The Liverpool Standard and General Commercial Advertiser.
Liverpool, Merseyside, England, 1836-10-28 00:00:00

LECTURE ON OPTICS, MECHANICS' INSTITUTION.

On Wednesday evening, Robert Addams, Esq., delivered the

seventh of his highly interesting lectures.
The phenomena uf vision, considered in all its generality and
in its connexion with the human mind, is a subject of scientific
enquiry full of difficulty, and as vet but imperfectly tinderstood.
Indeed it is hardly to be supposed that we shall in our present
condition, he Able to determine the absolute connexion be,
tween mind and, matter.; but we ought not to suffer a .belief in
the impossibility of obtaining information on these matters to
stand as a barrier to our obtaining more information than we

now possess.
The eve is an optical instrument, composed of several parts.
Expressed generally, there are three coatings or cases for the
eye, and three fluids or semi-liquids. The eye, whets divided
perpendicularly from the front to the back, will show, on the
outside, a coating which is called " the sclerotic coat." Now,
this is a thick membrane of the eye, having very few blood-
vessels in it, and, when the eye is lifeless, it is of a leathery'kind
of consistency and more difficult to cut than thick buff-leather;
a pair of scissors, sharp as they maya be, will be found incapable
of cutting it, and a knife, sharp as a razor, will make little im-
pression on the lower parts of it ; therefore, it is admirably
adapted to preserve the eve. To the front of this coat is attach-
ed a transparent shell, like a watch glass, called " the cornea,"
from its having been supposed to be formed of horn; but it is
snore like bladde- than horn. On the inside of this outer tough
coat is another, composed of a black substance, called " the
choroid" coat, which is continued to the front portion of the
eve, where it terminates, or, rather, becomes a new adjunct to
the eye, called the iris, that part of the eye which determines
its colour. In that part of the eye there is a central hole,
which, in the human creature, is round, but in other animals it
assumes a varied form. We find there is much wisdom in the
formation of this entrance, according to the habits of different
creatures. The human head is placed on the shoulders verti-
cally, and can readily be turned round without much motion of
the body, consequently, the round shape is given to man as the
one which will best enable him to see in-every direction; hut,
in the case of horses, cows,.sheen, &c., whose heads are at the
extremity of an horizontal neck, it would be inconvenient to
turn the head round to see objects to the right and left. The
hole or pupil in their eye is horizontally elongated, which en-
ables them to see to the right and left much more readily than
the human race. But, in the squirrel and the cat,' the elonga-
tion is upward. Now, the squirrel has to leap from tree to tree,
and it is necessary that its sight should be most perfect in a verti-
cal direction. We might advert to main. others; but these will
strew the wisdom of Providence in adapting the parts of the
body to their purposes. At the inside of this coat (which is so
tender that a pin will scratch it) is another membrane, called
the retina. This interior part is of very great importance, to
being the nervous part of the eye, or that part of the eve in
which the sensation of light takes place. •It is of a structure
like the meshes of a net, as its name implies. This retina has
its origin in a part of the eye called the optical nerve, which
passes into the head through a sheath, and branches out into the
brain, becoming a part of the brain itself. In the interior of
these coats, we have three humours, or liquids—perhaps better
called humours, as they are not liquid like water. The most
conspicuous of these will be the vitreous humour which occupies
the greater portion of the interior part of the eye. Now, this
humour is not so liquid as water nor so thick or dense as jelly.
It is perfectly colourless, and as transparent as the purest water
that can be lound. It is, moreover, enclosed in a memlirane, or
bag, forming one interior globular mirror, excepting at the de-
pression in front. We then come to another, which is called
the lens of the eye, or, for the sake of distinction, the crystalline
lens. It is much thicker than the calf's-foot jelly which we
find in shops, transparent as water itself, and made up of fibres
twisted backwards and forwards. The crystalline lenses of
some creatures are composed of layers of shells. 'We then
come to the front of the eye, and. in the space between the
crystalline lens and the cornea, which is the outside part of the
whole, there is dispersed a humour which is called the aqueous
humour. This is as liquid as water, and when a lancet is used
to pierce its covering it flows out and leaves the coils a flaccid,
its use being to push out the cornea in a lens-like manner.
Mr. Addams illustrated the foregoing description by means of
a beautiful mechanical representation of the eye, of similar size
to that iu a human being, in which the various coatings,
humours, muscles, nerves, and membranes were faithfully re-
presented by ivory, glass, &c. The eve is furnished with mus-
cles by which it can be moved in any 'direction without the ne-
cessity of stirring the head. When one of these muscles has
not the same power of connection or elongation as the rest it
does not sufficiently counterbalance them, and causes 010 com-
plaint called squinting. Mr. Addams here dissected the eye
of an ox, and explained the various parts of it to the audience.
The crystalline lens is similar to the cross lens of the telescope,
more round on the one side than the other, and magnifies like
the lens of a microscope.
The structure of the eye being thus described, we must pro-
ceed to consider it as an optical instrument; and, first, we can-
not but admire the arrangement of the humours of the eye, so
efficient for the leading of light from the front of the eye to the
retina, where the image is formed devoid of the errors of spheri-
cal aberration. In a former lecture the achromatic telescope
of Dollond was adverted to, in which three glasses were used to
prevent aberration, and to correct the prismatic colours. These
glasses were of different forms. and of two descriptions of glass.
Now in the eye the humours are of different degrees of density.
The aqueous, and the vitreous, and the crystalline lens possess
different degrees of refractive power. The vitreous lens is round
on one side and concave on the other; the crystalline is rounder
on one side than on the other. We find great advantage from
this in correcting the phenomena of spherical aberration.
Here, then, we have the aqueous humour, which is concave on
the one side, and convex on the other. These have different
refractive powers, the aqueous index being la,Kaa' the vitreous
being rather more than 1,3.39. But in the crystalline lens we
find a peculiar adaptation which man has never been able to
adopt, that of having the centre of greater density than the
surface. In making a glass, we are obliged to submit to have
it of one density throughout the whole; but if we were able to
make glasses denser towards the centre, we should be able to
obtain a better focus than we are at present able to do. In the
crystalline lens the index of refraction is 1,376 in front, 1,399
its the centre, and 1,383 at the track, by which means spherical
aberration is entirely removed. In the telescope we have a
black disc, called the diaphragm, in order to prevent the light
which comes in at the mouth of the telescope from striking
against the sides of the brass, and, by being reflected, injuring
the image. In the eye we find a black screen placed appa-
rently for the same purpose. In the telescope and microscope
we ought to have adjustments for the eye-glass so as to be able
to accommodate them to any distance. 'The seine arrangement
is provided in the eye. If we look at a lamp which is near, the
eye requires adjustment, and if at a distant object, it requires
another adjustment, This is managed by meaus of a capsule
attached to certain muscles in contact with the pupil of the eye.
The eye is frequently exposed to light of various degrees of in-
tensity, and an arrangement is made by which we can see at
midday and by twilight. This consists in the contraction or
or enlargement. of a membrane, by which 'Aare or less light is
admitted through the central hole. This is remarkable in cats'
eyes when the sun is shining strong; the contraction is then
very great. As two images of an object are formed in the re-
tina by our eyes, it may be asked why we do not see two objects'?
We do see two objects, but we see them exactly in the same
position. one lapped over the other, so that they appear only
one, that is, provided the eye be properly adjusted. Now it is
well known that we can see bodies very well with one eye, and
the question might arise why there should be two eyes. It is,
perhaps; in part for the sake of symmetry and beauty of ap-
pearance, as we have two arms, two ears, &e. Two eyes may
be better with regard to appearance than one ; but vet if all
mankind were formed with but one eye, we should not 'find any
deformity iu it; therefore that does not at all follow. But we
may conceive that we have two eyes, because being a precious
organ, should we lose one, we might have the other. Besides
we can certainly see better with two eyes than with one. This
is plainly shown even by brutes. A giontleman kept a favourite
toad and raven in the same garden. The toad used to get iu
the pathway of the ants, and by darting out his tongue secure a
great number of them. It, however, happened that the raven
and toad disagreeing, the former picked out the eve of the let-
ter, after which the toad was unable to capture his prey as well
as before. Two eves give us a more ample view than we other-
wise should have. We can see half a circle, or 180°, when both
our eyes are open, whilst we can see little more than 1000 with
one eye alone.
'There is a partof the retina which is insensible to light, where
the optical nerve passes into the shell. The nerve becomes a bun-
dle oftibres, which will not vibrate to the undulations of the mther
This fact is proved by several experiments. If we place
three spots two feet asunder on the wall, and stand 10 feet
from them, we shall see one of these three objects disappear al-
together. whilst steadily looking with one eye at either of the
others. The fact is that the eye is so situated that the light from
the invisible spot falls upon the optical nerve and disappears.
We find every thing beautifully perfect in the organization of
man, and this nerve accordingly is past the axis,of vision, nearer
the nose, and therefore out of the way of the light coming.
Every part of the retina excepting that, is sensible to light,
but there is only one part of it which is perfect, and we always
turn that part, which is towards the axis of the eye, to any
thing which we wish to scrutinize particularly. In reading
writing, or print, the image of every distinct letter is formed
upon the retina.
It may appear singular how we should see a person erect,
when his image on the back part of the eye is turned upside
down. The fact is, that we see through the inside of the eye,
and not as through a telescope tube. There are many creatures
who are so sensitive, that they can see light when there appears
to be none. The cat, rat, and mouse can see light when the walls
are of a three fold thickness ; else how are we to suppose that
rats will run round the wainscoating of a house, on a ledge, not
half an inch in diameter? What is sight but undulation ;
therefore, as vibration is continually going on every where,
there may be light enough generated to act upon some retina,
even in the darkest dungeons. Mr. Addams then explained
satisfactorily, by the aid of an apparatus, why we 0 not see ob-
jects reversed, as their image is fotrnerd in the retina.
he li
When the eyes do not perform office
by age, rays do not fall as they ought to do uponpon the nerves.
Thus in short-sighted persons they t come o focus before they
reach the retina, and in age they have not reached the focus
when they came to the retina. By the aid of spectacles these
defects are obviated, and the rays caused to fall properly. The
eye may be diseased with regard to external bodies, by sup-
posing them to he in motion when the eye itself unconsciously
moves. The lecturer had looked at a 'body which he believed
to be stationary, and had seen it run up, apparently with great
velocity. This happened in Scotland a few years ago. He was
k
coming down Loch Ness, and the pas et slopped a short tune
for the purpose of allowing the passengers to inspect a beauti-
ful waterfall. Mr. Addams was observing the manner in which
the waves of this waterfall succeeded one another, and kept
his eye fixed on a particular part of the water for that purpose.
This put his eye hs motion in an oppysite direction to the
water, and having satisfied himself, he turned round to the
rock on his left, and to his surprise the whole of it appeared to
be going up with the seine velocity Ts the waterfall was coining
down. A similar effect is felt on the railway, if the eye looks
for a few moments at external objects, whilst the carriages
move rapidly along.
The abberration of light is "a phenomenon attended with an
apparent displacement of the stars in the heavens. When we
look at a star it appears to he a little farther east than its true
place by a very minute portion. Now, supposing that a star
was perpendicular to an observer, and a ray or wave to leave
it towards his eye, by the time that it reached him the earth
would have travelled in her orbicular path, so that that the per-
pendicular ray would not come near the observer. Besides this
action of the earth, the diurnal motion is productive of a varied
aberration which together with the orbicular aberration, roust
be taken into consideration by astronomers.
In the next lecture to be delivered to-morrow evening, I%tr..
Addams will treat on optical deceptions, the duratiipocnt:f n
n
pressions made no the retina—light and shade— "rned 4 the
presentations, and on the optical spectra occasio
super-position of illuminated surfaces.
re I-•veS'

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