Thomas Reid.

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

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foundeveryobjectappeardouble; but, byde-
grees, the most familiar ones became single ;
and, in time, all objects became so, without
any amendment of the distortion."

I observe here, that it is not said that
the two appearances gradually approached,
and at last united, without any amendment
of the distortion. This would indeed have
been a decisive proof of a change in the
corresponding points of the retina, and yet
of such a change as could not be accounted
for from custom. But this is not said ; and,
if it had been observed, a circumstance so
remarkable would have been mentioned by
Mr Cheselden, as it was in the other case
by Dr Hepburn. We may, therefore, take
it for granted, that one of the appearances
vanished by degrees, without approaching to



the other. And this I conceive might hap-
pen several ways. First, The sight of the
distorted eye might gradually decay by the
hurt ; so the appearances presented by that
eye would gradually vanish. Secondly, A
small and unperceived change in the man-
ner of directing the eyes, might occasion
his not seeing the object with the dis-
torted eye, as appears from § 15, Art. 10.
Thirdly, By acquiring the habit of direct-
ing one and the same eye always to the ob-
ject, the faint and oblique appearance pre-
sented by the other eye, might be so little
attended to when it became familiar, as not
to be perceived. One of these causes, or
more of them concurring, might produce
the effect mentioned, without any change of
the corresponding points of the eyes.

For these reasons, the facts mentioned
by Dr Smith, although curious, seem not
to be decisive.

The following facts ought to be put in
the opposite scale. First, in the famous
case of the young gentleman couched by Mr
Cheselden, after having had cataracts on
both eyes untilhe was [above] thirteen years
of age, it appears that he saw objects single
from the time he began to see with both
eyes. Mr Cheselden's words are, "And
now, being lately couched of his other eye,
he says, that objects, at first, appeared
large to this eye, but not so large as they
did at first to the other ; and, looking upon
the same object with both eyes, he thought
it looked about twice as large as with the
first couched eye only, hut not double, that
we can anywise discover."

Secondly, The three young gentlemen
mentioned in the last section, who had
squinted, as far as I know, from infancy,
as soon as they learned to direct both eyes to
an obj ect, saw it single. In these four cases,
it appears evident that the centres of the
retince corresponded originally, and before
custom could produce any such effect ; for
Mr Cheselden's young gentleman had never
been accustomed to see at all before he was
couched ; and the other three had never
been accustomed to direct the axes of both
eyes to the object.

Thirdly, from the facts recited in § 13,
it appears, that, from the time we are
capable of observing the phsenomena of
single and double vision, custom makes no
change in them.

I have amused myself with such observ-
ations for more than thirty years ; and in
every case wherein I saw the object double
at first, I see it so to this day, notwith-
standing the constant experience of its being
single. In other cases, where I know there
are two objects, there appears only one,
after thousancls^pf experiments.

Let a man look at a familiar object
through a polyhedron, or multiplying-glass,

every hour of his life, the number of visibU.
appearances will be the same at last as at
first ; nor does any number of experiments,
or length of time, make the least change.

Effects produced by habit, must vary
according as the acts by which the habit is
acquired are more or less frequent ; but
the phsenomena of single and double vision
are so invariable and uniform in all men,
are so exactly regulated by mathematical
rules, that I think we have good reason to
conclude that they are not the effect of cus-
tom, but of fixed and immutable laws of

Section XVIII.
or DR porterfield's account op single


Bishop Berkeley and Dr Smith seem to
attribute too much to custom in vision, Dr
Porterfield too little.

This ingenious writer thinks, that, by an
original law of our nature, antecedent to
custom and experience, we perceive visible
objects in their true place, not only as to
their direction, but likewise as to their dis-
tance from the eye ; and, therefore, he
accounts for our seeing objects single, with
two eyes, in this manner. Having the
faculty of perceiving the object with each
eye in its true place, we must perceive it
with both eyes in the same place; and,
consequently, must perceive it single.

He is aware that this principle, although
it accounts for our seeing objects single
with two eyes, yet does not at all account
for our seeing objects double ; and, whereas
other writers on this subject take it to be a
sufficient cause for double vision that we
have two eyes, and only find it difficult to
assign a cause for single vision, on the
contrary, Dr Porterfield's principle throws
all the difficulty on the other side.

Therefore, in order to account for the
phsenomena of double vision, he advances
another principle, without signifying whe-
ther he conceives it to be an original law of
our nature, or the effect of custom. It is,
That our natural perception of the distance
of objects from the eye, is not extended to
all the objects that fall within the field of
vision, but limited to that which we directly
look at ; and that the circumjacent objects,
whatever be their real distance, are seen at
the same distance with the object we look
at ; as if they were all in the surface of a
sphere, whereof the eye is the centre.

Thus, single vision is accounted for by
our seeing the true distance of an object
which we look at ; and double vision, by a
false appearance of distance in objects
which we do not directly look at.



We agree with this .earned and inge-
nious author, that it is by a natural and
original principle that we see visible objects
in a certain direction from the eye, and
honour him as the author of this discovery :*
but we cannot assent to either of those
principles by which he explains single and
double vision— for the following reasons : —

1. Our having a natural and original
perception of the distance of objects from
the eye, appears contrary to a well-attested
fact : for the young gentleman couched by
Mr Cheselden imagined, at first, that what-
ever he saw touched his eye, as what he
felt touched his hand.-f

2. The perception we have of the distance
of objects from the eye, whether it be from
nature or custom, is not so accurate and
determinate as is necessary to produce sin-
gle vision. A mistake of the twentieth or
thirtieth part of the distance of a small
object, such as a pin, ought, according to
Dr Porterfield's hypothesis, to make it ap-
pear double. Very few can judge of the
distance of a visible object with such
accuracy. Yet we never find double vision
produced by mistaking the distance of the
object. There are many cases in vision,
even with the naked eye, wherein we mis-
take the distance of an object by one half
or more : why do we see such objects single ?
When I move my spectacles from my eyes
toward a small object, two or three feet dis-
tant, the object seems to approach, so as to
be seen at last at about half its real distance ;
but it is seen single at that apparent distance,

* To this honour Porterfield has no title. The law
of Me line of visible direction, was a-common theory
long' before the publication of Jiis writings j for it was
maintained by Kepler, Gassendi, Scheiner, Rohault,
Regis, Du Hamel, Mariotte, De Chales, Musschen-
broek, Molyneux, &c. &c, and many of these main-
tained -that this law was an original principle or in.
stitution of our nature. — H.

+ We must be careful not, like Reid and,philo-
sophersin general, to confound the perceptions of
mere externality or outness, and the knowledge 'we
have of. distance, through the eye. The former may
he, and probably is, natural; while the latter, in a but unappretiable measure, is acquired. In the
case of Cheselden— that in which the blindness pre.
vious to the recovery of eight was most perfect, and,
therefore, the most instructive upon record — the
patient, though he had little or no perception of
distance, i. e. of the degree of externality, had still
a perception of that externality absolutely. The
objects, he said, seemed to " touch his eyes, as what
he felt did his skin j" but they did not appear to him
as if in his eyes, far less as a mere affection of the or-
gan. This, however, is erroneously assumed by Mr
Fearn. This natural perception of Outness, which
is the foundation of our acquired- knowledge of dis.
tance, seems given us in the'natura! perception we
have .of the direction of the rays of light.

In like manner, we must not confound, as is com-
monly done, the fact of the eye affording us a per-
ception of extension and plain-figure, or outline,
in thaperception -of colours, and the fact of its being
the vehicle of intimations in regard to the compa-
rative magnitude and cubical forms of the objects
from which these rays proceed. The-one is a know-
ledge by sense — natural, immediate, and infallible ;
the other, like that of distance, is, by inference, ac-
quired, mediate, and at best always insecure.— H.

as well as when we see it with the naked
eye at its real distance. And when we look
at an object with a binocular telescope, pro-
perly fitted to the eyes, we see it single,
while it appears fifteen or twenty times
nearer than it is. There are then few cases
wherein the distance of an object from the
eye is seen so accurately as is necessary for
single vision, upon this hypothesis : this
seems to be a conclusive argument against
the account given of single vision. We find,
likewise, that false judgments or fallacious
appearances of the distance of an object, do
not produce double vision : this seems to
be a conclusive argument against the account
given of double vision.

3. The perception we have of the linear
distance of objects seems to be wholly the
effect of experience. This, I think, hath
been proved by Bishop Berkeley and by
Dr Smith ; *and when we come to point out
the means of judging of distance by sight,
it will appear that they are all furnished by

4. Supposing that, by a law of our nature,
the distance of objects from the eye were
perceived most accurately, as well as their
direction, it will not follow that we must
see the object single. Let us consider what
means such a law of nature would furnish
for resolving the question, Whether the
objects of the two eyes are in one and the
same place, and consequently are not two,
but one ?

Suppose, then, two right lines, one drawn
from the centre of one eye to its object, the
other drawn, in like manner, from the centre
of the other eye to its object. This law of
nature gives us the direction or position of
each of these right lines, and the length of
each ; and this is all that it gives. These
are geometrical data, and we may learn from
geometry what is determined by their means.
Is it, then, determined by these data, Whe-
ther the two right lines terminate in one
and the same point, or not ? No, truly.
In order to determine this, we must have
three other data. We must know whether
the two right lines are in one plane ; we
must know what angle they make ; and we
must know the distance between the centres
of the eyes. And when these things are
known, we must apply the rules of trigono-
metry, before we can resolve the question,
Whether the objects of the two eyes are in
one and the same place ; and, consequently,
whether they are two or one ?

5. That false appearance of distance into
which double vision is resolved, cannot be
the effect of custom, for constant experience
contradicts it. Neither hath it the features
of a law of nature, because it does not
answer any good purpose, nor, indeed, any
purpose at all, but to deceive us. But why
should we seek for arguments, in a question




concerning what appears to us, or does not
appear ? The question is, At what distance
do the objects now in my eye appear ? Do
they all appear at one distance, as if placed
in the concave surface of a sphere, the eye
being in the centre ? Every man, surely,
may know this with certainty ; and, if he
will but give attention to the testimony of
his eyes, needs not ask a philosopher how
visible objects appear to him. Now, it is
very true, that, if I look up to a star in the
heavens, the other stars that appear at the
same time, do appear in this manner : yet
this phsenomenon does not favour Dr Por-
terfield's hypothesis ; for the stars and
heavenly bodies do not appear at their true
distance's when we look directly to them,
any more than when they are seen obliquely :
a id if this phsenomenon be an argument for
Dr Portorfield's second principle, it must
destroy the first.

The true cause of this pheenomenon will
be given afterwards ; therefore, setting it
aside for the present, let us put, another
case. I sit in my room, and direct my
eyes to the door, which appears to be
about sixteen feet distant : at the same
time, I see many other objects faintly and
obliquely — the floor, floor-cloth, the table
which I write upon, papers, standish,
candle, &c. Now, do all these objects ap-
pear at the same distance of sixteen feet ?
Upon the closest attention, I find they do

Section XIX.

of dr briggs's theory, and sir isaac
newton's conjecture on this sub-

I am afraid the reader, as well as the
writer, is already tired of the subject of
single and double vision. The multitude
of theories advanced by authors of great
name, and the multitude of facts, observed
without sufficient skill in optics, or related
without attention to the most material and
decisive circumstances, have equally contri-
buted to perplex it.

In order to bring it to some issue, I have,
in the 13th section, given a more full
and regular deduction than had been given
heretofore, of the phaenomena of single and
double vision, in those whose sight is per-
fect ; and have traced them up to one ge-
neral principle, which appears to be a law
of vision in human eyes that are perfect and
in their natural state.

In the 14th section, I have made it ap-
pear, that this law of vision, although ex-
cellently adapted to the fabric of human
eyes, cannot answer the purposes of vision
in some other animals ; and therefore, very

probably, is not common to all animals.
The purpose of the 15th and 16th sections
is, to inquire, Whether there be any de-
viation from this law of vision in those
who squint ? — a question which is of real
importance in the medical art, as well as
in the philosophy of vision; but which,
after all that hath been observed and
written on the subject, seems not to be
ripe for a determination, for want of pro-
per observations. Those who have had
skill to make proper observations, have
wanted opportunities ; and those who have
had opportunities, have wanted skill or
attention. I have therefore thought it
worth while to give a distinct account of
the observations necessary for the deter-
mination of this question, and what con-
clusions may be drawn from the facts ob-
served. I have likewise collected, and set
in one view, the most conclusive facts that
have occurred in authors, or have fallen
under my own observation.

It must be confessed that these facts,
when applied to the question in hand, make
a very poor figure ; and the gentlemen of
the medical faculty are called upon, for the
honour of their profession, and for the bene-
fit of mankind, to add to them.

All the medical, and all the optical writers
upon the strabismus that I have met with,
except Dr Jurin, either affirm, or take it
for granted, that squinting persons see the
object with both eyes, and yet see it single.
Dr Jurin affirms that squinting persons
never see the object with both eyes ; and
that, if they did, they would see it double.
If the common opinion be true, the cure of
a squint would be as pernicious to the sight
of the patient, as the causing of a perma-
nent squint would be to one who naturally
had no squint ; and, therefore, no physi-
cian ought to attempt such a cure, no
patient ought to submit to it. But, if Dr
.Turin's opinion be true, most young people
that squint may cure themselves, by taking
some pains ; and may not only remove the
deformity, but, at the same time, improve
their sight. If the common opinion be
true, the centres, and other points of the two
retina, in squinting persons, do not corre-
spond, as in other men, and Nature, in them,
deviates from her common rule. But, if
Dr Jurin's opinion be true, there is reason
to think that the same general law of vision
which we have found in perfect human eyes,
extends also to those which squint.

It is impossible to determine, by reason-
ing, which of these opinions is true; or
whether one may not be found true in some
patients, and the other in others. Here,
experience and observation are our only
guides ; and a deduction of instances is the
only rational argument. It might, there-
fore, have been exuected, that the patrons



of the contrary opinions should have given
instances in support of them that are clear
and indisputable ; but I have not found one
such instance on either side of the question,
in all the authors I have met with. I have
given three instances from my own observ-
ation, in confirmation of Dr Jurin's opinion,
which admit of no doubt ; and one which
leans rather to the other opinion, but is
dubious. And here I must leave the matter
to further observation.

In thel7th section, I have endeavoured to
shewthat the correspondence and [or] sym-
pathy of certain points of the two retina,
into which we have resolved all the phseno-
mena of single and double vision, is not, as
Dr Smith conceived, the effect of custom,
nor can [it] be changed by custom, but is a
natural and original property of human
eyes ; and, in the last section, that it is not
owing to an original and natural perception
of the true distance of objects from the eye,
as Dr Forterfield imagined. After this re-
capitulation, which is intended to relieve the
attention of the reader, shall we enter into
more theories upon this subject ?

That of Dr Briggs — first published in
English, in the " Philosophical Transac-
tions," afterwards in Latin, under the title
of *' Nova Visionis Theoria," with a prefa-
tory epistle of Sir Isaac Newton to the
author — amounts to this, That the fibres of
the optic nerves, passing from correspond-
ing points of the retina to the thalami ner-
vorum opticorum, having the same length,
the same tension, and a similar situation,
will have the same tone ; and, therefore,
their vibrations, excited by the impression
of the rays of light, will be like unisons in
music, and will present one and the same
image to the mind : but the fibres passing
from parts of the retinm which do not cor-
respond, having different tensions and tones,
will have discordant vibrations ; and, there-
fore, present different images to the mind.

I shall not enter upon a particular exam-
ination of this theory. It is enough to ob-
serve, in general, that it is a system of con-
jectures concerning things of which we are
entirely ignorant ; and that all such theories
in philosophy deserve rather to be laughed
at, than to be seriously refuted.

From the first dawn of philosophy to this
day, it hath been believed that the optic
nerves are intended to carry the images of
visible objects from the bottom of the eye to
the mind ; and that the nerves belonging to
the organs of the other senses have a like
office. * But how do we know this ? We
conjecture it ; and, taking this conjecture
for a truth, we consider how the nerves may
best answer this purpose. The system of
the nerves, for many ages, was taken to be a

• This statement is far too unqualified H'.

hydraulic engine, consisting of a bundle of
pipes, which carried to and fro a liquor called
animal spirits. About the time of Dr
Briggs, it was thought rather to be a stringed
instrument, composed of vibrating chords,
each of which had its proper tension and
tone. But some, with as great probability,
conceived it to be a wind instrument, which
played its part by the vibrations of an elastic
aether in the nervous fibrils.

These, I think, are all the engines into
which the nervous system hath been moulded
by philosophers, for conveying the images
of sensible things from the organ to the
srnsorium. And, for all that we know of
the matter, every man may freely choose
which he thinks fittest for the purpose ; for,
from fact and experiment, no one of them
can claim preference to another. Indeed,
they all seem so unhandy engines for carry-
ing images, that a man would be tempted to
invent a new one.

Since, therefore, a blind man may guess
as well in the dark as one that sees, I beg
leave to offer another conjecture touching
the nervous system, which, I hope, will
answer the purpose as well as those we have
mentioned, and which recommends itself by
its simplicity. Why may not the optic
nerves, for instance, be made up of empty
tubes, opening their mouths wide enough to
receive the rays of light which form the
image upon the retina, and gently convey-
ing them safe, and in their proper order, to
the very seat of the soul, until they flash in
her face ? It is easy for an ingenious phi-
losopher to fit the caliber of these empty
tubes to the diameter of the particles of
light, so as they shall receive no grosser
kind of matter ; and, if these rays should be
in danger of mistaking their way, an expe-
dient may also be found to prevent this ;
for it requires no more than to bestow upon
the tubes of the nervous system a peristal-
tic motion, like that of the alimentary tube.

It is a peculiar advantage of this hypo-
thesis, that, although all philosophers be-
lieve that the species or images of things
are conveyed by the nerves to the soul, yet
none of their hypotheses shew how this
may be done. For how can the images of
sound, taste, smell, colour, figure, and all
sensible qualities, be made out of the vibra-
tions of musical chords, or the undulations
of animal spirits, or of aether ? We ought
not to suppose means inadequate to the
end. Is it not as philosophical, and more
intelligible, to conceive, that, as the stomach
receives its food, so the soul receives her
images by a kind of nervous deglutition ?
I might add, that we need only continue
this peristaltic motion of the nervous tubes
from the sensorium to the extremities of the
nerves that serve the muscles, in order to
account for muscular motion.

n 3



Thus Nature will be consonant to her-
self ; and, as sensation will be the convey-
ance of the ideal aliment to the mind, so
muscular motion will be the expulsion of
the recrementitious part of it. For who
can deny, that the images of things con-
veyed by sensation, may, after due con-
coction, become fit to be thrown off by
muscular motion ? I only give hints of
these things to the ingenious, hoping that in
time this hypothesis may be wrought up into
a system as truly philosophical as that of ani-
mal spirits, or the vibration of nervous fibres.

To be serious : In the operations of na-
ture, I hold the theories of a philosopher,
which are unsupported by fact, in the same
estimation with the dreams of a man asleep,
or the ravings of a madman. We laugh at
the Indian philosopher, who, to account
for the support of the earth, contrived the
hypothesis of a huge elephant, and, to
support the elephant, a huge tortoise.
If we will candidly confess the truth, we
know as little of the operation of the nerves,
as he did of the manner in which the earth
is supported ; and our hypotheses about
animal spirits, or about the tension and
vibrations of the nerves, are as like to be
true, as his about the support of the earth.
His elephant was a hypothesis, and our
hypotheses are elephants. Every theory
in philosophy, which is built on pure con-
jecture, is an elephant ; and every theory
that is supported partly by fact, and partly
by conjecture, is like Nebuchadnezzar's
image, whose feet were partly of iron and
partly of clay.

Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 41 of 114)