DOI Page / Citation link:
https://doi.org/10.11588/diglit.18826#0056
42
ON PERSPECTIVE.
[lect. 1.
PLATE IX.
The ftructure of the eye, is in fact, the fource and origin of
Perfpective, and all appearances of objects are regulated by it,
and are conformed to its principles. We have therefore thought
it advifeable to explain in fome degree, the nature and conduc-
tion of the human eye.
No. I. This figure reprcfents the eve as a globe inclofed in
its membranes, but having an aperture through which the rays
of light paf> into it: the chief body of this globe is filled with a
kind of gelid humour, whofe convergent powers .are not very
firong : but nearer to the orifice whereat the light enters, is a
kind or lens, whofe convergent powers are coniiderable, and
this is ol principal ufe in producing correct vifion.
This figure is meant to fhew that the ravs of light, which by
pafling through thefe humours, produce vifion, are directed to
oppolite parts of the eye from thofe at which they enter, fu that
A above is depicted on the retina (which lines the inner cavity of
the eye, and is the immediate leat of vifionj in a below ; B at b,
C at c ; D at d, and E below, at e above ; only the center C re-
taining its original direction : it follows, that objects are depicted
on the retina inverfely.
No. II. This figure (hews alfo that objects are depicted in-
veriely ; at the lame time it hints at the nature of that converg-
ing power which the humours of the eye poffefs, whereby the
ravs of light arc directed precifely to reach the retina, and nei-
ther to exceed the diftance of that membrane, nor to tall fhort of
it. The proportion of thefe powers we fhali fee in another plate.
No. III. Shews the nature, and caufes, of the apparent di-
minution of objects. We obferve, that A A occupies on the
circle of the retina, a much greater portion (as a a on the line
a a) than BB does, which only occupies the fpace bb, cutting off
a fpace on the fmaller line a a, proportionate to fo much of the
line A A, as is cut off by the line B B. On the fame principle
C C is narrower (as cc, j than B B ; and as C C occupies but a
fmall portion*6f the line A A, fo it occupies but a fmall portion
of the circle of the eye, or of the line a a. This is one reafon
why diltant objects appear fainter than thofe which are at hand :
but other reafons are given in the lecture.
No. IV. Is an inltance of ocular deception ; but in fome
refpects rather artificial, than natural; it reprefents a vellel con-
taining a piece of money, fo placed at the bottom of it, that the
eye cannot perceive it, becaufe its beams fhoot over it: to
render it vihble to the eye preferring its nation, the vellel is
filled with water, the refraction of the rays in the water enables
the eye to difeovcr it. Many deceptions of the eve are prac-
tifed by glades, &e. in optics; but they do not properly belong
to the nature of perfpefiive, thougji they flow from iinhlar prin-
ciples.
PLATE
ON PERSPECTIVE.
[lect. 1.
PLATE IX.
The ftructure of the eye, is in fact, the fource and origin of
Perfpective, and all appearances of objects are regulated by it,
and are conformed to its principles. We have therefore thought
it advifeable to explain in fome degree, the nature and conduc-
tion of the human eye.
No. I. This figure reprcfents the eve as a globe inclofed in
its membranes, but having an aperture through which the rays
of light paf> into it: the chief body of this globe is filled with a
kind of gelid humour, whofe convergent powers .are not very
firong : but nearer to the orifice whereat the light enters, is a
kind or lens, whofe convergent powers are coniiderable, and
this is ol principal ufe in producing correct vifion.
This figure is meant to fhew that the ravs of light, which by
pafling through thefe humours, produce vifion, are directed to
oppolite parts of the eye from thofe at which they enter, fu that
A above is depicted on the retina (which lines the inner cavity of
the eye, and is the immediate leat of vifionj in a below ; B at b,
C at c ; D at d, and E below, at e above ; only the center C re-
taining its original direction : it follows, that objects are depicted
on the retina inverfely.
No. II. This figure (hews alfo that objects are depicted in-
veriely ; at the lame time it hints at the nature of that converg-
ing power which the humours of the eye poffefs, whereby the
ravs of light arc directed precifely to reach the retina, and nei-
ther to exceed the diftance of that membrane, nor to tall fhort of
it. The proportion of thefe powers we fhali fee in another plate.
No. III. Shews the nature, and caufes, of the apparent di-
minution of objects. We obferve, that A A occupies on the
circle of the retina, a much greater portion (as a a on the line
a a) than BB does, which only occupies the fpace bb, cutting off
a fpace on the fmaller line a a, proportionate to fo much of the
line A A, as is cut off by the line B B. On the fame principle
C C is narrower (as cc, j than B B ; and as C C occupies but a
fmall portion*6f the line A A, fo it occupies but a fmall portion
of the circle of the eye, or of the line a a. This is one reafon
why diltant objects appear fainter than thofe which are at hand :
but other reafons are given in the lecture.
No. IV. Is an inltance of ocular deception ; but in fome
refpects rather artificial, than natural; it reprefents a vellel con-
taining a piece of money, fo placed at the bottom of it, that the
eye cannot perceive it, becaufe its beams fhoot over it: to
render it vihble to the eye preferring its nation, the vellel is
filled with water, the refraction of the rays in the water enables
the eye to difeovcr it. Many deceptions of the eve are prac-
tifed by glades, &e. in optics; but they do not properly belong
to the nature of perfpefiive, thougji they flow from iinhlar prin-
ciples.
PLATE