their axes are directed, do also appear single: and in this case, it
is evident to those who understand the principles of optics, that the
pictures of an object to which the eyes are not directed, but which
is at the same distance as that to which they are directed, fall both
on the same side of the centre, that is, both to the right, or both
to the left, and both at the same distance from the centre. Hence it
is plain, that points in the retina, which are similarly situated
with respect to the centres, are corresponding points.
An object which is much nearer, or much more distant from the eyes,
than that to which their axes are directed, appears double. In this
case, it will easily appear, that the pictures of the object which is
seen double, do not fall upon points which are similarly situated.
From these facts, we are led to the following conclusion, viz. that
the points of the two retinas, which are similarly situated with
respect to the centres, correspond with each other, and that the
points which are dissimilarly situated, do not correspond. The truth
of this general conclusion is founded upon a a very full induction,
which is all the evidence we can have for a fact of this nature.
The next thing that seems to merit our attention, is, to inquire,
whether this correspondence between certain points of the two retinas
which is necessary to single vision, is the effect of custom, or an
original property of the human eyes.
We have a strong argument in favour of its being an original
property, from the habit we get of directing our eyes accurately to
an object; we get this habit by finding it necessary for perfect and
distinct vision; because thereby, the two images of the object
falling upon corresponding points, the eyes assist each other in
vision, and the object is seen better by both eyes together, than by
one: but when the eyes are not accurately directed, the two images of
the object fall upon points which do not correspond, whereby the
sight of the one eye disturbs that of the other. Hence it is not
unreasonable to conclude, that this correspondence between certain
points of the retina is prior to the habits we acquire in vision:
and, consequently, natural and original.
We have all acquired the habit of directing our eyes in one
particular manner, which causes single vision; now if the Author of
Nature had ordained that we should see objects single, only when our
eyes are thus directed, there is an obvious reason why all mankind
should agree in the habit of directing them in this manner; but, if
single vision were the effect of custom, any other habit of directing
the eyes would have answered the purpose; we therefore, on this
supposition, can give no reason why this particular habit should be
so universal.
Bishop Berkeley maintains a contrary opinion, and thinks that our
seeing objects single with both eyes, as well as our seeing them
erect, by inverted images, depends upon custom. In this he is
followed by Dr. Smith, who observes, that the question, why we see
objects single with both eyes, is of the same nature with this, why
we hear sounds single with both ears; and that the same answer will
serve for both; whence he concludes, that as the second of these
phenomena is the effect of custom, so also is the first. But I think,
that the questions are not so much of the same sort, as that the same
answer will serve for both; and, moreover, that our hearing single
with both ears is not the effect of custom. No person will doubt that
things which are produced by custom, may be undone by disuse, or by a
contrary custom. On the other hand, it is a strong argument, that an
effect is not owing to custom, but to the constitution of nature,
when a contrary custom, long continued, is found neither to change
nor weaken it. Now it appears, that from the time we are able to
observe the phenomena of single and double vision, custom makes no
change in them, every thing which at first appeared double, appearing
so still in the same circumstances. Dr. Smith has adduced some facts
in favour of his opinion, which, though curious, seem by no means
decisive. But in the famous case of the young man couched by Mr.
Cheselden, after having had cataracts in both his eyes till his
thirteenth year, it appears that he saw objects single from the time
he began to see with both eyes. And the three young gentlemen
mentioned by Dr. Reid, who had squinted, as far as he could learn,
from infancy, as soon as they learned to direct both eyes to an
object, saw it single.
In these cases it is evident that the centres of the retina
corresponded originally, for Mr. Cheselden's young man had never seen
at all before he was couched, and the other three had never been
accustomed to direct the axes of both eyes to the same point. These
facts render it probable, that this correspondence is not the effect
of custom, but of fixed and immutable laws of nature.
With regard to the cause of this correspondence, many theories have
been proposed, but as none of them can be looked on in any other
light than as probable conjectures, I think it would be to little
purpose to notice them. That of the illustrious Newton is the most
ingenious of any, and though it has more the appearance of truth than
any other, that great man has proposed it under the modest form of a
query.
Having given a short account of the principal phenomena of vision, I
proceed next to treat of some of the diseases to which this sense is
subject, I shall first take notice of the deformity called squinting.
_Of Squinting._
Though this is a subject which well deserves our particular
attention, yet having spoken of such a variety of subjects in the
preceding part of this lecture, I have not time for many observations
on this. I shall just mention the principal opinions, concerning the
cause of this deformity, and point out that which seems to me most
probable. This subject is certainly very worthy the attention of the
physician, as it is a case concerning which he may often be
consulted, and which it may be sometimes in his power to cure.
A person is said to squint, when the axes of both his eyes are not
directed to the same object.
This defect consists in the wrong direction of one of the eyes only.
I have never met with an instance in which both eyes had a wrong
direction, neither have I seen one accurately described by any
author.
The generality of writers on this subject have supposed this defect
to proceed from a disease of, or want of proper correspondence in,
the muscles of the eyes, which not acting in proper concert with one
another, as in persons free from this blemish, are not able to point
both eyes to the same object. But this, I think, is very seldom the
cause, for when the other eye is shut, the distorted eye can be moved
by the action of its muscles, in all possible directions, as freely
as that of any other person, which shows that it is not owing to a
defect in the muscles, neither is it owing to a want of
correspondence in the muscles of both eyes; for when both eyes are
open, and the undistorted eye is moved in any direction whatever, the
other always accompanies it, and is turned the same way at the same
instant of time.
I shall next take notice of the hypothesis of M. de la Hire, who
supposes, that in the generality of mankind, that part of the retina
which is seated in and about the axis of the eye, is of a more
delicate sense and perception than what the rest of the coat is
endowed with; and therefore we direct both axes to the same object,
chiefly in order to receive the picture on that part of the retina
which can best perceive it; but in persons who squint, he conceives
the most sensible part of the retina of one eye, not to be placed in
the axis, but at some distance from it: and that, therefore, this
more sensible part of the retina is turned towards the object, to
which the other eye is directed, and thus causes squinting. This
ingenious hypothesis has been followed by Dr. Boerhaave, and many
other eminent physicians. If it be true, then if the sound eye be
shut, and the distorted eye alone be used to look at an object, it
must still be as much distorted as before, for the same reason: but
the contrary is true in fact; for if you desire such a person to
close his other eye and look at you with that which is usually
distorted, he will immediately turn the axis of it directly towards
you. If you bid him open the undistorted eye, and look at you with
both eyes, you will find the axis of this last pointed at you and the
other turned away, and drawn close to the nose, or perhaps to the
upper eye lid. From these facts, and some others mentioned by Dr.
Jurin, I think we may conclude that this defect is seldom, if ever,
occasioned by such a preternatural make of the eye, as M. de la Hire
supposed.
From the most accurate observations it will appear, that by far the
most common cause of squinting, is a defect in the distorted eye. Dr.
Reid examined above twenty people that squinted, and found in all of
them a defect in the sight of one eye; M. Buffon likewise, from a
great number of observations, has found that the true and original
cause of this blemish, is an inequality in the goodness, or in the
limits of distinct vision, in the two eyes. Dr. Porterfield says this
is generally the case with people who squint; and I have found it so
in all that I have had an opportunity of examining.
With regard to the nature of this defect, the distorted eye is
sometimes more convex, and sometimes more flat, than the sound one;
sometimes it does not depend upon the convexity, but upon a weakness,
or some other affection, of the retina, of the nature of which we are
ignorant.
It will be easy to conceive how this inequality of goodness in the
two eyes, when in a certain degree, must necessarily occasion
squinting, and that this blemish is not a bad habit, but a necessary
one, which the person is obliged to learn, in order to see with
advantage. When the eyes are equally good, an object will appear more
distinct and clear when viewed with both eyes than with only one; but
the difference is very little. The ingenious Dr. Jurin, who has made
some beautiful experiments to ascertain this point, has shown, that
when the eyes are equal in goodness, we see more distinctly with both
than with one, by about one thirteenth part only. But M. Buffon has
found that when the eyes are unequal, the case will be quite
different. A small degree of inequality will make the object, when
seen with the better eye alone, appear equally bright or clear, as
when seen with both eyes; a little greater inequality will make the
object appear less distinct when seen with both eyes, than when it is
seen with the stronger eye alone; and a still greater inequality will
render the object, when seen with both eyes, so confused, that in
order to see it distinctly, one will be obliged to turn aside the
weak eye, and put it into a situation where it cannot disturb the
sight of the other. The truth of this may be easily proved by
experiment. Let a person take a convex lens, and hold it about half
an inch before one of his eyes; he will, by these means, render them
very unequal. and if he attempt to read with both eyes, he will
perceive a confusion in the letters, caused by this inequality; which
confusion will disappear as soon as he shuts the eye which is covered
with the lens, and looks only with the other.
Squinting is a necessary consequence of this inequality in the
goodness of the two eyes; for a person whose eyes are to a certain
degree unequal, finds that, when he looks at an object, he sees it
very indistinctly; every conformation, or change of direction of his
eyes which lessens the evil, will be agreeable; and he will acquire a
habit of turning his eye towards the nose, not for the sake of seeing
better with it, but in order to avoid, as much as possible, seeing at
all with the distorted eye; for which purpose it will be drawn either
under the upper eye lid, so that the pupil may be entirely or
partially covered; or directly towards the nose, in which case the
image of the object will fall at a distance from the axis of the eye:
and it is a fact well known to philosophers, that we never naturally
attend to an image which is at a distance from the axis; so that in
this situation it will give little disturbance to the sight of the
other.
It is easy to show that a squinting person very seldom, if ever, sees
an object with the distorted eye. Indeed in above forty cases that I
have examined, I found that when I placed an opaque body between the
undistorted eye and the object, it immediately disappeared, nor were
they able to see it at all, till they directed the axis of the
distorted eye to the object. I find the same observation made by Dr.
Reid and M. Buffon.
M. Buffon takes notice of a fact which I have often observed; viz.
that many persons have their eyes very unequal without squinting.
When the difference is very considerable, the weak eye does not turn
aside, because it can see almost nothing, and therefore cannot
disturb the vision of the good eye. Also, when the inequality is but
small, the weak eye will not turn aside, as it affords very little
disturbance to the sight of the other: when the inequality consists
in the difference of convexity, or difference of the limits of
distinct vision, having the limits of distinct vision in each eye
given, it may be calculated with some degree of accuracy what degree
of inequality is necessary to produce squinting. It seems then that
there are certain limits with regard to the inequality of the eyes,
necessary to produce this deformity; and that if the inequality be
either greater or less than these limits, the person will not squint.
Having now endeavoured to show what is the most common cause of
squinting, I shall briefly attempt to point out those cases in which
we may expect to effect a cure, and afterwards give a very short
account of the most likely methods of doing it.
We cannot have great hopes of success, when there is a very great
defect in the distorted eye. When the eyes are of different
convexities, there is no other way of removing the deformity, than by
bringing them to an equality by means of glasses, and then the person
would only look straight when he used spectacles. When this defect is
owing to a weakness in the distorted eye, it may sometimes be cured:
M. Buffon observes that a weak eye acquires strength by exercise, and
that many persons, whose squinting he had thought to be incurable, on
account of the inequality of their eyes, having covered their good
eye for a few minutes only, and consequently being obliged to
exercise their bad one for that short time, were themselves surprised
at the strength it had acquired, and on measuring their view
afterwards, he found it to be more extended, and judged the squinting
to be curable. In order therefore to judge with any certainty of the
possibility of a cure, it ought always to be tried whether the
distorted eye will grow better by exercise; if it does not, we can
have little hopes of success; but when the eyes do not differ much in
goodness, and it is found that the distorted eye acquires strength by
exercise, a cure may then be attempted: and the best way of doing it,
(according to M. Buffon) is to cover the good eye for some time, for,
in this condition, the distorted eye will be obliged to act, and turn
itself towards objects, which by degrees will become natural to it.
When the eyes are nearly brought to an equality by exercise, but
cannot both be directed to the same point, Dr. Jurin's method may be
practised, which is as follows.
If the person is of such an age, as to be capable of observing
directions, place him directly before you, and let him close the
undistorted eye, and look at you with the other; when you find the
axis of this fixed directly upon you, bid him endeavour to keep it in
that situation, and open the other eye; you will now see the
distorted eye turn away from you towards his nose, and the axis of
the other will be pointed towards you, but with patience and repeated
trials he will, by degrees, be able to keep the distorted eye fixed
upon you, at least for some time after the other is opened, and when
you have brought him to keep the axis of both eyes fixed upon you, as
you stand directly before him, it will be time to change his posture,
and set him, first a little to one side of you, and then to the
other, and so practise the same thing. And when, in all these
situations, he can perfectly and readily turn the axes of both eyes
towards you, the cure is effected. An adult person may practice all
this before a mirror, without a director, though not so easily as
with one: but the older he is, the more patience will be necessary.
With regard to the success of this method, M. Buffon says, that
having communicated his scheme to several persons, and, among others,
to M. Bernard de Jussieu, he had the satisfaction to find his opinion
confirmed by an experiment of that gentleman, which is related by Mr.
Allen, in his Synopsis Universae Medicinae. Dr. Jurin tells us that
he had attempted a cure in this manner with flattering hopes of
success, but was interrupted by the young gentleman's falling ill of
the small pox, of which he died. Dr. Reid likewise tried it with
success on three young gentlemen, and had brought them to look
straight when they were upon their guard. Upon the whole this seems
by much the most rational method of attempting to cure the deformity.
The only remaining morbid affections of the eye which I shall take
notice of in this lecture, are two, which produce the indistinct
vision of an object, by directly opposite means. The first is caused
by the cornea, and crystalline, or either of them, being too convex,
or the distance between the retina and crystalline being too great.
It is evident, that from any of these causes, or all combined, the
distinct picture of an object, at an ordinary distance, will fall
before the retina, and therefore the picture on the retina itself
must be confused, which will render the vision confused and
indistinct; whence, in order to see things distinctly, people whose
eyes are so formed are obliged to bring the object very near their
eyes; by which means the rays fall upon the eye in a more diverging
state, so that a distinct picture will be formed on the retina, by
which the object will be distinctly seen: from the circumstance of
such persons being obliged to hold objects near their eyes, in order
to see them distinctly, they are called short sighted.
If a short sighted person look at an object through a small hole made
in a card, he will be able to see even remote objects, with tolerable
distinctness, for this lessens the circles of dissipation on the
retina, and thus lessens the confusion in the picture. For the same
purpose, we commonly observe short sighted people, when they wish to
see distant objects more distinctly, almost shut their eye lids: and
it is from this, says Dr. Porterfield, that short sighted persons
were anciently called myopes.
The sight of myopes is remedied by a concave lens of proper
concavity, which, by increasing the divergency of the rays, causes
them to be united into a focus on the retina: and they do not require
different glasses for different distances, for, if they have a lens
which will make them see distinctly at the distance most commonly
used by other persons, for example, at the distance at which persons
whose eyes are good generally read, they will, by the help of the
same glass, be able to see distinctly at all the distances at which
good sighted people can see distinctly: for the cause of
shortsightedness, is not a want of power to vary the conformation of
the eye, but is owing to the whole quantity of refraction being too
great for the distance of the retina from the cornea.
The other defect to be mentioned, is of an opposite nature, and
persons labouring under it are called long sighted, or presbytae: it
is caused by the cornea and crystalline, or either of them, being too
flat in proportion to the distance between the crystalline and
retina: whence it follows, that the rays which come from an object at
an ordinary distance, will not be sufficiently refracted, and,
consequently, will not meet at the retina, but beyond it, which will
render the picture on the retina confused, and vision indistinct.
Whence, in order to read, such persons are obliged to remove the book
to a great distance, which lessens the divergency of the rays falling
on the eye, and makes them converge to a focus sooner, so as to paint
a distinct image on the retina.
The presbytical eye is remedied by a convex lens of proper convexity,
which makes the rays converge to a focus sooner, and thus causes
distinct vision: the sight of such persons is even more benefited by
a convex lens, than that of myopes by a concave one; for a convex
lens not only makes the picture of the object on the retina distinct,
but also more bright, by causing a greater quantity of light to enter
the pupil; while a concave one, at the same time that it renders
vision distinct, diminishes the quantity of light.
Long sighted persons commonly become more so as they advance in
years, owing to a waste of the humours of the eye; and even many
people whose sight was very good in their youth, cannot see without
spectacles when they grow old. The same waste in the humours of the
eye, is the reason why shortsighted persons commonly become less so
as they advance in years; so that many who were shortsighted in their
youth, come to see very distinctly when they grow old. Dr. Smith
seems to doubt this, and thinks that it is rather a hypothesis than a
matter of fact. I have however myself seen several instances in
confirmation of it; and it is very natural to suppose, that since
short and long sight depend upon directly opposite causes, and since
the latter is increased by age, the former must be diminished by it.
LECTURE IX.
THE LAWS OF ANIMAL LIFE.
In the preceding lectures I have taken a view, first of the general
structure and functions of the living body, and next of the different
organs called senses, by means of which we become acquainted with
external objects. I shall next endeavor to show that, through the
medium of these different senses, external objects affect us in a
still different manner, and by their different action, keep us alive:
for the human body is not an automaton; its life, and its different
actions, depend continually on impressions made upon it by external
objects. When the action of these ceases, either from their being
withdrawn, or from the organization necessary to perceive them, being
deranged or injured, the body becomes a piece of dead matter; becomes
obedient to the common laws of chemical attraction, and is decomposed
into its pristine elements, which, uniting with caloric, form gases;
which gases, being carried about in the atmosphere, or dissolved in
water, are absorbed by plants, and contribute to their nourishment.
These are devoured by animals, which in their turn die, and are
decompounded; thus, in the living world, as well as in the inanimate,
every thing is subject to change, and to be renewed perpetually.
"Look nature through, 'tis revolution all,
All change, no death; day follows night; and night,
The dying day; stars rise, and set, and rise;
Earth takes th' example; see the summer gay,
With her green chaplet, and ambrosial flowers,
Droops into pallid autumn; winter gray,
Horrid with frost, and turbulent with storm,
Blows autumn and his golden fruits away,
Then melts into the spring; soft spring with breath
Favonian, from warm chambers of the south
Recals the first. All to reflourish, fades;
As in a wheel, all sinks to reascend."
The subject on which we are entering is of the utmost importance;
for, by pointing out the manner in which life is supported and
modified by the action of external powers, it discovers to us the
true and only means of promoting health and longevity; for the action
of these powers is generally within our own direction; and if the
action of heat, food, air, and exercise, were properly regulated, we
should have little to fear from the attacks of diseases.