Thomas Reid.

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

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bit, like all others, may be got by frequent
exercise. He may practise before a mirror
when alone, and in company he ought to have
those about him who will observe and ad-
monish him when he squints.

10. What is supposed in the 9th article
is not merely imaginary ; it is really the
case of some squinting persons, as will
appear in the next section. Therefore, it
ought further to be inquired, How it comes
to pass that such a person sees an object
which he looks at, only with one eye, when
both are open ? In order to answer this
question, it may be observed, first, Whether,
when he looks at an object, the diverging
eye is not drawn so close to the nose, that it
can have no distinct images ? Or, secondly,
whether the pupil of the diverging eye is not
covered wholly, or in part, by the upper eye-
lid ? Dr Jurin observed instances of these
cases in persons that squinted, and assigns
them as causes of their seeing the object
only with one eye. Thirdly, it may be
observed, whether the diverging eye is not
so directed, that the picture of the object
falls upon that part of the retina where the

optic nerve enters, and where there is no
vision ? This will probably happen in a
squint wherein the axes of the eyes converge
so as to meet about six inches before the

11. In the last place, it ought to be
inquired, Whether such a person hath any
distinct vision at all with the diverging
eye, at the time he is looking at an object
with the other ?

It may seem very improbable that he
should be able to read with the diverging
eye when the other is covered, and yet, when
both are open, have no distinct vision with
it at all. But this, perhaps, will not appear
so improbable if the following considerations
are duly attended to.

Let us suppose that one who saw per-
fectly, gets, by a blow on the head, or some
other accident, a permanent and involun-
tary squint. According to the laws of vi-
sion, he will see objects double, and will see
objects distant from one another confounded
together ; but, such vision being very dis-
agreeable, as well as inconvenient, he will
do everything in his power to remedy it.
For alleviating such distresses, nature often
teaches men wonderful expedients, which
the sagacity of a philosopher would be un-
able to discover. Every accidental motion,
every direction or conformation of his eyes,
which lessens the evil, will be agreeable ;
it will be repeated until it be learned to
perfection, and become habitual, even with-
out thought or design. Now, in this case,
what disturbs the sight of one eye is the
sight of the other ; and all the disagreeable
appearances in vision would cease if the
light of one eye was extinct. The sight of
one eye will become more distinct and
more agreeable, in the same proportion as
that of the other becomes faint and in-
distinct. It may, therefore, be expected,
that every habit will, by degrees, be ac-
quired which tends to destroy distinct vi-
sion in one eye while it is preserved in the
other. These habits will be greatly facili-
tated if one eye was at first better than the
other ; for, in that case, the best eye will
always be directed to the object which he
intends to look at, and every habit will be
acquired which tends to hinder his seeing
it at all, or seeing it distinctly by the other
at the same time.

I shall mention one or two habits that
may probably be acquired in such a, case ;
perhaps there are others which we cannot
so easily conjecture. First, By a small in-
crease or diminution of his squint, he may
bring it to correspond with one or other of
the cases mentioned in the last article.
Secondly, The diverging eye may be brought
to such a conformation as to be extremely
short-sighted, and consequently to have no
distinct vision of objects at a distance. I



knew this to be the case of one person that
squinted ; but cannot say whether the
short-sightedness of the diverging eye was
original, or acquired by habit.

We see, therefore, that one who squints,
and originally saw objects double by reason
of that squint, may acquire such habits
that, when he looks at an object, he shall
see it only with one eye ; nay, he may ac-
quire such habits that, when he looks at an
object with his best eye, he shall have no
distinct vision with the other at all. Whether
this is really the case — being unable to de-
termine in the instances that have fallen
under my observation — I shall leave to fu-
ture inquiry.

I have endeavoured, in the foregoing
articles, to delineate such a process as is
proper in observing the phsenomena of
squinting. I know well by experience, that
this process appears more easy in theory,
than it will be found to be in practice ;
and that, in order to carry it on with success,
some qualifications of mind are necessary
in the patient, which are not always to be
met with. But, if those who have proper
opportunities and inclination to observe
such phsenomena, attend duly to this pro-
cess, they may be able to furnish facts less
vague and uninstructive than those we meet
with, even in authors of reputation. By
such facts, vain theories may be exploded,
and our knowledge of the laws of nature,
which regard the noblest of our senses,

Section XVI.


Having considered the phsenomena of
squinting, hypothetically, and their connec-
tion with corresponding points in the re-
tincB, I shall now mention the facts I have
had occasion to observe myself, or have
met with in authors, that can give any light
to this subject.

Having examined above twenty persons
that squinted, I found in all of them a de-
fect in the sight of one eye. Four only
had so much of distinct vision in the weak
eye, as to be able to read with it, when the
other was covered. The rest saw nothing
at all distinctly with one eye.

Dr Porterfield says, that this is generally
the case of people that squint : and I sus-
pect it is so more generally than is com-
monly imagined. Dr Jurin, in a very
judicious dissertation upon squinting,
printed in Dr Smith's " Optics," observes,
that those who squint, and see with both
eyes, never see the same object with both
at the same time ; that, when one eye is
directs' strni'^'.t forward to an object, the

other is drawn so close to the nose that the
object cannot at all be seen by it, the
images being too oblique and too indistinct
to affect the eye. In some squinting per-
sons, he observed the diverging eye drawn
under the upper eyelid, while the other
was directed to the object. From these
observations, he concludes that " the eye is
thus distorted, not for the sake of seeing
better with it, but rather to avoid seeing at
all with it as much as possible." From all
the observations he had made, he was satis-
fied that there is nothing peculiar in the
structure of a squinting eye ; that the fault
is only in its wrong direction; and that
this wrong direction is got by habit. There-
fore, he proposes that method of cure which
we have described in the eighth and ninth
articles of the last section. He tells ns,
that he had attempted a cure, after this
method, upon a young gentleman, with
promising hopes of success ; but was in-
terrupted by his falling ill of the small-
pox, of which he died.

It were to be wished that Dr Jurin had
acquainted us whether he ever brought the
young man to direct the axes of both eyes
to the same object, and whether, in that
case, he saw the object single, and saw it
with both eyes ; and that he had likewise
acquainted ns, whether he saw objects
double when his squint was diminished.
But as to these facts he is silent.

I wished long for an opportunity of trying
Dr Jurin's method of curing a squint, with-
out finding one ; having always, upon ex-
amination, discovered so great a defect in
the sight of one eye of the patient as dis-
couraged the attempt.

But I have lately found three young
gentlemen, with whom I am hopeful this
method may have success, if they have
patience and perseverance in using it. Two
of them are brothers, and, before I had
access to examine them, had been practis-
ing this method by the direction of their
tutor, with such success that the elder looks
straight when he is upon his guard : the
younger can direct both his eyes to one
object ; but they soon return to their usual

A third young gentleman, who had never
heard of this method before, by a few days
practice, was able to direct both his eyes to
one object, but could not keep them long in
that direction. All the three agree in this,
that, when both eyes are directed to one ob-
ject, they see it and the adjacent objects
single; but, when they squint, they see
objects sometimes single and sometimes
double. I observed of all the three, that
when they squinted most— that is, in the

way they had been accustomed to the axes

of their eyes converged so as to meet five
or six inches before the nose. It is pro:



bable that, in this case, the picture of the
object in the diverging eye, must fall upon
that part of the retina where the optic
nerve enters; and, therefore, the object
could not be seen by that eye.

All the three have some defect in the
sight of one eye, which none of them knew
until I put them upon making trials ; and
when they squint, the best eye is always
directed to the object, and the weak eye is
that which diverges from it. But when the
best eye is covered, the weak eye is turned
directly to the object. Whether this defect
of sight in one eye, be the effect of its hav-
ing been long disused, as it must have been
when they squinted ; or whether some ori-
ginal defect in one eye might be the occasion
of their squinting, time may discover. The
two brothers have found the sight of the
weak eye improved by using to read with it
while the other is covered. The elder can
read an ordinary print with the weak eye ;
the other, as well as the third gentleman,
can only read a large print with the weak
eye. I have met with one other person
only who squinted, and yet could read a
large print with the weak eye. He is a
young man, whose eyes are both tender and
weak-sighted, but the left much weaker than
the right. When he looks at any object,
he always directs the right eye to it, and
then the left is turned towards the nose so
much that it is impossible for him to see
the same object with both eyes at the same
time. When the right eye is covered, he
turns the left directly to the object ; but he
sees it indistinctly, and as if it had a mist
about it.

I made several experiments, some of them
in the company and with the assistance of
an ingenious physician, in order to discover
whether objects that were in the axes of the
two eyes, were seen in one place confounded
together, as in those who have no involun-
tary squint. The object placed in the axis
of the weak eye was a lighted candle, at the
distance of eight or ten feet. Before the
other eye was placed a printed book, at such
a distance as that he could read upon it.
He said, that while he read upon the book,
he saw the candle but very faintly. And
from what we could learn, these two objects
did not appear in one place, but had all that
angular distance in appearance which they
had in reality.*

If this was really the case, the conclusion
to be drawn from it is, that the correspond-
ing points in his eyes are not situate in the
same manner as in other men ; and that, if
he could be brought to direct both eyes to
one object, he would see it double. But,
considering that the young man had never
been accustomed to observations of this

* See Weill— (" h» Essays," &c., p. 26.)— H.

kind, and that the sight of one eye was so
imperfect, I do not pretend to draw this
conclusion with certainty from this single

All that can be inferred from these facts
is, that, of four persons who squint, three
appear to have nothing preternatural in the
structure of their eyes. The centres of their
retinas, and the points similarly situate with
regard to the centres, do certainly corre-
spond in the same manner as in other men —
so that, if they can be brought to the habit
of directing their eyes right to an object,
they will not only remove a deformity, but
improve their sight. With regard to the
fourth, the case is dubious, with some pro-
bability of a deviation from the usual course
of nature in the situation of the correspond-
ing points of his eyes.

Seotion XVII.


It appears from the phaenomena of single
and double vision, recited in § 13, that
our seeing an object single with two eyes,
depends upon these two things : — First,
Upon that mutual correspondence of certain
points of the retina which we have often
described ; Secondly, Upon the two eyes
being directed to the object so accurately
that the two images of it fall upon corre-
sponding points. These two things must
concur in order to our seeing an object
single with two eyes ; and, as far as they
depend upon custom, so far only can single
vision depend upon custom.

With regard to the second — that is, the
accurate direction of both eyes to the ob-
ject — I think it must be acknowledged
that this is only learned by custom. Na-
ture hath wisely ordained the eyes to move
in such manner that their axes shall
always be nearly parallel ; but hath left it
in our power to vary their inclination a
little, according to the distance of the ob-
ject we look at. Without this power,
objects would appear single at one parti-
cular distance only ; and, at distances much
less or much greater, would always appear
double. The wisdom of nature is conspi-
cuous in giving us this power, and no less
conspicuous in making the extent of it ex-
actly adequate to the end.

The parallelism of the eyes, in general,
is therefore the work of nature ; but that
precise and accurate direction, which must
be varied according to the distance of the
object, is the effect of custom. The power
which nature hath left us of varying the
inclination of the optic axes a little, is
turned into a habit of giving them always



that inclination which is adapted to the
distance of the object.

But it may be asked, What gives rise to
this habit ? The only answer that can be
given to this question is, that it is found
necessary to perfect and distinct vision. A
man who hath lost the sight of one eye,
very often loses the habit of directing it
exactly to the object he looks at, because
that habit is no longer of use to him. And
if he should recover the sight of his eye,
he would recover this habit, by finding it
useful. No part of the human constitution
is more admirable than that whereby we
acquire habits which are found useful, with-
out any design or intention. Children
must see imperfectly at first ; but, by using
their eyes, they learn to use them in the
best manner, and acquire, without intend-
ing it, the habits necessary for that pur-
pose. Every man becomes most expert in
that kind of vision which is most useful to
him in his particular profession and man-
ner of life. A miniature painter, or an
engraver, sees very near objects better than
a sailor ; but the sailor sees very distant
objects much better than they. A person
that is short-sighted, in looking at distant
objects, gets the habit of contracting the
aperture of his eyes, by almost closing his
eyelids. Why ? For no other reason,
but because this makes him see the object
more distinct. In like manner, the reason
why every man acquires the habit of direct-
ing both eyes accurately to the object, must
be, because thereby he sees it more per-
fectly and distinctly.

It remains to be considered, whether that
correspondence between certain points of
the retina, which is likewise necessary to
single vision, be the effect of custom, or an
original property of human eyes.

A strong argument for its being an ori-
ginal property, may be drawn from the
habit, just now mentioned, of directing the
eyes accurately to an object. This habit
is got by our finding it necessary to perfect
and distinct vision. But why is it neces-
sary ? For no other reason but this, be-
cause 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 together, than it
could be by one ; but when the eyes are not
accurately directed, the two images of an
object fall upon points that do not corre-
spond, whereby the sight of one eye disturbs
the sight of the other, and the object is
seen more indistinctly with both eyes than
it would be with one. Whence it is rea-
sonable to conclude, that this correspond-
ence of certain points of the retina, is prior
to the habits we acquire in vision, and con-
sequently is natural and original. We have
all acquired the habit of directing our eyes

always in a particular manner, which causes
single vision. Now, if nature' hath ordained
that we should have single vision only, when
our eyes are thus directed, there is an ob-
vious reason why all mankind should agree
in the habit of directing them in this manner.
But, if single vision is the effect of custom,
any other habit of directing the eyes would
have answered the purpose ; and no account
can be given why this particular habit should
be so universal ; and it must appear very
strange, that no one instance hath been
found of a person who had acquired the
habit of seeing objects single with both eyes,
while they were directed in any other man-

The judicious Dr Smith, in his excellent
system of optics, maintains the contrary
opinion, and offers some reasonings and
facts in proof of it. He agrees with Bishop
Berkeley^ in attributing it entirely to cus-
tom, that we see obj ects single with two eyes,
as well as that we see objects erect by in-
verted images. Having considered Bishop
Berkeley's reasonings in the 1 1th section,
we shall now beg leave to make some
remarks on what Dr Smith hath said upon
this subject, with the respect due to an
author to whom the world owes, not only
many valuable discoveries Of Ins own, but
those of the brightest mathematical genius
of this age, which, with great labour, he
generously redeemed from oblivion.

He observes, that the question, Why we
see objects single with two eyes ? is of the
same sort with this, Why we hear sounds
single with two ears ? — and that the same
answer must serve both. The inference
intended to be drawn from this observation
is, that, as the second of these phsenomena
is the effect of custom, so likewise is the

Now, I humbly conceive that the ques-
tions are not so much of the same sort,
that the same answer must serve for
both ; and, moreover, that our hearing
single with two ears, is not the effect of

* This objection did not escape Dr Smith himself;
but Reid seems to have overlooked his answer.
*' When we view," he says, " an abject steadily, we
have acquired a habit of directing the optic axes to
the point in view ; because its pictures, falling upon
the middle points of the retinas, are then distincter
than if they fell upon any other places ; and, since
the pictures of the whole object are equal to one
another, and are both inverted with respect to the
optic axes, it follows that the pictures of any col-
lateral point are painted upon corresponding points of
the retinas."

This answer is rendered more plausible from the
subsequent anatomical discovery of Soemmering.
He found that, in that part of the retina which lies
at the axis of the eye, there is, in man, and in other
animals of acute vision, an 'opening, real or appar-
ent, (foramen centrale,) the dimensions of which
are such that the images of distincter vision would
seem to be enclosed within it H.

t This is an inadvertency. Berkeley hazards no
such opinion in any of his works.— H.



Two or more visible objects, although
perfectly similar, and seen at the very same
time, may be distinguished by their visible
places; but two sounds perfectly similar,
and heard at the same time, cannot be dis-
tinguished ; for, from the nature of sound,
the sensations they occasion must coalesce
into one, and lose all distinction. If, there-
fore, it is asked, Why we hear sounds single
with two ears ? I answer, Not from custom ;
but because two sounds which are perfectly
like and synchronous, have nothing by
which they can be distinguished. But will
this answer fit the other question ? I think

The object makes an appearance to each
eye, as the sound makes an impression upon
each ear : so far the two senses agree. But
the visible appearances may be distin-
guished by place, when perfectly like in other
respects ; the sounds cannot be thus dis-
tinguished : and herein the two senses dif-
fer. Indeed, if the two appearances have
the same visible place, they are, in that
case, as incapable of distinction as the sounds
were, and we see the object single. But
when they have not the same visible place,
they are perfectly distinguishable, and we
see the object double. We see the object
single only, when the eyes are directed in
one particular manner; while there are many
other ways of directing them within the
sphere of our power, by which we see the
object double.

Dr Smith justly attributes to custom that
well-known fallacy in feeling, whereby a
button pressed with two opposite sides of
two contiguous fingers laid across, is felt
double. I agree with him, that the cause
of this appearance is, that those opposite
sides of the fingers have never been used
to feel the same object, but two different
objects, at the same time. And I beg leave
to add, that, as custom produces this phse-
nomenon, so a contrary custom destroys it ;
for, if a man frequently accustoms himself
to feel the button with his fingers across, it
will at last be felt single ; as I have found
by experience.

It may be taken for a general rule, that
things which are produced by custom, may
be undone or changed 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. I take this to be the best rule
by which we can determine the question
presently* under consideration. I shall,
therefore, mention two facts brought by
Dr Smith, to prove that the corresponding
points of the retinue have been changed by

* See note * at p. 96, a — H.

custom ; and then I shall mention soma
facts tending to prove, that there are cor-
responding points of the retina of the eyes
originally, and that custom produces no
change in them.

" One fact is related upon the authority
of Martin Folkes, Esq., who was informed
by Dr Hepburn of Lynn, that the Rev. Mr
Foster of Clinch wharton, in that neighbour-
hood, having been blind for some years of a
gutta serena, was restored to sight by sali-
vation ; and that, upon his first beginning
to see, all objects appeared to him double ;
but afterwards, the two appearances ap-
proaching by degrees, he came at last to
see single, and as distinctly as he did before
he was blind."

Upon this case, I observe, First, That it
does not prove any change of the corre-
sponding points of the eyes, unless we sup-
pose, what is not affirmed, that Mr Foster
directed his eyes to the object at first, when
he saw double, with the same accuracy, and
in the same manner, that he did afterwards,
when he saw single. Secondly, If we should
suppose this, no account can be given, why
at first the two appearances should be seen
at one certain angular distance rather than
another ; or why this angular distance should
gradually decrease, until at last the appear-
ances coincided. How could this effect be
produced by custom ? But, Thirdly, Every
circumstance of this case may be accounted
for on the supposition that Mr Foster had
corresponding points in the retina of his
eyes from the time he began to see, and that
custom made no change with regard to them.
We need only further suppose, what is
common in such cases, that, by some years'
blindness, he had lost the habit of directing
his eyes accurately to an object, and that he
gradually recovered this habit when he came
to see.

The second fact mentioned by Dr Smith,
is taken from Mr Cheselden's " Anatomy,"
and is this : — " A gentleman who, from a
blow on the head, had one eye distorted,

Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 40 of 114)