particularly such as are hard or elastic, which receive the waves or
pulses of the air and reflect them back again; these reflected
pulses, striking the ear along with the original, strengthen the
original sound. Hence it is, that the voice of a speaker is louder,
and more distinctly heard, in a room than in the open air. I said
that these reflected sounds entered the ear at the same time with the
original: this however is not strictly the case, for they must enter
the ear after the original, because the sound has a greater space to
move over: but they enter the ear so quickly after the original that
our sense cannot distinguish the difference. If however the
reflecting body should be placed at such a distance, that the
reflected sound should enter the ear some considerable or sensible
time after the original, an echo or distinct sound would be heard.
It appears from experiment that the ear of an experienced musician
can only distinguish such sounds as follow each other at the rate of
nine or ten in a second, or any lower rate; and therefore that we may
have a distinct perception of the direct and reflected sound, there
should at least be an interval of 1/9 of a second; but in this time
sound passes over one hundred and twenty seven feet, and
consequently, unless the space between the sounding body and the
reflecting surface, added to that between the reflecting surface and
the ear, be greater than one hundred and twenty seven feet, no echo
will be heard, because the reflected sound will enter the ear so soon
after the original, that the difference cannot be distinguished; and
therefore it will only serve to augment the original sound.
From what has been said, it is evident, in order that a person may
hear the echo of his own voice, that he should stand at least sixty
three, or sixty four feet from the reflecting obstacle, so that the
sound may have time to move over at least one hundred and twenty
seven feet before it come to his ear, otherwise he could not
distinguish it from the original sound.
But though the first reflected pulses may produce no echo, both on
account of their being too few in number, and too rapid in their
return to the ear; yet it must be evident that the reflecting surface
may be so formed, that the pulses, which come to the ear after two or
more reflections, may, after having passed over one hundred and
twenty seven feet or more, arrive at the ear in sufficient numbers to
produce an echo, though the distance of the reflecting surface from
the ear be less than the limit of echoes. This is instanced by the
echoes that we hear in several caves or caverns.
The sense of hearing is more apt to be vitiated or diseased than any
of the other senses, which indeed is not surprising, when we consider
that its organ is complex, consisting of many minute parts, which are
apt to be deranged.
It sometimes becomes too acute, and this may arise either from too
great an irritability of the whole nervous system, which often occurs
in hysteria, also in phrenitis, and some fevers; or from an inflamed
state of the ear itself.
The sense of hearing becomes diminished, and often entirely
abolished; and this may arise from various causes, such as an
original defect in the external ear, or the meatus auditorius, or
both; the meatus auditorius is often blocked up with wax or other
substances, which being removed, the hearing becomes perfect.
Deafness may likewise arise from a rigidity of the membrane of the
tympanum, from its being erodedor ruptured, or from an obstruction
of the Eustachian tube. It may likewise arise from a paralysis or
torpor of the auditory nerve, or from some diseased state of the
labyrinth, or from a vitiated state of the brain and nerves. There
is a kind of nervous deafness which comes on suddenly, and often
leaves the patient as suddenly.
There are various instances, however, in which the membane of the
tympanum has been lacerated or destroyed, without a total loss of
the sense of hearing, or indeed any great diminution of it. A
consideration of these circumstances induced Mr. Astley Cooper to
think of perforating it, in cases of deafness arising from a
permanent obstruction of the Eustachian tube, and he has often
performed this operation with great success. Of this he has given an
account in the last part of the Philosophical Transactions. This
operation ought however only to be performed in case of the closure
of the Eustachian tube. Cases of this kind may be distinguished by
the followingcriteria. If a person on blowing the nose violently,
feel a swelling in the ear, from the membrane of the tympanum being
forced outwards, the tube is open; and though the tube be closed,
if the beating of a watch placed between the teeth, or pressed against
the side of the head, cannot be heard, the operation cannot relieve,
as the sensibility of the auditory nerve must have been destroyed. In
a closed Eustachian tube, there is no noise in the head, like that
accompanying nervous deafness.
There is one species of deafness, which occurs very frequently, and
happens generally to old persons, though sometimes to the delicate
and irritable in the earlier periods of life. Anxiety and distress of
mind have been known to produce it. Its approach is generally
gradual, the patient hears better at one time than at another; a
cloudy day, a warm room, agitated spirits, or the operation of fear,
will produce a considerable diminution in the powers of the organ. In
the open air the hearing is better than in a confined situation; in a
noisy, than in a quiet society; in a coach when it is in motion, than
when it is still. A pulsation is often felt in the ear; a noise
resembling sometimes the roaring of the sea, and at others the
ringing of distant bells is heard. This deafness generally begins
with a diminished secretion of the wax of the ear, which the patient
attributes to cold. It may be cured, particularly at its
commencement, by the application of such stimulants as are capable of
exciting a discharge from the ceruminous glands; for which purpose
thev should be introduced into the meatus auditorius.
In some cases of this kind, where the auditory nerve has been in some
degree torpid, or rather perhaps where there has been a kind of
paralysis, or want of action, in the muscles which brace the membrane
of the tympanum, and keep the chain of bones in their proper state; a
person has not been able to hear, except during a considerable noise.
Willis mentions the case of a person who could only hear when a drum
was beaten near her; and we are told of a woman who could not hear a
word except when the sound of a drum was near, in which case she
could hear perfectly well. When she married, her husband hired a
drummer for his servant. In instances of this kind the noise probably
excites the action of the torpid muscles, which then put the
apparatus in a proper condition to hear.
LECTURE VIII.
VISION.
In order to understand properly the theory of vision, it will be
necessary to premise an anatomical description of the eye: but I
shall content myself with as short a one as will suffice to explain
the effects it produces on the rays of light, so as to produce the
distinct vision of an object.
The shape of the eye is nearly spherical; it is composed of several
coats or tunics, one within another; and is filled with transparent
humours of different densities.
The proper coats of the eye are reckoned five in number; viz. the
sclerotica, cornea, choroides, iris or uvea, and the retina.
After the tunica conjunctiva, or adnata, (a membrane, which, having
lined the eyelids in the manner of a cuticle, surrounds the anterior
part of the globe) is removed, we perceive a white, firm, membrane,
called the sclerotica, which takes its rise from that part of the
globe where the optic nerve enters, and surrounds the whole eye,
except a little in the fore part; which fore part has a membrane,
immediately to be described, called the cornea. The tunica
sclerotica, viewed through the conjunctiva, forms what is called the
white of the eye. Some anatomists have supposed that this coat is a
continuation of the dura mater, which surrounds the optic nerve; but
later observations have shown this opinion to be ill founded. The
tunica sclerotica consists of two layers, which are with difficulty
separated.
The next coat is the cornea, so called from its resemblance to
transparent horn; it arises where the sclerotic coat ends, and forms
the fore part of the eye. The cornea is a segment of a lesser sphere
than the rest of the eye, and consequently makes it more prominent on
the fore part: it is transparent, and firmly connected by its edges
to the sclerotica.
Immediately adherent to the sclerotica, within, is the choroides,
which takes its rise from that part of the eye where the optic nerve
enters, and accompanies the sclerotica to the place where it is
joined to the cornea; here it is very closely connected to the
sclerotica, where it forms that annulus, called ligamentum ciliare;
then leaving the sclerotic coat, it is turned inwards, and surrounds
the crystalline lens; but as this circle, where it embraces the
crystalline, is much narrower than where the membrane leaves the
sclerotic coat, it becomes beautifully corrugated, which folds or
corrugations have been, by the more ancient anatomists, improperly
called ciliary processes.
To the same part of the choroid coat, where the ciliary ligament
begins, is fixed a moveable and curious membrane, called the iris;
this membrane has a perforation in the middle, called the pupil, for
the admission of the rays of light. The iris is composed of two kinds
of fibres: those of the one sort tend, like the radii of a circle,
towards its centre, and the others form a number of concentric
circles round the same centre. The pupil is of no constant magnitude,
for when a very luminous object is viewed, the circular fibres of the
iris contract, and diminish its orifice; and, on the contrary, when
objects are dark and obscure, those fibres relax, and suffer the
pupil to enlarge, in order to admit a greater quantity of light into
the eye: it is thought that the radial fibres also assist in
enlarging the pupil. The iris is variously coloured in different
persons, but according to no certain rule; though in general, they
who have light hair, and a fair complexion, have the iris blue or
grey; and, on the contrary, they whose hair and complexion are dark,
have the iris of a deep brown; but whether this difference in colour
occasions any difference in the sense, is not yet discovered. In the
human eye the whole choroid coat, and even the interior surface of
the iris or uvea, is lined with a black mucus; this mucus, or as it
is called, pigmentum, is darkest in young persons, and becomes more
light coloured as we advance in years. In many animals, but more
particularly those which catch their prey in the night, this
pigmentum is of a bright colour: its use will appear afterwards.
The last, and innermost coat of the eye, is the retina, it differs
much from the above mentioned coats, being very delicate and tender.
It is nothing but an expansion of the medullary part of the optic
nerve, which is inserted into each eye, nearer the nose, and a little
higher, than the axis. This coat has been thought by many to end
where the choroides, going inwards, towards the axis of the eye,
forms the ciliary ligament; Dr. Monro thinks that it is not continued
so far, and we cannot see with what advantage it could have been
continued to the ciliary ligament, since none of the rays of light,
passing through the pupil, could fall upon that part of it. In the
middle of the optic nerve is found the branch of an artery, from the
internal carotid, which is diffused and ramified in a beautiful
manner along the retina. From this artery, a small branch goes
through the middle of the vitreous humour, and giving off branches on
every side, expands itself upon the capsule of the crystalline lens.
We shall now consider the humours of the eye, which are three in
number, the aqueous, the crystalline, and the vitreous; all
transparent, and in general colourless; but of different densities.
The aqueous humour, so called from its resemblance to water, fills up
all the space between the cornea and the crystalline humour. It is
partly before and partly behind the uvea, and is divided by that
membrane into two parts, which are called the chambers of the aqueous
humour; which chambers communicate with each other by means of the
pupil.
The next humour is the crystalline; it is situated between the
aqueous and vitreous humours, and is connected to the choroid coat by
the ciliary ligament: it is not the least of all the humours, as has
been generally supposed, the aqueous and it being of equal weights;
but its substance is more firm and solid than that of the other
humours: its figure is that of a double convex lens; but the fore
part next the pupil is not so convex as its other side, which is
contiguous to the vitreous humour; the diameter of the sphere, of
which its anterior segment is a part, being in general about seven or
eight lines, whereas the diameter of the sphere, of which its
posterior segment forms a portion, is commonly only about five or six
lines. It is covered with a fine transparent capsule, which is called
arachnoides. This humour is situated exactly behind the pupil, but
not in the centre of the eye, as was supposed by Vesalius, being a
good deal nearer its forepart. The convexity of its posterior surface
is received into an equal concavity of the vitreous humour. It is not
of an equal density throughout, but is much more hard and dense
towards its centre than externally, the reason of which will appear
hereafter. Till we arrive at about our thirtieth year, this humour
continues perfectly transparent, and colourless; about that time it
generally has a little tinge of yellow, and this colour increases
with age.
The third humour of the eye, is the vitreous; it is the largest of
all the humours, filling up the whole of that part of the eye which
lies behind the crystalline humour. It is thicker than the aqueous,
but thinner than the crystalline humour; on its back part is spread
the retina, and in the middle of its fore part is a small cavity, in
which the whole posterior surface of the crystalline lens lies; this
humour is also enclosed in a very fine capsule, called tunica vitrea;
this capsule at the edge of the crystalline humour is divided into
two membranes, of which the one is continued over the whole anterior
surface of the vitreous humour, and lines that cavity into which the
back part of the crystalline is received; the other passes over the
crystalline humour, and covers all its fore part, by which means
these two humours are closely connected together. The weights of the
aqueous, crystalline, and vitreous humours in a human eye, are,
according to the accurate Petit, at a medium, to each other, as 1, 1,
and 25.
It was thought necessary to premise this general description of the
structure of the eye, in order that what we are going to add in the
remaining part of this Lecture may be the more easily comprehended. A
more distinct idea will perhaps be had from a contemplation of the
following figure, which represents the section of an eye by a
vertical plane passing through its centre.
[FIGURE]
EXPLANATION.
NOO represents the optic nerve.
The outmost line ALLB represents the sclerotic coat, and the
part ACB the transparent cornea.
The line ALLB, immediately within the former, represents the
choroides; the part APB is the iris or uvea, in which the hole at P
is the pupil.
The line FOOG is the retina.
The cavity ACBEMDA is the aqueous humour.
DE is the crystalline lens or humour.
The space DFOOGE, lying behind the crystalline, represents the
vitreous humour.
BE and AD is the ligamentum ciliare.
_Nature and Properties of Light._
After this short description of the human eye, I shall next proceed
to take notice of some of the properties of light; but shall confine
myself to such as are absolutely necessary for explaining the
phenomena of vision, as far as that can be done from optical
principles.
1. It is, I believe, generally at present agreed, that light consists
of exceedingly small particles of matter, projected with great
velocity in all directions from the luminous or radiant body. This
hypothesis, to which no solid objection has yet been made, appears to
be more simple than any other; and is so consistent with all the
phenomena yet observed, that we have great reason to think it true:
however, as it is not absolutely and directly demonstrated, it may
have been wrong in optical writers to have given this hypothesis (for
it can only be called a hypothesis) as a definition of light.
2. The space through which light passes is, by opticians, called a
medium, and it is observed, that, when light passes through a medium,
either absolutely void, or containing matter of an uniform density,
and of the same kind, it always proceeds in straight lines.
3. Those rays of light which come directly from a luminous body to
the eye, only give us a perception of light; but when they fall upon
other bodies, and are from them reflected to the eye, they give us an
idea or perception of those bodies.
4. When a ray of light passes out of one medium into another of
different density, it is bent out of its course, and is said to be
refracted. We must, however, except those rays which fall in a
direction perpendicular to the surface of the refracting medium; as
the refractive force acts in the same direction in which those rays
move, they will not be turned out of their course, but proceed in the
same direction they had before they entered the refracting medium.
When a ray passes out of a rarer into a denser medium, it will be
refracted, or bent towards a line which is perpendicular to the
surface which separates the media at the point where it falls; but
when it passes out of a denser into a rarer medium, it will be bent
from the perpendicular.
5. Whenever the rays, which come from all the points of any object,
meet again in so many points, after they have been made to converge
by refraction, there they will form the picture of the object,
distinct, and of the same colours, but inverted. This is beautifully
demonstrated by a common optical instrument, the camera obscura. If a
double convex lens, be placed in the hole of a window shutter in a
dark room, and a sheet of white paper be placed at a certain distance
behind the lens; a beautiful, but inverted picture of the external
objects will be formed: but if the paper be held nearer, or more
remote than this distance, so that the rays from each point shall not
meet at the paper, but betwixt it and the lens, or beyond the paper,
the picture will be indistinct and confused.
_Of the Manner in which Vision is performed._
From the just mentioned properties of light, and the description we
have given of the eye, it will not be difficult to explain the theory
of vision, so far as it depends upon optical principles. For the eye
may, with great propriety, be compared to a camera obscura; the rays
which flow from external objects, and enter the eye, painting an
inverted picture of those objects on the retina: if you carefully
dissect from the bottom of an eye, newly taken out of the head of an
animal, a small portion of the tunica sclerotica and choroides, and
place this eye in a hole made in the window shutter of a dark
chamber, so that the bottom of the eye may be towards you; the
pictures or images of external objects will be painted on the retina
in lively colours, but inverted.
In order to see how the several parts of the eye contribute to
produce this effect, let us follow the rays proceeding from a
luminous point, and see what will happen to them from the
beforementioned properties of light.
Since the rays of light flow from every visible point of a body in
every direction, some of them, issuing from this point, will fall
upon the cornea, and, entering a medium of greater density, will be
refracted towards the perpendicular, and as they fall upon a convex
spherical surface, nearly in a parallel state, the pupil being so
extremely small, it is evident, from the principles of optics, that
they will be made to converge: those which fall very obliquely will
either be reflected, or falling upon the uvea, or pigmentum nigrum,
which covers the ciliary ligaments, will be suffocated, and prevented
from entering the internal parts of the eye: those which fall more
directly, as was before said, become converging, in which state they
fall upon the anterior surface of the crystalline humour, which,
having a greater refracting power than the aqueous humour, and its
surface being convex, will cause them to converge still more, in
which state they will fall upon the posterior surface of the
crystalline, or anterior surface of the vitreous humour; which having
a less refractive power than the crystalline, they will be refracted
from the perpendicular; but, as they fall upon a concave surface, it
is evident, from the principles of optics, that they will be made to
converge still more: in which state they will go on to the retina,
and if the eye is well formed, the refraction of these several
humours will be just sufficient to bring them to a point or focus on
the retina.
The same thing will happen to rays flowing from every other visible
point of the object: the rays which flow from every point will be
collected into a corresponding point on the retina, and,
consequently, will paint the image of that object inverted; the rays
coming from the superior part of any object, being collected on the
inferior part of the retina, and vice versa, as is manifest from the
principles of optics.
If the rays are accurately, or very nearly, collected into a focus on
the retina, distinct vision will be produced; but if they be made to
converge to a point before or beyond the retina, the object will be
seen indistinctly; this is proved by holding a convex or concave
glass before the eye of a good sighted person: in the former case,
the rays will be made to converge to a point before they arrive at
the retina, and in the latter, to a point beyond it. In these cases,
it is plain that the rays which flow from a point in the object, will
not form a point, but a circular spot, upon the retina, and these
various circles intermixing with other, will render the image very
indistinct. This is well illustrated by the camera obscura, where if
you hold the paper nearer or more remote than the focal distance of
the lens, the picture will be indistinct.
So far then, in the theory of vision, are we led by the principles of
optics, and we can with certainty, by their assistance, affirm, that
if the eye is sound, and the image of an object distinctly painted
upon the retina, it will be seen distinctly, erect, and of its proper
colours: so far we can proceed on safe and sure grounds, but if we
venture further, we shall find ourselves bewildered in the regions of
hypothesis and fancy. The machinery by which nature connects the
material and immaterial world is hidden from our view; in most cases
we must be satisfied with knowing that there are such connexions, and
that these connexions invariably follow each other, without our being
able to discover the chain that goes between them. It is to such
connexions that we give the name of laws of nature; and when we say
that one thing produces another by a law of nature, this signifies no
more, than that one thing, which is called the cause, is constantly
and invariably followed by another, which we call the effect, and
that we know not how they are connected. But there seems a natural
propensity in the mind of man, to endeavour to account for every
phenomenon that falls under his view, which has given rise to a
number of absurd and romantic conjectures in almost every branch of
science. From this source has risen the vibration of the fibres of
the optic nerve, or the undulation of a subtile ether, or animal
spirits, by which attempts have been made to explain the theory of
vision; but all of them are absurd and hypothetical.
Kepler was the first who had any distinct notion of the formation of
the pictures of objects on the bottom of the eye; this discovery he
published about the year 1600. Joannes Baptista Porta had indeed got
some rude notion of it prior to the time of Kepler, but as he knew
nothing of the refraction made by the humours of the eye, his
doctrine was lame and defective, for he imagines that the images are
painted on the surface of the crystalline humour.