Thomas Reid.

The works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters online

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animal, whether it has many eyes or few,
whether of one structure or of another, sees
objects single, and in their true and proper
direction. And, since there is a prodigious
variety in the structure, the motions, and
the number of eyes in different animals and
insects, it is probable that the laws by
which vision is regulated, are not the same
in all, but various, adapted to the eyes which
nature hath given them.

Mankind naturally turn their eyes al-
ways the same way, so that the axes of the
two eyes meet in one point. They natur-
ally attend to, or look at that object only
which is placed in the point where the axes
meet. And whether the object be more or
less distant, the configuration of the eye is
adapted to the distance of the object, so as
to form a distinct picture of it.

When we use our eyes in this natural
way, the two pictures of the object we look
at are formed upon the centres of the two
retinae ; and the two pictures of any con-
tiguous object are formed upon the points
of the retina which are similarly situate
with regard to the centres. Therefore, in
order to our seeing objects single, and in
their proper direction, with two eyes, it is

only refer the jeader who is curious in such points,
to the following recent publications :— J. Mueller,
'* Zur Vergleichenden Physiologie de- Gesichtssin.
nes," &c., 1826.— Volkmann, "Neue Beytraege Bur
Physiologie des Gesich'tssinnes," 183fi.— Heermann,
" Ueber dieBildungder Gesichtsvorstellungcn,"&c a



sufficient that we be so constituted, that
objects whose pictures are formed upon
the centres of the two retina, or upon
points similarly situate with regard to these
centres, shall be seen in the same visi-
ble place. And this is the constitution
which nature hath actually given to human

When we distort our eyes from their
parallel direction, which is an unnatural
motion, but may be learned by practice ; or
when we direct the axes of the two eyes to
one point, and at the same time direct our
attention to some visible object much nearer
or much more distant than that point, which
is also unnatural, yet may be learned : in
these cases, and in these only, we see one
object double, or two objects confounded in
one. In these cases, the two pictures of
the same object are formed upon points of
the retina which are not similarly situate,
and so the object is seen double ; or the
two pictures of different objects are formed
upon points of the retina which are simi-
larly situate, and so the two objects are
seen confounded in one place.

Thus it appears, that the laws of vision
in the human constitution are wisely adapted
to the natural use of human eyes, but not
to that use of them which is uunatural. We
see objects truly when we use our eyes in
the natural way ; but have false appearances
presented to us when we use them in a way
that is unnatural. We may reasonably
think that the case is the same with other
animals. But is it not unreasonable to
think, that those animals which naturally
turn one eye towards one obj ect, and another
eye towards another object, must thereby
have such false appearances presented to
them, as we have when we do so against
nature ?

Many animals have their eyes by nature
placed adverse and immoveable, the axes
of the two eyes being always directed to
opposite points. Do objects painted on the
centres of the two retinae appear to such
animals as they do to human eyes, in one
and the same visible place ? I think it is
highly probable that they do not ; and that
they appear, as they really are, in opposite

If we judge from analogy in this case,
it will lead us to think that there is a certain
correspondence between points of the two
retinas in such animals, but of a different
kind from that which we have found in
human eyes. The centre of one retina will
correspond with the centre of the other,
in such manner that the objects whose
pictures are formed upon these correspond-
ing points, shall appear not to be in the
same place, as in human eyes, but in op-
posite places. And in the same manner
will the superior part of one retina corre-

spond with the inferior part of the other,
and the anterior part of one with the pos-
terior part of the other.

Some animals, by nature, turn their eyes
with equal facility, either the same way or
different ways, as we turn oiir hands and
arms. Have such animals corresponding
points in their retina, and points which do
not correspond, as the human kind has ?
I think it is probable that they have not ;
because such a constitution in them could
serve no other purpose but to exhibit false

If we judgefrom analogy, it will lead us
to think, that, as such animals move their
eyes in a manner similar to that in which
we move our arms, they have an immediate
and natural perception of the direction they
give to their eyes, as we have of the direc-
tion we give to our arms ; and perceive the
situation of visible objects by their eyes, in
a manner similar to that in which we per-
ceive the situation of tangible objects with
our hands.

We cannot teach brute animals to use
their eyes in any other way than in that
which nature hath taught them ; nor can
we teach them to communicate to us the
appearances which visible objects make to
them, either in ordinary or in extraordinary
cases. We have not, therefore, the same
means of discovering the laws of vision in
them, as in our own kind, but must satisfy
ourselves with probable conjectures ; and
what we have said upon this subject, is
chiefly intended to shew, that animals to
which nature hath given eyes differing in
their number, in their position, and in
their natural motions, may very probably
be subjected to different laws of vision,
adapted to the peculiarities of their organs
of vision.

Section XV.


Whether there be corresponding points
in the retinae of those who have an invo-
luntary squint ? and, if there are, Whether
they be situate in the same manner as in
those who have no squint ? are not ques-
tions of mere curiosity. They are of real
importance to the physician who attempts
the cure of a squint, and to the patient who
submits to the cure. After so much has
been said of the strabismus, or squint, both
by medical and by optical writers, one might
expect to find abundance of facts for deter-
mining these questions. Yet, I confess, I
have been disappointed in this expectation,
after taking some pains both to make ob-
servations, and to collect those which have
been made by others.



Nor will this appear very strange, if we
eonsider, that to make the observations
which are necessary for determining these
questions, knowledge of the principles of
optics, and of the laws of vision, must
concur with opportunities rarely to be met

Of those who squint, the far greater
part have no distinct vision with one eye. *
When this is the case, it is impossible,
and indeed of no importance, to determine
the situation of the corresponding points.
When both eyes are good, they commonly
differ so much in their direction, that the
same object cannot be seen by both at the
same time ; and, in this case, it will be
very difficult to determine the situation
of the corresponding points ; for such per-
sons will probably attend only to the ob-
jects of one eye, and the objects of the other
will be as little regarded as if they were not

We have before observed, that, when we
look at a near object, and attend to it, we
do not perceive the double appearances of
more distant objects, even when they are
in the same direction, and are presented
to the eye at the same time. It is probable
that a squinting person, when he attends to
the objects of one eye, will, in like manner,
have his attention totally diverted from the
objects of the other ; and that he will per-
ceive them as little as we perceive the
double appearances of objects when we use
our eyes in the natural way. Such a per-
son, therefore, unless he is so much a phi-
losopher as to have acquired the habit of
attending very accurately to the visible ap-
pearances of objects, and even of objects
which he does not look at, will not be able
to give any light to the questions now under

It is very probable that hares, rabbits,
birds, and fishes, whose eyes are fixed in
an adverse position, have the natural fa-
culty of attending at the same time to vi-
sible objects placed in different, and even
in contrary directions ; because, without
this faculty, they could not have those ad-
vantages from the contrary direction of
their eyes, which nature seems to have in-
tended. But it is not probable that those
who squint have any such natural faculty ;
because we find no such faculty in the rest
of the species. We naturally attend to ob-
jects placed in the point where the axes of
the two eyes meet, and to them only. To
give attention to an object in a different di-
rection is unnatural, and not to be learned
without pains and practice.

* On this imperfection of vision ii rested the
theory of Squinting, proposed by Buflbn, and now
generally adopted. The defective eye is turned aside,
because, if it were directed to the object, together
with the perfect one, a confused impression would
be produced, — H.

A very convincing proof of this may be
drawn from a fact now well known to phi-
losophers : when one eye is shut, there is
a certain space within the field of vision,
where we can see nothing at all — the space
which is directly opposed to that part of the
bottom of the eye where the optic nerve
enters. This defect of sight, in one part
of the eye, is common to all human eyes,
and hath been so from the beginning of the
world ; yet it was never known, until the
sagacity of the Abb6 Mariotte discovered
it in the last century. And now when it is
known, it cannot be perceived, but by means
of some particular experiments, which re-
quire care and attention to make them

What is the reason that so remarkable
a defect of sight, common to all mankind,
was so long unknown, and is now perceived

with so much difficulty ? It is surely this

That the defect is at some distance from
the axis of the eye, and consequently in a
part of the field of vision to which we never
attend naturally, and to which we cannot
attend at all, without the aid of some par-
ticular circumstances.

From what we have said, it appears,
that, to determine the situation of the cor-
responding points in the eyes of those who
squint, is impossible, if they do not see dis-
tinctly with both eyes ; and that it will be
very difficult, unless the two eyes differ so
little in their direction, that the same object
may be seen with both at the same time.
Such patients I apprehend are rare ; at
least there are very few of them with whom
I have had the fortune to meet : and there-
fore, for the assistance of those who may
have happier opportunities, and inclination
to make the proper use of them, we shall con-
sider the case of squinting, hypothetically,
pointing out the proper articles of inquiry,
the observations that are wanted, and the
conclusions that may be drawn from them.

1. It ought to be inquired, Whether the
squinting person sees equally well with
both eyes ? and, if there be a defect in one,
the nature and degree of that defect ought
to be remarked. The experiments by which
this may be done, are so obvious, that I
need not mention them. But I would ad-
vise the observer to make the proper ex-
periments, and not to rely upon the testi-
mony of the patient ; because I have found
many instances, both of persons that squint-
ed, and others who were found, upon trial,
to have a great defect in the sight of one
eye, although they were never aware of it
before. In all the following articles, it is
supposed that the patient sees with both
eyes so well as to be able to read with
either, when the other is covered.

2. It ought to be inquired, Whether,
when one eye is covered, the other is turned



directly to the object ? This ought to be
tried in both eyes successively. By this
observation, as a touchstone, we may try
the hypothesis concerning squinting, in-
vented by M. de la Hire, and adopted by
Boerhaave, and many others of the medical

The hypothesis is, That, in one eye of-
a squinting person, the greatest sensibility
and the most distinct vision is not, as in
other men, in the centre of the retina, but
upon one side of the centre ; and that he
turns the axis of this eye aside from the
object, in order that the picture of the object
may fall upon the most sensible part of the
retina, and thereby give the most distinct
vision. If this is the cause of squinting,
the "(minting eye will be turned aside from
the joject, when the other eye is covered,
as well as when it is not.

A trial so easy to be made, never was
made for more than forty years ; but the
hypothesis was very generally received —
so prone are men to invent hypotheses,
and so backward to examine them by facts.
At last, Dr Jurin having made the trial,
found that persons Iwho squint turn the
axis of the squinting eye directly to the
obj ect, when the other eye is covered. This
fact is confirmed by Dr Porterfield ; and I
have found it verified in all the instances
that have fallen under my observation.

3. It ought to be inquired, Whether the
axes of the two eyes follow one another, so
as to have always the same inclination, or
make the same angle, when the person
looks to the right or to the left, upward or
downward, or straight forward. By this
observation we may judge whether a squint
is owing to any defect in the muscles which
move the eye, as some have supposed. In
the following articles, we suppose that the
inclination of the axes of the eyes is found
to be always the same.

4. It ought to be inquired, Whether the
person that squints sees an object single or
double ?

If he sees the object double, and if the
two appearances have an angular distance,
equal to the angle which the axes of his
eyes make with each other, it may be con-
cluded that he hath corresponding points in
the retina of his eyes, and that they have
the same situation as in those who have no
squint. If the two appearances should
have an angular distance which is always
the same, but manifestly greater or less
than the angle contained under the optic
axes, this would indicate corresponding
points in the retina, whose situation is not
the same as in those who have no squint ;
but it is difficult to judge accurately of the
angle which the optic axes make.

A squint.too small to be perceived, may
occasion double vision of objects : for. if we

speak strictly, every person squints more
or less, whose optic axes do not meet ex-
actly in the object which he looks at. . Thus,
if a man can only bring the axes of his
eyes to be parallel, but cannot make them
converge in the least, he must have a small
squint in looking at near objects, and will
see them double, while he sees very distant
objects single. Again, if the optic axes
always converge, so as to meet eight or ten
feet before the face at farthest, such a per-
son will see near objects single ; but when
he looks at very distant objects, he will
squint a little, and see them double.

An instance of this kind is related by
Aguilonius in his " Optics," who says, that
he had seen a young man to whom near
objects appeared single, but distant objects
appeared double.

Dr Bi-iggs, in his " Nova Visionis Theo-
ria," having collected from authors several
instances of double vision, quotes this from
Aguilonius, as the most wonderful and un-
accountable of all, insomuch that he sus-
pects some imposition on the part of the
young man : but to those who understand
the laws by which single and double vision
are regulated, it appears to be the natural
effect of a very small squint.*

Double vision may always be owing to a
small squint, when the two appearances
are seen at a small angular distance,
although no squint was observed : and I do
not . remember any instances of double
vision recorded by authors, wherein any
account is given of the angular distance of
the appearances.

In almost all the instances of double
vision, there is reason to suspect a squint
or distortion of the eyes, from the concomi-
tant circumstances, which we find to be
one or other of the following — the approach
of death or of a deliquium, excessive drink-
ing or other intemperance, violent headache,
blistering the head, smoking tobacco, blows
or wounds in the head. In all these cases,
it is reasonable to suspect a distortion of
the eyes, either from spasm, or paralysis in
the muscles that move them. But, although
it be probable that there is always a squint
greater or less where there is double vision,
yet it is certain that there is not double
vision always where there is a squint. I
know no instance of double vision that con-
tinued for life, or even for a great number of
years. We shall therefore suppose, in the
following articles, that the squinting person
sees objects single.

5. The next inquiry, then, ought to be,
Whether the object is seen with both eyes
at the same time, or only with the eye

« It is observed by Purkinje and Volkmann, that
short-sighted persons, under certain conditions, see
distant objects double. Is the case of Aguilonius
more than an example of this ?— H.



whose axis is directed to it ? It hath been
taken for granted, by the writers upon the
strabismus, before Dr Jurin, that those who
squint commonly see objects single with
both eyes at the same time ; but I know
not one fact advanced by any writer which
proves it. Dr Jurin is of a contrary opi-
nion ; and, as it is of consequence, so it is
very easy, to determine this point, in parti-
cular instances, by this obvious experiment.
While the person that squints looks steadily
at an object, let the observer carefully re-
mark the direction of both his eyes, and
observe their motions ; and let an opaque
body be interposed between the object and
the two eyes successively. If the patient,
notwithstanding this interposition, and with-
out changing the direction of his eyes, con-
tinues to see the object all the time, it may
be concluded that he saw it with both eyes
at once. But, if the interposition of the
body between one eye and the object makes
it disappear, then we may be certain that it
was seen by that eye only. In the two
following articles, we shall suppose the first
to happen, according to the common hypo-

6. Upon this supposition, it ought to be
inquired, Whether the patient sees an ob-
ject double in those circumstances wherein
it appears double to them who have no
squint ? Let him, for instance, place a
candle at the distance of ten feet; and
holding his finger at arm's-length between
him and the candle, let him observe, when
he looks at the candle, whether he sees his
finger with both eyes, and whether he sees
it single or double ; and when he looks at
his finger, let him observe whether he sees
the candle with both eyes, and whether
single or double.

By this observation, it may be deter-
mined, whether to this patient, the phseno-
mena of double as well as of single vision
are the same as to them who have no squint.
If they are not the same — if he sees objects
single with two eyes, not only in the cases
wherein they appear single, but in those
also wherein they appear double to other
men — the conclusion to be drawn from this
supposition is, that his single vision does not
arise from corresponding points in the re-
tina of his eyes ; and that the laws of vision
are not the same in him as in the rest of

7. If, on the other hand, he sees objects
double in those cases wherein they appear
double to others, the conclusion must be,
that he hath corresponding points in the
retina of his eyes, but unnaturally situate.
And their situation may be thus determined.

When he looks at an object, having the
axis of one eye directed to it, and the axis
of the other turned aside from it, let us
suppose a right line to pass from the object

through the centre of the diverging eye.
We shall, for the sake of perspicuity, call
this right line, the natural axis of the eye;
and it will make an angle with the real
axis, greater or less, according as his squint
is greater or less. We shall also call that
point of the retina in which the natural
axis cuts it, the natural centre of the retina ;
which will be more or less distant from the
real centre, according "as the squint is
greater or less.

Having premised these definitions, it will
be evident to those who understand the
principles of optics, that in this person the
natural centre of one retina corresponds
with the real centre of the other, in the
very same manner as the two real centres
correspond in perfect eyes ; and that the
points similarly situate with regard to the
real centre in one retina, and the natural
centre in the other, do likewise correspond,
in the very same manner as the points si-
milarly situate with regard to the two reiJi
centres correspond in perfect eyes.

If it is true, as has been commonly af-
firmed, that one who squints sees an object
with both eyes at the same time, and yet
sees it single, the squint will most probably
be such as we have described in this article.
And we may further conclude, that, if a
person affected with such a squint as we
have supposed, could be brought to the
habit of looking straight, his sight would
thereby be greatly hurt; for he would
then see everything double which he saw
with both eyes at the same time ; and ob-
jects distant from one another would appear
to be confounded together. His eyes are
made for squinting, as much as those of
other men are made for looking straight ;
and his sight would be no less injured by
looking straight, than that of another man
by squinting. He can never see perfectly
when he does not squint, unless the corre-
sponding points of his eyes should by custom
change their place ; but how small the pro-
bability of this is will appear in the 17th

Those of the medical faculty who attempt
the cure of a squint, would do well to con-
sider whether it is attended with such symp-
toms as are above described. If it is, the
cure would be worse than the malady: for,
every one will readily acknowledge that it
is better to put up with the deformity of a
squint, than to purchase the cure by the
loss of perfect and distinct vision.

8. We shall now return to Dr Jurin's
hypothesis, and suppose that our patient,
when he saw objects single notwithstanding
his squint, was found, upon trial, to have
seen them only with one eye.

We would advise such a patient to en-
deavour, by repeated efforts, to lessen his
squint, and to bring the axes of his eyes



nearer to a parallel direction. We have
naturally the power of making small varia-
tions in the inclination of the optic axes ;
and this power maybe greatly increased by

In the ordinary and natural use of our
eyes, we can direct their axes to a fixed
star ; in this ease they must be parallel :
we can direct them also to an object six
inches distant from the eye; and in this
case the axes must make an angle of fif-
teen or twenty degrees. We see young
people in their frolics learn to squint, mak-
ing their eyes either converge or diverge,
when they will, to a very considerable de-
gree. Why should it be more difficult for
a squinting person to learn to look straight
when he pleases ? If once, by an effort of
his will, he can but lessen his squint, fre-
quent practice will make it easy to lessen
it, and will daily increase his power. So
that, if he begins this practice in youth, and
perseveres in it, he may probably, after
«ome time, learn to direct both his eyes to
one object.

When he hath acquired this power, it
will be no difficult matter to determine, by
proper observations, whether the centres of
the retina, and other points similarly situate
with regard to the centres, correspond, as
in other men.

9. Let us now suppose that he finds this
to be the case ; and that he sees an object
single with both eyes, when the axes of
both are directed to it. It will then concern
him to acquire the habit of looking straight,
as he hath got the power, because he will
thereby not only remove a deformity, but
improve his sight ; and I conceive this ha-

Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 39 of 114)