of planes,,
and of lines, to project perpendicular from a picture ;
we therefore tranfpofe their places, and imagine the
eye and ns fyfiem of rays turned upward, and the
flab, &c. (/. e. all before the ground line of a picture)
turned downward; and this reftores the parallelifm,
and produces the fame effects ; fo that now a Angle
fheet of paper contains the whole procefs.
Right lines, having a regular, and determinate per-
fpecfive tendency, are eafily put into perfpeclive re-
prefentation • and angular figures, being compofed of
right
the picture, the two perpendicular fides of the board
apparently tend to that point: i.e. the center; the
neareft part of the plane to the eye: the other two
fides of the fquare being parallel to the picture (the
glafs in this experiment) fuffer no change of form from
any perfpeclivity connected with them, except an
apparent diminution of magnitude, as they recede:
the mod diftant being the fmalleft. By the bye, this
want of perfpeclivity in the parallel fides of a fquare,
obliges us to feek for fome line which may have a
determinate, and exact, relation to a fquare, and alfo
to the horizon ; this we obtain, by means of the dia-
gonals, whofe angular declination from the fides being
45 degrees, gives 45 degrees from the center, on the
horizontal line, for their vanifning point; as is tlhrf-
trated in the examples.
But, my chief defign in this experiment is to fhevv,
that the effects we have been noticing: arife from the
paraUelifni of the vifual rays and the ground plane.
Now, in defign, we cannot caufe a variety of planes,,
and of lines, to project perpendicular from a picture ;
we therefore tranfpofe their places, and imagine the
eye and ns fyfiem of rays turned upward, and the
flab, &c. (/. e. all before the ground line of a picture)
turned downward; and this reftores the parallelifm,
and produces the fame effects ; fo that now a Angle
fheet of paper contains the whole procefs.
Right lines, having a regular, and determinate per-
fpecfive tendency, are eafily put into perfpeclive re-
prefentation • and angular figures, being compofed of
right