Thomas Reid.

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discovered tno curious fact of this local insensibility.

knowledge, therefore, that the retina is not
the last and most immediate instrument of
the mind in vision. There are other mate-
rial organs, whose operation is necessary to
seeing, even after the pictures upon the
retina are formed. If ever we come to
know the structure and use of the choroid
membrane, the optic nerve, and the brain,
and what impressions are made upon them
by means of the pictures on the retina,
some more links of the chain may be brought
within our view, and a more general law
of vision discovered ; but, while we know
so little of the nature and office of these
more immediate instruments of vision, it
seems to be impossible to trace its laws be-
yond the pictures upon the retina.

Neither do I pretend to say, that there
may not be diseases of the eye, or accidents,
which may occasion our seeing objects in a
direction somewhat different from that men-
tioned above. I shall beg leave to mention
one instance of this kind that concerns my-

In May 1761, being occupied in making
an exact meridian, in order to observe the
transit of Venus, I rashly directed to the
sun, by my right eye, the cross hairs of a
small telescope. I had often done the like
in my younger days with impunity ; but I
suffered by it at last, which I mention as a
warning to others.

I soon observed a remarkable dimness in
that eye ; and for many weeks, when I was
in the dark, or shut my eyes, there ap-
peared before the right eye a lucid spot,
which trembled much like the image of the
sun seen by reflection from water. This
appearance grew fainter, and less frequent,
by degrees ; so that now there are seldom
any remains of it. But some other very
sensible effects of this hurt still remain.
For, First, The sight of the right eye con-
tinues to be more dim than that of the left.
Secondly, The nearest limit of distinct
vision is more remote in the right eye than
in the other; although, before the time
mentioned, they were equal in both these
respects, as I had found by many trials.
But, thirdly, what I chiefly intended to
mention is, That a straight line, in some
circumstances, appears to the right eye to
have a curvature in it. Thus, when I look
upon a music book, and, shutting my left
eye, direct the right to a point of the mid-

and who ingeniously employed it in support of bit
opinion, that the choroid, not the retina, is the
proximate organ in vision. But not only is the ab-
sence of the choroid not to be viewed as the cause ol
this phenomenon : it is not even to be attributed to
the entrance of the optic nerve. For it is proved
that the impassive portion of the retina does not
occupy above a third part of the disc, corresponding
o the circumference of that nerve ; and the conjec-
'"1? ?f Rudolph, seems probable, that the insensi-
bility islirmted to the spot where the arteria centralit
enters.— H,



die line of the five which compose the staff
of music, the middle line appears dim, in-
deed, at the point to which the eye is di-
rected, hut straight ; at the same time, the
two lines above it, and the two below it,
appear to be bent outwards, and to be more
distant from each other and from the middle
line, than at other parts of the staff, to
which the eye is not directed. Fourthly,
Although I have repeated this experiment
times innumerable, within these sixteen
months, I do not find that custom and ex-
perience takes away this appearance of cur-
vature in straight lines. Lastly, This ap-
pearance of curvature is perceptible when
I look with the right eye only, but not when
I look with both eyes; yet I see better
with both eyes together, than even with
the left eye alone.

I have related this fact minutely as it is,
without regard to any hypothesis ; because
I think such uncommon facts deserve to be
recorded. I shall leave it to others to con-
jecture the cause of this appearance. To
me it seems most probable, that a small
part of the retina towards the centre is
shrunk, and that thereby the contiguous
parts are drawn nearer to the centre, and
to one another, than they were before ; and
that objects, whose images fall on these
parts, appear at that distance from each
other which corresponds, not to the interval
of the parts in their present preternatural
contraction, but to their interval in their
natural and sound, state.

Section XIII.


Another phsenomenon of vision which
deserves attention, is our seeing objects
single with two eyes.* There are two pic-

* The opinions relative to single vision with two
eyes, may, I think,be reduced to two supreme classes.
The one attempts to shew that there is no difficulty
10 be solved ; the other attempts to solve the difficulty
which is admitted. — Under the former class, there
are, as L recollect, three hypotheses. The Jirst sup.
poses that we see only with one eye — that man is in
reality a Cyclops ; the second supposes that the two
impressions are not, in fact, made at the same instant
In both eyes, and, consequently, that two simulta-
neous impressions are not conveyed to the brain and
mind ; the third supposes that, although a separate
impression be made on each retina, yet that these
several impressions are, as it were, fused into one
before they reach the common sensory, in conse.
quence of a union of the optic nerves. — The hypo-
theses of the latter class which, I think, may also be
reduced to three, all admit that there are simultaneous
impressions on the two retimB, and that these im-
pressions are separately conveyed to the termination
of the organic apparatus ; but still hold that, in the
mind, there is determined only a single perception.
One opinion allows the perception to have been origi-
nally twofold, and saves the phenomenon, by suppos-
ing that it became single through the influence of cus-
tom and association. Another explains it more sub-
jectively, by an ultimate and inexplicable law of our

tures of the object, one on each retina ,
and each picture by itself makes us see an
object in a certain direction from the eye ;
yet both together commonly make us see
only one object. All the accounts or solu-
tions of this phsenomenon given by anato-
mists and philosophers seem to be unsatisfac-
tory. I shall pass over the opinions of Galen,
of Gassendus, of Baptista Porta, and of Ro-
hault. The reader may see these examined
and refuted by Dr Porterfield. I shall ex-
amine Dr Porterfield's own opinion, Bishop
Berkeley's, and some others. But it will be
necessary first to ascertain the facts : for, if
we mistake the phsenomena of single and
double vision, it is ten to one but this mis-
take will lead us wrong in assigning the
causes. This likewise we ought carefully to
attend to, which is acknowledged in theory
by all who have any true judgment or just
taste in inquiries of this nature, but is very
often overlooked in practice — namely, that,
in the solution of natural phsenomena, all
the length that the human faculties can
carry us, is only this, that, from particular
phsenomena, we may, by induction, trace
out general phsenomena, of which all the
particular ones are necessary consequences.
And when we have arrived at the most
general phsenomena we can reach, there
we must stop. If it is asked, Why such a
body gravitates towards the earth ? all the
answer that can be given is, Because all
bodies gravitate towards the earth. This
is resolving a particular phsenomenon into
a general one. If it should again be asked,
Why do all bodies gravitate towards the
earth ? we can give no other solution of this
phsenomenon, but that all bodies whatso-
ever gravitate towards each other. This
is resolving a general phsenomenon into a
more general one. If it should be asked,
Why all bodies gravitate to one another ? we
cannot tell ; but, if we could tell, it could
only be by resolving this universal gravita-
tion of bodies into some other phaenomenon
still more general, and of which the gravi-
tation of all bodies is a particular instance.
The most general phsenomena we can reach,
are what we call laws of nature ; so that the
laws of nature are nothing else but the most
general facts relating to the operations of
nature, which include a great many parti-
cular facts under them. And if, in any case,
we should give the name of a law of nature
to a general phsenomenon, which human
industry shall afterwards trace to one more
general, there is no great harm done. The
most general assumes the name of a law of
nature when it is discovered, and the less
general is contained and comprehended in
it. Having premised these things, we pro-
ceed to consider the phsenomena of single

constitution j and the last, more'objectisely, on some
intelligible principle of optics.— H.




and double vision, in order to discover some
general principle to which they all lead, and
of which they are the necessary conse-
quences. If we can discover any such
general principle, it must either be a law of
nature, or the necessary consequence of
some law of nature ; and its authority will
be equal whether it is the first or the last.

1. We find that, when the eyes are sound
and perfect, and the axes of both directed
to one point, an object placed in that point is
seen single — and here we observe, that in
this case the two pictures which shew the
object single, are in the centres of the
retina. When two pictures of a small
object are formed upon points of the retina,
if they shew the object single, we shall, for
the sake of perspicuity, call such two points
of the retina, corresponding points ; and
where the object is seen double, we shall
call the points of the retina on which the
pictures are formed, points that do not cor-
respond* Now, in this first phsenomenon,
it is evident, that the two centres of the
retina are corresponding points.

2. Supposing the same things as in the
last phsenomenon, other objects at the same
distance from the eyes as that to which
their axes are directed, do also appear
single. Thus, if I direct my eyes to a
candle placed at the distance of ten feet,
and, while I look at this caudle, another
stands at the same distance from my eyes,
within the field of vision, I can, while I
look at the first candle, attend to the ap-
pearance which the second makes to the
eye ; and I find that in this case it always
appears single. It is here to be observed,
that the pictures of the second candle do
not fall upon the centres of the retina, but
they both fall upon the same side of the
centres — that is, both to the right, or both
to the left ; and both are at the same dis-
tance from the centres. This might easily
be demonstrated from the principles of
optics. Hence it appears, that in this
second phsenomenon of single vision, the
corresponding points are points of the two
retinae, which are similarly situate with
respect to the two centres, being both upon
the same side of the centre, and at the same
distance from it. It appears likewise, from
this phsenomenon, that every point in one
retina corresponds with that which is simi-
larly situate in the other.

• It is to be noticed that Reid uses the terms, cor.
responding points in a sense opposite to that of
Smith, ana someoptical writers; thpy use it anatomi.
cally, he physiologically. Two points are anatomi.
cally correspondent, when on opposite sides of the
body they severally hold the same relation to the
centre. J. Mueller, and other recent physiologists,
employ these terms in the same signification as Reid.
An argument a priori has been employed against
the doctrine here maintained, on the ground that
the congruent points in the opposite eyes are not
snatomically corresponding points.— H.

3^ Supposing still the same things, ob.
jects which are much nearer to the eyes, or
much more distant from them, than that
to which the two eyes are directed, appear
double. Thus, if the candle is placed at
the distance of ten feet, and I hold my finger
at arms-length between my eyes and the can-
dle — when I look at the candle, I see my fin-
ger double ; and when I look at my finger,
1 see the candle double ; and the same thing
happens with regard to all other objects at
like distances which fall within the sphere
of vision. In this phsenomenon, it is evi-
dent to those who understand the prin-
ciples of optics, that the pictures of the ob-
jects which are seen double, do not fall upon
points of the retina which are similarly sit-
uate, but that the pictures of the objects
seen single do fall upon points similarly
situate. Whence we infer, that, as the points
of the two retina, which are similarly situate
with regard to the centres, do correspond,
so those which are dissimilarly situate do
not correspond.

4. It is to be observed, that, although, in
such cases as are mentioned in the last
phsenomenon, we have been accustomed
from infancy to see objects double which
we know to be single ; yet custom, and ex-
perience of the unity of the object, never
take away this appearance of duplicity.

5. It may, however, be remarked that
the custom of attending to visible appear-
ances has a considerable effect, and makes
the phsenomenon of double vision to be more
or less observed and remembered. Thus
you may find a man that can say, with a
good conscience, that he never saw things
double all his life ; yet this very man, put
in the situation above mentioned, with his
finger between him and the candle, and de-
sired to attend to the appearance of the
object which he does not look at, will, upon
the first trial, see the candle double, when
he looks at his finger ; and his finger double,
when he looks at the candle. Does he now
see otherwise than he saw before ? No,
surely; but he now attends to what he
never attended to before. The same double
appearance of an object hath been a thou-
sand times presented to his eye before now,
but he did not attend to it ; and so it is as
little an object of his reflection and memory,
as if it had never happened.

When we look at an object, the circum-
jacent objects may be seen at the same
time, although more obscurely and indis-
tinctly: for the eye hath a considerable
field of vision, which it takes in at once.
But we attend only to the object we look at.
The other objects which fall within the field
of vision, are not attended to ; and therefore
are as if they were not seen. If any of
them draws our attention, it naturally draws
the eyes at the same time : for, in the com-



mon course of life, the eyes always follow
the attention : or if at any time, in a revery,
they are separated from it, we hardly at
that time see what is directly before us.
Hence we may see the reason why the man
we are speaking of thinks that he never
before saw an object double. When he
looks at any object, he sees it single, and
takes no notice of other visible objects at
that time, whether they appear single or
double. If any of them draws his attention,
it draws his eyes at the same time ; and, as
soon as the eyes are turned towards it, it
appears single. But, in order to see things
double — at least, in order to have any reflec-
tion or remembrance that he did so — it is
necessary that he should look at one object,
and at the same time attend to the faint
appearance of other objects which are within
the field of vision. This is a practice which
perhaps he never used, nor attempted ; and
therefore he does not recollect that ever he
saw an object double. But when he is put
upon giving this attention, he immediately
sees objects double, in the same manner, and
with the very same circumstances, as they
who have been accustomed, for the greatest
part of their lives, to give this attention.

There are many phsenomena of a similar
nature, which shew that the mind may not
attend to, and thereby, in some sort, not
perceive objects that strike the senses. I
had occasion to mention several instances
of this in the second chapter ; and I have
been assured, by persons of the best skill in
music, that, in hearing a tune upon the
harpsichord, when they give attention to
the treble, they do not hear the bass ; and
when they attend to the bass, they do not
perceive the air of the treble. Some per-
sons are so near-sighted, that, in reading,
they hold the book to one eye, while the
other is directed to other objects. Such
persons acquire the habit of attending, in
this case, to the objects of one eye, while
they give no attention to those of the other.

6. It is observable, that, in all cases
wherein we see an object double, the two
appearances have a certain position with
regard to one another, and a certain appar-
ent or angular distance. This apparent
distance is greater or less in different cir-
cumstances ; but, in the same circumstances,
it is always the same, not only to the same,
but to different persons.

Thus, in the experiment above mentioned,
if twenty different persons, who see perfectly
with both eyes, shall place their finger and
the candle at the distances above expressed,
and hold their heads upright, looking at the
finger, they will see two candles, one on the
right, another on the left. That which is
seen on the right, is seen by the right eye,
and that which is seen on the left, by the
left eye ; and they will see them at the same

apparent distance from each other. If,
again, they look at the candle, they will
see two fingers, one on the right, and the
other on the left ; and all will see them at
the same apparent distance ; the finger
towards the left being seen by the right eye,
and the other by the left. If the head is
laid horizontally to one side, other circum-
stances remaining the same, one appearance
of the object seen double, will be directly
above the other. In a word, vary the cir-
cumstances as you please, and the appear-
ances are varied to all the spectators in one
and the same manner.

7. Having made many experiments in
order to ascertain the apparent distance of
the two appearances of an object seen double,
I have found that in all cases this apparent
distance is proportioned to the distance be-
tween the point of the retina, where the
picture is made in one eye, and the point
which is situated similarly to that on which
the picture is made on the other eye ; so
that, as the apparent distance of two objects
seen with one eye, is proportioned to the
arch of the retina, which lies between their
pictures, in like manner, when an object is
seen double with the two eyes, the apparent
distance of the two appearances is propor-
tioned to the arch of either retina, which
lies between the picture in that retina, and
the point corresponding to that of the pic-
ture in the other retina.

8. As, in certain circumstances, we in-
variably see one object appear double, so,
in others, we as invariably see two objects
unite into one, and, in appearance, lose
their duplicity. This is evident in the ap-
pearance of the binocular telescope. And
the same thing happens when any two simi-
lar tubes are applied to the two eyes in a,
parallel direction ; for, in this case, we see
only one tube. And if two shillings are
placed at the extremities of the two tubes,
one exactly in the axis of one eye, and the
other in the axis of the other eye, we shall
see but one shilling. If two pieces of coin,
or other bodies, of different colour, and of
different figure, be properly placed in the
two axes of the eyes, and at the extremi-
ties of the tubes, we shall see both the
bodies in one and the same place, each' as
it were spread over the other, withouthid-
ing it j and the colour will be that which is
compounded of the two colours.*

* Thia last statement is incorrect; it misrepresents*
if it does not reverse, the observation of Du Tour-
But, though Reid's assertion be inaccurate, there is
great difference (probably from the different consti.
tution ot their organs) in the phattiomeno-, as re-
ported by various observers. None, seemingly,
(the reverse of what Reid says,) in looking, e. g.,
with one eye through a blue, and with the other
through a yellow glass, experience a comple-
mentary sensation of green. But some see both
colours at once; some only one colour— a colour,
however, which corresponds neither to yellow nor tc
blue, and, at the same time, is not gieen. Jn rr.y



9. From these pheenomena, and from all
the trials I have been able to make, it ap-
pears evidently, that, in perfect human eyes,
the centres of the two retina correspond and
harmonize wth one another, and that every
other point in one retina doth correspond
and harmonize with the point which is
similarly situate in the other ; in such man-
ner, that pictures falling on the corre-
sponding points of the two retina, shew
only one object, even when there are really
two ; and pictures falling upon points of
the retina which do not correspond, shew
us two visible appearances, although there
be but one object : so that pictures, upon
corresponding points of the two retinas, pre-
sent the same appearance to the mind as
if they had both fallen upon the same point
of one retina ; and pictures upon points of
the two retina, which do not correspond,
present to the mind the same apparent
distance and position of two objects, as if
one of those pictures was carried to the
point corresponding to it in the other retina.
This relation and sympathy between cor-
responding points of the two retina, I do
not advance as an hypothesis, but as a
general fact or phcenomenon of vision. All
the pheenomena before mentioned, of single
or double vision, lead to it, and are neces-
sary consequences of it. It holds true in-
variably in all perfect human eyes, as far
as I am able to collect from innumerable
trials of various kinds made upon my own
eyes, and many made by others at my de-
sire. Most of the hypotheses that have
been contrived to resolve the pheenomena
of single and double vision, suppose this
general fact, while their authors were not
aware of it. Sir Isaac Newton, who was
too judicious a philosopher, and too accu-
rate an observer, to have offered even a
conjecture which did not tally with the facts
that had fallen under his observation, pro-
poses a query with respect to the cause of
it — " Optics," Query, 15. The judicious
Dr Smith, in his " Optics," Book 1, § 137,
hath confirmed the truth of this general
phenomenon from his own experience, not
only as to the apparent unity of objects
whose pictures fall upon the corresponding
points of the retina, but also as to the ap-
parent distance of the two appearances of
the same object when seen double.*

own eye, I can see either of these phenomena,
under certain conditions, at will. Johannes Mueller,
Weber, Volkmann, and Heermann, are the most
recent observers. 1 may also notice, that the
congruence between the corresponding points (in
Reid's sense) of the two retinje, is admitted for the
perception of figure, but not for the sensations of
tight and colour. — H,

» It might be proper here to say something of the
strictures of Dr Wells on Reid's doctrine of single
vision ; but, as the matter is, after all, of no high
psychological. importance, while the whole theory of
the form of the Horopter is, in consequence of
Mueller's observations, anew under discussion, I shall

This general phsenomenon appears, there-
fore, to be founded upon a very full induc-
tion, which is all the evidence we can have
for a fact of this nature. Before we make
an end of this subject, it will be proper to
inquire, First, Whether those animals whose
eyes have an adverse position in their heads,
and look contrary ways, have such corre-
sponding points in their retinae? Secondly,
What is the position of the corresponding
points in imperfect human eyes — I mean in
those that squint ? And, in the last place,
Whether this harmony of the correspond-
ing points in the retinae, be natural and
original, or the effect of custom ? And, if
it is original, Whether it can be accounted
for by any of the laws of nature already
discovered ? or whether it is itself to be
looked upon as a law of nature, and a part
of the human constitution ?

Section XIV.


It is the intention of nature,in giving eyes
to animals, that they may perceive the
situation of visible objects, or the direction
in which they are placed — it is probable,
therefore, that, in ordinary cases, every

Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 38 of 114)