DOI Page / Citation link:
https://doi.org/10.11588/diglit.18826#0030
<24 ON PERSPECTIVE. [LECT. I.
object is changed: therefore, we regard only one
eye in this bufinefs, and that eye we confider as a
point. A word more on this fubject:—At a certain
diftance, the rays from each eye coincide, and unite,
fo as to anfvver the purpofe of one eye, by tranfmit-
ing to the mind one image only of the object infpectedj
this remark will hereafter appear of importance ; fince,
whoever places himfelf to view a picture nearer than
this diftance, contravenes the defign of the Artift,
whofe calculation is intimately connected with this
circumftance.
As it would be perplexing to treat on points, or
fmall objects, when extenfive, and larger objects, are
more diftinct, I fubmit the following principles to
confideration.
A plane is a furface; a mere ideal extended fuper-
ficies, having no thicknefs: this meet of card paper,
is therefore almoft a plane, but not quite ; as having
ibme fubftance. This drawing-board is almoft a plane,
but, for the fame reafon not altogether; however, it
may ferve to elucidate the nature of planes ; except-
ing, that as planes are mere geometrical ideas, they
may be fuppofed in any direction whatever; or any
number of them may crofs each other at all points
with facility; a facility to which mahogany has no
pretenfions. Permit me however to call it a plane.
Now for its application :
The direct central beam of the eye, whether we
confider it as a ray of light, iffuing from the eve, or
entering into the eye, is, in either cafe, (naturally)
diametrically oppofed to the horizon; in other words,
the horizon is the height of the eye; I fpeak of a
fair
object is changed: therefore, we regard only one
eye in this bufinefs, and that eye we confider as a
point. A word more on this fubject:—At a certain
diftance, the rays from each eye coincide, and unite,
fo as to anfvver the purpofe of one eye, by tranfmit-
ing to the mind one image only of the object infpectedj
this remark will hereafter appear of importance ; fince,
whoever places himfelf to view a picture nearer than
this diftance, contravenes the defign of the Artift,
whofe calculation is intimately connected with this
circumftance.
As it would be perplexing to treat on points, or
fmall objects, when extenfive, and larger objects, are
more diftinct, I fubmit the following principles to
confideration.
A plane is a furface; a mere ideal extended fuper-
ficies, having no thicknefs: this meet of card paper,
is therefore almoft a plane, but not quite ; as having
ibme fubftance. This drawing-board is almoft a plane,
but, for the fame reafon not altogether; however, it
may ferve to elucidate the nature of planes ; except-
ing, that as planes are mere geometrical ideas, they
may be fuppofed in any direction whatever; or any
number of them may crofs each other at all points
with facility; a facility to which mahogany has no
pretenfions. Permit me however to call it a plane.
Now for its application :
The direct central beam of the eye, whether we
confider it as a ray of light, iffuing from the eve, or
entering into the eye, is, in either cafe, (naturally)
diametrically oppofed to the horizon; in other words,
the horizon is the height of the eye; I fpeak of a
fair