Encyclopaedia Britannica > Volume 15, NIC-PAR

21 8
OfYisior. ^ da will be the
optics.
m a u vvm ray refracted by the first lens.
Through the focus of the second lens draw the perpen¬
dicular^ e, cutting AB in *; and draw e b through the
centre of the second lens. BD parallel to b e will be
the next refracted ray. Through the focus * of the
third lens draw the perpendicular */, cutting in/,
and draw f c through the centre of the third lens. ^
parallel to/c, will be the refracted ray } and so on.
Sect. V. On Vision.
Having described how the rays of light, flowing from
obiects, and passing through convex glasses, are collect¬
ed into points, and form the images of external objects ,
it will be easy to understand how the rays are refracted
by the humours of the eye, and are thereby collected
into innumerable points on the retina on which they
form the images of the objects from which they flow.
For the different humours of the eye, and particularly
or uie — ^ ' - i #
the crystalline, are to be considered as a convex glass,
and the rays in passing through them as affected in t ic
same manner in the one as in the other. A description
of the coats and humours, &c. has been given in Ana¬
tomy ; but it will be proper to repeat as much of
the description as will be sufficient for our present pur-
Plate 1 The eye is nearly globular, and consists of three coats
ccclxxx. an(l three humours. The part UHHG of the ou er
fig‘ 3' coat, is called the sclerotica ; the rest, DEFG, the cornea
Description Next within this coat is that caJW the cW« »Wh
of the eye. serves as it were for a lining to the other, and joins
with the iris, m n, m n. The iris is composed of two
sets of muscular fibres-, the one of a circular form, which
contracts the bole in the middle called the pupil, when
the light would otherwise be too strong for the eye-, and
the other of radical fibres, tending everywhere from the
circumference of the iris towards the middle of the
pupil j which fibres, bv their contraction, dilate and
enlarge the pupil when the light is weak, m order to
let in a greater quantity of it. The third coat is only a
fine expansion of the optic nerve L, which spreads lute
net work all over the inside of the choroides, and is
theiefore called the retrna; upon which are thrown the
images of all visible objects.
Under the cornea is a fine transparent fluid like water,
thence called the aqueous humour. It gives a protube¬
rant figure to the cornea, fills the two cavities m m and
n n which communicate by the pupil P \ and has the
same limpidity, specific gravity, and refracting power,
as water. At the back of this lies the crystalline hu¬
mour II, which is shaped like a double convex glass ;
and is a little more convex on the back than the fore
part. It converges the rays, which pass through it
from every visible object to its focus at tho bottom of the
eye> This humour is transparent like crystal, is of the
consistence of hard jelly, and is to the specific gravity
of water as 11 to xo. It is enclosed in a fine transparent
membrane, called the capsule of the crystalline lens,
from which proceed radial fibres o o, called the ciliary
ligaments, all around its edge, and join to the circum¬
ference of the iris.
At the back of the crystalline, lies the vitreous hu¬
mour KK, which is transparent like glass, and is largest
of all in quantity, filling the whole orb of the eye, and
Part L
giving it a globular shape. It is much of a consistence Of Vision,
with the white of an egg, and very little exceeds the' y J
specific gravity and refractive power of water. _ ^
As every point of an object ABC, sends out rays in The objects
all directions, some rays, from every point on the side on the ret.
next the eye, will fall upon the cornea between E and ^
;p . and by passing on through the pupil and humours ot verle(j.
the eye they will be converged to as many points on
the retina or bottom of the eye, and will form upon it a
distinct inverted picture c b a, of the object. Thus, the o- •
pencil of rays q r s that flows from the point A of the
object, will be converged to the point a on the retina -,
those from the point B will be converged to the point
b; those from the point C will be converged to the point
c; and so of all the intermediate points : by which
means the whole image a 6 c is formed, and the object-
made visible ", though it must be owned, that the me¬
thod by which this sensation is conveyed by the optic
nerve from the eye to the brain, and there discerned, is
above the reach of our comprehension.
That vision is effected in this manner, may be de¬
monstrated experimentally. Take a bullock’s eye whilst
it is fresh 5 and having cut off the three coats from the
back part, quite to the vitreous humour, put a piece
of white paper over that part, and hold the eye to¬
wards any bright object, and you will see an inverted
picture of the object upon the paper, or the same thing
may be better accomplished by paring the sclerotic coat
so thin that it becomes a little transparent, and retains
the vitreous humour. 134
Since the image is inverted, many have wondered why the?
why the object appears upright. But we are to consider, are seen
1. That inverted is only a relative term : and, 2, l hat 1 k
there is a very great difference between the real object
and the image by which we perceive it. When all
the parts of a distant prospect are painted upon the
retina, they are all right with respect to one another,
as well as the parts of the prospect itself-, and we can
only judge of an object’s being inverted, when it is
turned reverse to its natural position with respect to
other objects which we see and compare it with.—If we
lay hold of an upright stick in the dark, we can tell
which is the upper or lower part of it, by moving oui
hand downward and upward 3 and know very well that
we cannot feel the upper end by moving our hand
downward. In the same manner we find by experience,
that upon directing our eyes towards a tall object, we
cannot see its top by turning our eyes downward, nor
its foot by turning our eyes upward; but must trace the
object the same way by the eye to see it irom bead to
foot, as we do by the hand to feel it; and as the judgy
ment is informed by the motion ot the hand in one case*
so it is also by the motion of the eye in the other.
In fig. 9. is exhibited the manner of seeing the same Fig. <?•
object ABC, by both the eyes D and E at once. 135
When any part of the image cb a falls upon the °p-A»
tic nerve E,. the corresponding part of the object be-”™
* ~ o\ 11 nf rtiFtir*. nerve 18. .1 „
comes invisible. On this account, the optic nerve is'both eyeS
wisely placed, not in the middle of the bottom ol the(joe9not
eye, but towards the side next the nose ; so that what-appear
ever part of the image falls upon the optic nerve °f one^e’he'
eye, may not fall upon the optic nerve of the otbci'-oplicnerve
Thus the point a of the image eba falls upon the opticis insensi.
nerve of the eye D, but not of the eye E ; and the point ble of.tig