Thomas Reid.

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

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experience, is attained by idiots. We need
not know what muscles we are to contract,
and what we are to relax, in order to fit
the eye to a particular distance of the object.

But, although we are not conscious of the
motions we perform, in order to fit the eyes
to the distance of the object, we are aon-
scious of the effort employed in producing
these motions ; and probably have some
sensation which accompanies them, to which
we give as little attention as to other sensa-
tions. And thus, an effort consciously ex-
erted, or a sensation consequent upon that
effort, comes to be conjoined with the dis-
tance of the object which gave occasion to
it, and by this conjunction becomes a sign
of that distance. Some instances of this
will appear in considering the means or
signs by which we learn to see the distance
of objects from the eye. In the enumera-
tion of these, we agree with Dr Porterfield,
notwithstanding that distance from the eye,
in his opinion, is perceived originally, but,
in our opinion, by experience only.

In general, when a near object affects the
eye in one manner, and the same object,
placed at a greater distance, affects it in a
different manner, these various affections
of the eye become signs of the correspond-
ing distances. The means of perceiving
distance by the eye will therefore be ex-
plained by shewing in what various ways
objects affect the eye differently, according
to their proximity or distance.

1. It is well known, that, to see objects
distinctly at various distances, the form of
the eye must undergo some change : and
nature hath given us the power of adapting
it to near objects, by the contraction of
certain muscles, and to distant objects by
the contraction of other muscles. As to
the manner in which this is done, and the
muscular parts employed, anatomists do not
altogether agree. The ingenious Dr Jurin,
in his excellent essay on distinct and indis-
" tinct vision, seems to have given the most
probable account of this matter ; and to him
I refer the reader.*

But, whatever be the manner in which
this change of- the form of the eye is ef-
fected, it is certain that young people have
commonly the power of adapting their eyes

* The mole in which the eye-is accommodated to its
various perceptions, is a subject which has obtained
much attention from the more recent physiologists. —


to all distances of the object, from six or
seven inches, to fifteen or sixteen feet ; so
as to have perfect and distinct vision at any
distance within these limits. From this it
follows, that the effort we consciously em-
ploy to adapt the eye to any particular dis-
tance of objects within these limits, will be
connected and associated with that dis-
tance, and will become a sign of it. When
the object is removed beyond the farthest
limit of distinct vision, it will be seen in-
distinctly ; but, more or less so, according
as its distance is greater or less ; so that
the degrees of indistinctness of the object
may become the signs of distances consi-
derably beyond the farthest limit of distinct

If we had no other mean but this, of per-
ceiving distance of visible objects, the most
distant would not appear to be above twenty
or thirty feet from the eye, and the tops of
houses and trees would seem to touch the
clouds ; for, in that case, the signs of all
greater distances being the same, they have
the same signification, and give the same
perception of distance.

But it is of more importance to observe,
that, because the nearest limit of distinct
vision in the time of youth, when we learn
to perceive distance by the eye, is about
six or seven inches, no object seen dis-
tinctly ever appears to be nearer than six
or seven inches from the eye. We can,
by art, make a small object appear dis-
tinct, when it is in reality not above half
an inch from the eye ; either by using a
single microscope, or by looking through
a small pin-hole in a card. When, by
either of these means, an object is made
to appear distinct, however small its dis-
tance is in reality, it seems to be removed
at least to the distance of six or seven
inches— that is, within the limits of distinct

This observation is the more important,
because it affords the only reason we can
give why an object is magnified either by a
single microscope, or by being seen through
a pin-hole ; and the only mean by which
we can ascertain the degree in which the
object will be magnified by either. Thus,
if the object is really half an inch distant
from the eye, and appears to be seven inches
distant, its diameter will seem to be enlarged

in the same proportion as its distance that

is, fourteen times.

2. In order to direct both eyes to an
object, the optic axes must have a greater
or less inclination, according as the object
is nearer or more distant. And, although
we are not conscious of this inclination,
yet we are conscious of the effort employed
in it. By this mean we perceive small
distances more accurately than we could
do by the conformation of the eye only



And, therefore, we find, that those who have
lost the sight of one eye are apt, even
within arm's-length, to make mistakes in
the distance of objects, which are easily
avoided by those who see with both eyes.
Such mistakes are often discovered in snuff-
ing a candle, in threading a needle, or in
filling a tea-cup.*

When a picture is seen with both eyes,
and at no great distance, the representation
appears not so natural as when it is seen
only with one. The intention of painting
being to deceive the eye, and to make things
appear at different distances which in reality
are upon the same piece of canvass, this
deception is not so easily put upon both
eyes as upon one ; because we perceive the
distance of visible objects more exactly and
determinately with two eyes than with one.
If the shading and relief be executed in the
best manner, the picture may have almost
the same appearance to one eye as the
obj ects themselves would have ; but it cannot
have the same appearance to both. This is
not the fault of the artist, but an unavoid-
able imperfection in the art. And it is
owing to what we just now observed, that
the perception we have of the distance of
objects by one eye is more uncertain, and
more liable to deception, than that which
we have by both.

The great impediment, and I think the
only invincible impediment, to that agree-
able deception of the eye which the painter
aims at, is the perception which we have of
the distance of visible objects from the eye,
partly by means of the conformation of the
eye, but chiefly by means of the inclination
of the optic axes. If this perception could
be removed, I see no reason why a picture
might not be made so perfect as to deceive
the eye in reality, and to be mistaken for
the original object. Therefore, in order to
judge of the merit of a picture, we ought,
as much as possible, to exclude these two
means of perceiving the distance of the
several parts of it.

In order to remove this perception of dis-
tance, the connoisseurs in painting use a
method which is very proper. They look
at the picture with one eye, through a tube
which excludes the view of all other objects.
By this method, the principal mean whereby
we perceive the distance of the object — to
wit, the inclination of the optic axes — is en-
tirely excluded. I would humbly propose,
as an improvement of this method of view-
ing pictures, that the aperture of the tube
next to the eye should be very small. If it is
as small as a pin-hole, so much the better,
providing there be light enough to see the
picture clearly. The reason of this proposal

* The same remark i3 made by many optical wri-
ters, old and new. — H,

is, that, when we look at an object through
a small aperture, it will be seen distinctly,
whether the conformation of the eye be
adapted to its distance or not ; and we have
no mean left to judge of the distance, but
the light and colouring, which are in the
painter's power. If, therefore, the artist
performs his part properly, the picture will
by this method affect the eye in the same
manner that the object represented would
do ; which is the perfection of this art.

Although this second mean of perceiving
the distance of visible objects be more de-
terminate and exact than the first, yet it
hath its limits, beyond which it can be 01
no use. For when the optic axes directed
to an object are so nearly parallel that, in
directing them to an object yet more distant,
we are not conscious of any new effort, nor
have any different sensation, there our per-
ception of distance stops ; and, as all more
distant objects affect the eye in the same
manner, we perceive them to be at the
same distance. This is the reason why the
sun, moon, planets, and fixed stars, when
seen not near the horizon, appear to be all
at the same' distance, as if they touched the
concave surface of a great sphere. The
surface of this celestial sphere is at that
distance beyond which all objects affect
the eye in the same manner. Why thif
celestial vault appears more distant towards
the horizon, than towards the zenith, will
afterwards appear.

3. The colours of objects, according as
they are more distant, become more faint
and languid, and are tinged more with the
azure of the intervening atmosphere : to
this we may add, that their minute parts
become more indistinct, and their outline
less accurately defined. It is by these
means chiefly, that painters can represent
objects at very different distances, upon the
same canvass. And the diminution of the
magnitude of an object would not have the
effect of making it appear to be at a great
distance, without this degradation of colour,
and indistinctness of the outline, and of the
minute parts. If a painter should make a
human figure ten times less than other
human figures that are in the same piece,
having the colours as bright, and the out-
line and minute parts as accurately defined,
it would not have the appearance of a man
at a great distance, hut of a pigmy or Lilli-

When an object hath a known variety of
colours, its distance is more clearly indi-
cated by the gradual dilution of the colours
into one another, than when it is of one
uniform colour. In the steeple which
stands before me at a small distance, the
joinings of the stones are clearly percepti-
ble ; the grey colour of the stone, and the
white cement are distinctly limited : when



I see it at a greater distance, the joinings

of the stones are less distinct, and the colours
of the stone and of the cement begin to
dilute into one another : at a distance still
greater, the joinings disappear altogether,
and the variety of colour vanishes.

In an apple-tree which stands at the dis-
tance of about twelve feet, covered with
flowers, I can perceive the figure and the
colour of the leaves and petals ; pieces of
branches, some larger, others smaller, peep-
ing through the intervals of the leaves —
some of them enlightened by the sun's rays,
others shaded ; and some openings of the
sky are perceived through the whole. When
I gradually remove from this tree, the ap-
pearance, even as to colour, changes every
minute. First, the smaller parts, then the
larger, are gradually confounded and mixed.
The colours of leaves, petals, branches,
and sky, are gradually diluted into each
other, and the colour of the whole becomes
more and more uniform. This change of
appearance, corresponding to the several dis-
tances, marks the distance more exactly than
if the whole object had been of one colour.

Dr Smith, in his " Optics," gives us a very
curious observation made by Bishop Berke-
ley, in his travels through Italy and Sicily.
He observed, That, in those countries,
cities and palaces seen at a great distance
appeared nearer to him by several miles
than they really were : and he very judi-
ciously imputed it to this cause, That the
purity of the Italian and Sicilian air, gave
to very distant objects that degree of
brightness and distinctness which, in the
grosser air of his own country, was to be
seen only in those that are near. The
purity of the Italian air hath been assigned
as the reason why the Italian painters
commonly give a more lively colour to the
sky than the Flemish. Ought they not,
for the same reason, to give less degrad-
ation of the colours, and less indistinct-
ness of the minute parts, in the representa-
tion of very distant objects ?

It is very certain that, as in air uncom-
monly pure, we are apt to think visible
objects nearer and less than' they really
are, so, in air uncommonly foggy, we are
apt to think them more distant and larger
than the truth. Walking by the sea-side
in a thick fog, I see an object which seems
to me to be a man on horseback, and at
the distance of about half a mile. My com-
panion, who has better eyes, or is more
accustomed to see such objects in such cir-
cumstances, assures me that it is a sea-
gull, and not a man on horseback. Upon
a second view, I immediately assent to his
opinion ; and now it appears to me to be a
sea -gull, and at the distance only of seventy
or eighty yards. The mistake made on this
occasion, and the correction of it, are both

so sudden, that we are at a loss whether
to call them by the name of judgment, or
by that of simple perception.

It is not worth while to dispute about
names 5 but it is evident that my belief,
both first and last, was produced rather by
signs than by arguments, and that the
mind proceeded to the conclusion in both
cases by habit, and not by ratiocination.
And the process of the mind seems to have
been this — First, Not knowing, or not
minding, the effect of a foggy air on the vis-
ible appearance of objects, the object seems
to me to have that degradation of colour,
and that indistinctness of the outline, which
objects have at the distance of half a mile ;
therefore, from the visible appearance as a
sign, I immediately proceed to the belief
that the object is half a mile distant.
Then, this distance, together with the vis-
ible magnitude, signify to me the real
magnitude, which, supposing the distance
to be half a mile, must be equal to that
of a man on horseback ; and the figure,
considering the indistinctness of the outline,
agrees with that of a man on horseback.
Thus the deception is brought about. But
when I am assured that it is a sea-gull, the
real magnitude of a sea-gull, together with
the visible magnitude presented to the eye,
immediately suggest the distance, which,
in this case, cannot be above seventy or
eighty yards : the indistinctness of the
figure likewise suggests the fogginess of the
air as its cause ; and now the whole chain
of signs, and things signified, seems stronger
and better connected than it was before ;
the half mile vanishes to eighty yards;
the man on horseback dwindles to a sea-
gull ; I get a new perception, and wonder
how I got the former, or what is become of
it ; for it is now so entirely gone, that I
cannot recover it.

It ought to be observed that, in order to
produce such deceptions from the clearness
or fogginess of the air, it must be uncom-
monly clear or uncommonly foggy ; for we
learn, from experience, to make allowance
for that variety of constitutions of the air
which we have been accustomed to observe,
and of which we are aware. Bishop
Berkeley therefore committed a mistake,
when he attributed the large appearance of
the horizontal moon to the faintness of her
light, occasioned by its passing through a
larger tract of atmosphere :* for we are so
much accustomed to see the moon in all
degrees of faintness and brightness, from
the greatest to the least, that we learn to
make allowance for it ; and do not imagine
her magnitude increased by the faintness of
her appearance. Besides, it is certain that
the horizontal moon seen through a tube

* Thia explanation was not original to Berkeley. — II



which cuts off the view of the interjacent
ground, and of all terrestrial objects, loses
all that unusual appearance of magnitude.

4. We frequently perceive the distance
of objects, by means of intervening or con-
tiguous objects, whose distance or magni-
tude is otherwise known. When I perceive
certain fields or tracts of ground to he be-
tween me and an object, it is evident that
these may become signs of its distance.
And although we have no particular in-
formation of the dimensions of such fields
or tracts, yet their similitude to others which
we know, suggests their dimensions.

We are so much accustomed to measure
with our eye the ground which we travel,
and to compare the judgments of distances
formed by sight, with our experience or in-
formation, that we learn by degrees, in this
way, to form a more accurate judgment of
the distance of terrestrial objects, than we
could do by any of the means before men-
tioned. An object placed upon the top of
a high building, appears much less than
when placed upon the ground, at the same
distance. When it stands upon the ground,
the intervening tract of ground serves as a
sign of its distance ; and the distance, to-
gether with the visible magnitude, serves
;is a sign of its real magnitude. But when
the object is placed on high, this sign of its
distance is taken away : the remaining
signs lead us to place it at a less distance ;
and this less distance, together with the
visible magnitude, becomes a sign of a less
real magnitude.

The two first means we have mentioned,
would never of themselves make a visible
object appear above a hundred and fifty,
or two hundred feet, distant ; because, be-
yond that there is no sensible change, either
of the conformation of the eyes, or of the
inclination of their axes. The third mean
is but a vague and undeterminate sign,
when applied to distances above two or three
hundred feet, unless we know the real colour
and figure of the object ; and the fifth
mean, to be afterwards mentioned, can
only be applied to objects which are fami-
liar, or whose real magnitude is known.
Hence it follows, that, when unknown ob-
jects, upon or near the surface of the earth,
are perceived to be at the distance of some
miles, it is always by this fourth mean that
we are led to that conclusion.

Dr Smith hath observed, very justly, that
the known distance of the terrestrial objects
which terminate our view, makes that part
of the Bky which is towards the horizon
uppear more distant than that which is to-
wards the zenith. Hence it comes to pass,
that the apparent figure of the sky is not
that of a hemisphere, but rather a less seg-
ment of a sphere. And, hence, likewise,
it comes to pass, that the diameter of the

sun or moon, or the distance between two
fixed stars, seen contiguous to a hill, or to
any distant terrestrial object, appears much
greater than when no such object strikes
the eye at the same time.

These observations have been sufficiently
explained and confirmed by Dr Smith. I
beg leave to add, that, when the visible
horizon is terminated by very distant ob-
jects, the celestial vault seems to be en-
larged in all its dimensions. When I view
it from a confined street or lane, it bears
some proportion to the buildings that sur-
round me ; but, when I view it from a large
plain, terminated on all hands by hills which
rise one above another to the distance of
twenty miles from the eye, methmks I see
a new heaven, whose magnificence declares
the greatness of its Author, and puts every
human edifice out of countenance ; for now
the lofty spires and the gorgeous palaces
shrink into nothing before it, and bear no
more proportion to the celestial dome than
their makers bear to its Maker.

5. There remains another mean by which
we perceive the distance of visible objects —
and that is, the diminution of their visible
or apparent magnitude. By experience, I
know what figure a man, or any other known
object, makes to my eye at the distance of
ten feet — I perceive the gradual and pro-
portional diminution of this visible figure, at
the distance of twenty, forty, a hundred
feet, and at greater distances, until it vanish
altogether. Hence a certain visible magni-
tude of a known object becomes the sign of
a certain determinate distance, and carries
along with it the conception and belief of
that distance.

In this process of the mind, the sign is
not a sensation ; it is an original percep-
tion. We perceive the visible figure and
visible magnitude of the object, by the ori-
ginal powers of vision ; but the visible figure
is used only as a sign of the real figure, and
the visible magnitude is used only as a sign
either of the distance, or of the real magni-
tude, of the object ; and, therefore, these
original perceptions, like other mere signs,
pass through the mind without any atten-
tion or reflection.

This last mean of perceiving the dis-
tance of known objects, serves to explain
some very remarkable phenomena in op-
tics, which would otherwise appear very
mysterious. When we view objects of
known dimensions through optical glasses,
there is no other mean left of determining
their distance, but this fifth. Hence it
follows, that known objects seen through
glasses, must seem to be brought nearer, in
proportion to the magnifying power of the
glass, or to be removed to a greater distance,
in proportion to the diminishing power of
the glass.



If a man who had never before seen ob-
jects through a telescope, were told that
the telescope, which he is about to use, mag-
nifies the diameter of the object ten times ;
when he looks through this telescope at a
man six feet high, what would he expect
to see ? Surely he would very naturally
expect to see a giant sixty feet high. But
he sees no such thing. The man appears
no more than six feet high, and conse-
quently no bigger than he really is ; but he
appears ten times nearer than he is. The
telescope indeed magnifies the image of
this man upon the retina ten times in dia-
meter, and must, therefore, magnify his
visible figure in the same proportion ; and,
as we have been accustomed to see him of
this visible magnitude when he was ten
times nearer than he is presently,* and in
no other case, this visible magnitude, there-
fore, suggests the conception and belief of
that distance of the object with which it
hath been always connected. We have
been accustomed to conceive this amplifi-
cation of the visible figure of a known ob-
ject, only as the effect or sign of its being
brought nearer : and we have annexed a
certain determinate distance to every de-
gree of visible magnitude of the object ;
and, therefore, any particular degree of vi-
sible magnitude, whether seen by the naked
eye or by glasses, brings along with it the
conception and belief of the distance which
corresponds to it. This is the reason
why a telescope seems not to magnify known
objects, but to bring them nearer to the

When we look through a pin-hole, or a
single microscope, at an object which is
half an inch from the eye, the picture of
the object upon the retina is not enlarged,
but only rendered distinct ; neither is the
visible figure enlarged : yet the object ap-
pears to the eye twelve or fourteen times
more distant, and as many times larger in
diameter, than it really is. Such a tele-
scope as we have mentioned amplifies the
image on the retina, and the visible figure
of the object, ten times in diameter, and yet
makes it seem no bigger, but only ten times
nearer. These appearances had been long
observed by the writers on optics ; they tor-
tured their invention to find the causes of
them from optical principles ; but in vain :
they must be resolved into habits of percep-
tion, which are acquired by custom, but
are apt to be mistaken for original percep-
tions. The Bishop of Cloyne first furnished
the world with the proper key for opening
up these mysterious appearances ; but he
made considerable mistakes in the applica-
tion of it. Dr Smith, in his elaborate and ju-
dicious treatise of " Optics," hath applied it

* Sec note * p. 96, a.— H.

to the apparent distance of objects seen with
glasses, and to the apparent figure of the
heavens, with such happy success, that there
can be no more doubt about the causes of
these phenomena.

Section XXIII.


The distance of objects from the eye is
the most important lesson in vision. Many

Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 44 of 114)