An Essay Towards a New Theory of Vision

G >> George Berkeley >> An Essay Towards a New Theory of Vision

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56. Now in order to discover by what means the magnitude of tangible
objects is perceived by sight. I need only reflect on what passes in my
own mind, and observe what those things be which introduce the ideas of
greater or lesser into my thoughts, when I look on any object. And these
I find to be, FIRST, the magnitude or extension of the visible object,
which being immediately perceived by sight, is connected with that other
which is tangible and placed at a distance. SECONDLY, the confusion or
distinctness. And thirdly, the vigorousness or faintness of the aforesaid
visible appearance. CETERIS PARIBUS, by how much the greater or lesser
the visible object is, by so much the greater or lesser do I conclude the
tangible object to be. But, be the idea immediately perceived by sight
never so large, yet if it be withal confused, I judge the magnitude of
the thing to be but small. If it be distinct and clear, I judge it
greater. And if it be faint, I apprehend it to be yet greater. What is
here meant by confusion and faintness hath been explained in sect. 35.

57. Moreover the judgments we make of greatness do, in like manner as
those of distance, depend on the disposition of the eye, also on the
figure, number, and situation of objects and other circumstances that
have been observed to attend great or small tangible magnitudes. Thus,
for instance, the very same quantity of visible extension, which in the
figure of a tower doth suggest the idea of great magnitude, shall in the
figure of a man suggest the idea of much smaller magnitude. That this is
owing to the experience we have had of the usual bigness of a tower and a
man no one, I suppose, need be told.

58. It is also evident that confusion or faintness have no more a
necessary connexion with little or great magnitude than they have with
little or great distance. As they suggest the latter, so they suggest the
former to our minds. And by consequence, if it were not for experience,
we should no more judge a faint or confused appearance to be connected
with great or little magnitude, than we should that it was connected with
great or little distance.

59. Nor will it be found that great or small visible magnitude hath any
necessary relation to great or small tangible magnitude: so that the one
may certainly be inferred from the other. But before we come to the proof
of this, it is fit we consider the difference there is betwixt the
extension and figure which is the proper object of touch, and that other
which is termed visible; and how the former is principally, though not
immediately taken notice of, when we look at any object. This has been
before mentioned, but we shall here inquire into the cause thereof. We
regard the objects that environ us in proportion as they are adapted to
benefit or injure our own bodies, and thereby produce in our minds the
sensation of pleasure or pain. Now bodies operating on our organs, by an
immediate application, and the hurt or advantage arising therefrom,
depending altogether on the tangible, and not at all on the visible,
qualities of any object: this is a plain reason why those should be
regarded by us much more than these: and for this end the visive sense
seems to have been bestowed on animals, to wit, that by the perception of
visible ideas (which in themselves are not capable of affecting or any
wise altering the frame of their bodies) they may be able to foresee
(from the experience they have had what tangible ideas are connected with
such and such visible ideas) and damage or benefit which is like to
ensue, upon the application of their own bodies to this or that body
which is at a distance. Which foresight, how necessary it is to the
preservation of an animal, everyone's experience can inform him. Hence it
is that when we look at an object, the tangible figure and extension
thereof are principally attended to; whilst there is small heed taken of
the visible figure and magnitude, which, though more immediately
perceived, do less concern us, and are not fitted to produce any
alteration in our bodies.

60. That the matter of fact is true will be evident to anyone who
considers that a man placed at ten foot distance is thought as great as
if he were placed at a distance only of five foot: which is true not with
relation to the visible, but tangible greatness of the object: the
visible magnitude being far greater at one station: than it is at the
other.

61. Inches, feet, etc., are settled stated lengths whereby we measure
objects and estimate their magnitude: we say, for example, an object
appears to be six inches or six foot long. Now, that this cannot be meant
of visible inches, etc., is evident, because a visible inch is itself no
constant, determinate magnitude, and cannot therefore serve to mark out
and determine the magnitude of any other thing. Take an inch marked upon
a ruler: view it, successively, at the distance of half a foot, a foot, a
foot and a half, etc., from the eye: at each of which, and at all the
intermediate distances, the inch shall have a different visible
extension, i.e. there shall be more or fewer points discerned in it. Now
I ask which of all these various extensions is that stated, determinate
one that is agreed on for a common measure of other magnitudes? No reason
can be assigned why we should pitch on one more than another: and except
there be some invariable, determinate extension fixed on to be marked to
the word inch, it is plain it can be used to little purpose; and to say a
thing contains this or that number of inches shall imply no more than
that it is extended, without bringing any particular idea of that
extension into the mind. Farther, an inch and a foot, from different
distances, shall both exhibit the same visible magnitude, and yet at the
same time you shall say that one seems several times greater than the
other. From all which it is manifest that the judgments we make of the
magnitude of objects by sight are altogether in reference to their
tangible extension. Whenever we say an object is great, or small, of this
or that determinate measure, I say it must be meant of the tangible, and
not the visible extension, which, though immediately perceived, is
nevertheless little taken notice of.

62. Now, that there is no necessary connexion between these two distinct
extensions is evident from hence: because our eyes might have been framed
in such a manner as to be able to see nothing but what were less than the
MINIMUM TANGIBILE. In which case it is not impossible we might have
perceived all the immediate objects of sight, the very same that we do
now: but unto those visible appearances there would not be connected
those different tangible magnitudes that are now. Which shows the
judgments we make of the magnitude of things placed at a distance from
the various greatness of the immediate objects of sight do not arise from
any essential or necessary but only a customary tie, which has been
observed between them.

63. Moreover, it is not only certain that any idea of sight might not
have been connected with this or that idea of touch, which we now observe
to accompany it: but also that the greater visible magnitudes might have
been connected with, and introduced into our minds lesser tangible
magnitudes and the lesser visible magnitudes greater tangible magnitudes.
Nay, that it actually is so we have daily experience; that object which
makes a strong and large appearance, not seeming near so great as
another, the visible magnitude whereof is much less, but more faint, and
the appearance upper, or which is the same thing painted lower on the
RETINA, which faintness and situation suggest both greater magnitude and
greater distance.

64. From which, and from sect. 57 and 58, it is manifest that as we do
not perceive the magnitudes of objects immediately by sight, so neither
do we perceive them by the mediation of anything which has a necessary
connexion with them. Those ideas that now suggest unto us the various
magnitudes of external objects before we touch them, might possibly have
suggested no such thing: or they might have signified them in a direct
contrary manner: so that the very same ideas, on the perception whereof
we judge an object to be small, might as well have served to make us
conclude it great. Those ideas being in their own nature equally fitted
to bring into our minds the idea of small or great, or no size at all of
outward objects; just as the words of any language are in their own
nature indifferent to signify this or that thing or nothing at all.

65. As we see distance, so we see magnitude. And we see both in the same
way that we see shame or anger in the looks of a man. Those passions are
themselves invisible, they are nevertheless let in by the eye along with
colours and alterations of countenance, which are the immediate object of
vision: and which signify them for no other reason than barely because
they have been observed to accompany them. Without which experience we
should no more have taken blushing for a sign of shame than of gladness.

66. We are nevertheless exceeding prone to imagine those things which are
perceived only by the mediation of others to be themselves the immediate
objects of sight; or, at least, to have in their own nature a fitness to
be suggested by them, before ever they had been experienced to coexist
with them. From which prejudice everyone, perhaps, will not find it easy
to emancipate himself, by any [but] the clearest convictions of reason.
And there are some grounds to think that if there was one only invariable
and universal languages in the world, and that men were born with the
faculty of speaking it, it would be the opinion of many that the ideas of
other men's minds were properly perceived by the ear, or had at least a
necessary and inseparable tie with the sounds that were affixed to them.
All which seems to arise from want of a due application of our discerning
faculty, thereby to discriminate between the ideas that are in our
understandings, and consider them apart from each other; which would
preserve us from confounding those that are different, and make us see
what ideas do, and what do not include or imply this or that other idea.

67. There is a celebrated phenomenon, the solution whereof I shall
attempt to give by the principles that have been laid down, in reference
to the manner wherein we apprehend by sight the magnitude of objects. The
apparent magnitude of the moon when placed in the horizon is much greater
than when it is in the meridian, though the angle under which the
diameter of the moon is seen be not observed greater in the former case
than in the latter: and the horizontal moon doth not constantly appear of
the same bigness, but at some times seemeth far greater than at others.

68. Now in order to explain the reason of the moon's appearing greater
than ordinary in the horizon, it must be observed that the particles
which compose our atmosphere intercept the rays of light proceeding from
any object to the eye; and by how much the greater is the portion of
atmosphere interjacent between the object and the eye, by so much the
more are the rays intercepted; and by consequence the appearance of the
object rendered more faint, every object appearing more vigorous or more
faint in proportion as it sendeth more or fewer rays into the eye. Now
between the eye and the moon, when situated in the horizon, there lies a
far greater quantity of atmosphere than there does when the moon is in
the meridian. Whence it comes to pass that the appearance of the
horizontal moon is fainter, and therefore by sect. 56 it should be
thought bigger in that situation than in the meridian, or in any other
elevation above the horizon.

69. Farther, the air being variously impregnated, sometimes more and
sometimes less, with vapours and exhalations fitted to retund and
intercept the rays of light, it follows that the appearance of the
horizontal moon hath not always an equal faintness, and by consequence
that luminary, though in the very same situation, is at one time judged
greater than at another.

70. That we have here given the true account of the phenomena of the
horizontal moon will, I suppose, be farther evident to anyone from the
following considerations. FIRST, it is plain that which in this case
suggests the idea of greater magnitude must be something which is itself
perceived; for that which is unperceived cannot suggest to our perception
any other thing. SECONDLY, it must be something that does not constantly
remain the same, but is subject to some change or variation, since the
appearance of the horizontal moon varies, being at one time greater than
at another. And yet, THIRDLY, it cannot be the visible figure or
magnitude, since that remains the same, or is rather lesser, by how much
the moon is nearer to the horizon. It remains therefore that the true
cause is that affection or alteration of the visible appearance which
proceeds from the greater paucity of rays arriving at the eye, and which
I term FAINTNESS: since this answers all the forementioned conditions,
and I am not conscious of any other perception that doth.

71. Add to this that in misty weather it is a common observation that the
appearance of the horizontal moon is far larger than usual, which greatly
conspires with and strengthens our opinion. Neither would it prove in the
least irreconcilable with what we have said, if the horizontal moon
should chance sometimes to seem enlarged beyond its usual extent, even in
more serene weather. For we must not only have regard to the mist which
happens to be in the place where we stand; we ought also to take into our
thoughts the whole sum of vapours and exhalations which lie betwixt the
eye and the moon: all which cooperating to render the appearance of the
moon more faint, and thereby increase its magnitude, it may chance to
appear greater than it usually does, even in the horizontal position, at
a time when, though there be no extraordinary fog or haziness, just in
the place where we stand, yet the air between the eye and the moon, taken
all together, may be loaded with a greater quantity of interspersed
vapours and exhalations than at other times.

72. It may be objected that in consequence of our principles the
interposition of a body in some degree opaque, which may intercept a
great part of the rays of light, should render the appearance of the moon
in the meridian as large as when it is viewed in the horizon. To which I
answer, it is not faintness anyhow applied that suggests greater
magnitude, there being no necessary but only an experimental connexion
between those two things. It follows that the faintness which enlarges
the appearance must be applied in such sort, and with such circumstances,
as have been observed to attend the vision of great magnitudes. When from
a distance we behold great objects, the particles of the intermediate air
and vapours, which are themselves unperceivable, do interrupt the rays of
light, and thereby render the appearance less strong and vivid: now,
faintness of appearance caused in this sort hath been experienced to
coexist with great magnitude. But when it is caused by the interposition
of an opaque sensible body, this circumstance alters the case, so that a
faint appearance this way caused doth not suggest greater magnitude,
because it hath not been experienced to coexist with it.

73. Faintness, as well as all other ideas or perceptions which suggest
magnitude or distance, doth it in the same way that words suggest the
notions to which they are annexed. Now, it is known a word pronounced
with certain circumstances, or in a certain context with other words,
hath not always the same import and signification that it hath when
pronounced in some other circumstances or different context of words. The
very same visible appearance as to faintness and all other respects, if
placed on high, shall not suggest the same magnitude that it would if it
were seen at an equal distance on a level with the eye. The reason
whereof is that we are rarely accustomed to view objects at a great
height; our concerns lie among things situated rather before than above
us, and accordingly our eyes are not placed on the top of our heads, but
in such a position as is most convenient for us to see distant objects
standing in our way. And this situation of them being a circumstance
which usually attends the vision of distant objects, we may from hence
account for (what is commonly observed) an object's appearing of
different magnitude, even with respect to its horizontal extension, on
the top of a steeple, for example, an hundred feet high to one standing
below, from what it would if placed at an hundred feet distance on a
level with his eye. For it hath been shown that the judgment we make on
the magnitude of a thing depends not on the visible appearance alone, but
also on divers other circumstances, any one of which being omitted or
varied may suffice to make some alteration in our judgment. Hence, the
circumstances of viewing a distant object in such a situation as is
usual, and suits with the ordinary posture of the head and eyes being
omitted, and instead thereof a different situation of the object, which
requires a different posture of the head taking place, it is not to be
wondered at if the magnitude be judged different: but it will be demanded
why an high object should constantly appear less than an equidistant low
object of the same dimensions, for so it is observed to be: it may indeed
be granted that the variation of some circumstances may vary the judgment
made on the magnitude of high objects, which we are less used to look at:
but it does not hence appear why they should be judged less rather than
greater? I answer that in case the magnitude of distant objects was
suggested by the extent of their visible appearance alone, and thought
proportional thereto, it is certain they would then be judged much less
than now they seem to be (VIDE sect. 79). But several circumstances
concurring to form the judgment we make on the magnitude of distant
objects, by means of which they appear far larger than others, whose
visible appearance hath an equal or even greater extension; it follows
that upon the change or omission of any of those circumstances which are
wont to attend the vision of distant objects, and so come to influence
the judgments made on their magnitude, they shall proportionably appear
less than otherwise they would. For any of those things that caused an
object to be thought greater than in proportion to its visible extension
being either omitted or applied without the usual circumstances, the
judgment depends more entirely on the visible extension, and consequently
the object must be judged less. Thus in the present case the situation of
the thing seen being different from what it usually is in those objects
we have occasion to view, and whose magnitude we observe, it follows that
the very same object, being an hundred feet high, should seem less than
if it was an hundred feet off on (or nearly on) a level with the eye.
What has been here set forth seems to me to have no small share in
contributing to magnify the appearance of the horizontal moon, and
deserves not to be passed over in the explication of it.

74. If we attentively consider the phenomenon before us, we shall find
the not discerning between the mediate and immediate objects of sight to
be the chief cause of the difficulty that occurs in the explication of
it. The magnitude of the visible moon, or that which is the proper and
immediate object of vision, is not greater when the moon is in the
horizon than when it is in the meridian. How comes it, therefore, to seem
greater in one situation than the other? What is it can put this cheat on
the understanding? It has no other perception of the moon than what it
gets by sight: and that which is seen is of the same extent, I say, the
visible appearance hath the same, or rather a less, magnitude when the
moon is viewed in the horizontal than when in the meridional position:
and yet it is esteemed greater in the former than in the latter. Herein
consists the difficulty, which doth vanish and admit of a most easy
solution, if we consider that as the visible moon is not greater in the
horizon than in the meridian, so neither is it thought to be so. It hath
been already shown that in any act of vision the visible object
absolutely, or in itself, is little taken notice of, the mind still
carrying its view from that to some tangible ideas which have been
observed to be connected with it, and by that means come to be suggested
by it. So that when a thing is said to appear great or small, or whatever
estimate be made of the magnitude of any thing, this is meant not of the
visible but of the tangible object. This duly considered, it will be no
hard matter to reconcile the seeming contradiction there is, that the
moon should appear of a different bigness, the visible magnitude thereof
remaining still the same. For by sect. 56 the very same visible
extension, with a different faintness, shall suggest a different tangible
extension. When therefore the horizontal moon is said to appear greater
than the meridional moon, this must be understood not of a greater
visible extension, but a of greater tangible or real extension, which by
reason of the more than ordinary faintness of the visible appearance, is
suggested to the mind along with it.

75. Many attempts have been made by learned men to account for this
appearance. Gassendus, Descartes, Hobbes, and several others have
employed their thoughts on that subject; but how fruitless and
unsatisfactory their endeavours have been is sufficiently shown in THE
TRANSACTIONS,[Phil. Trans. Num. 187. p. 314] where you may
see their several opinions at large set forth and confuted, not without
some surprize at the gross blunders that ingenious men have been forced
into by endeavouring to reconcile this appearance with the ordinary
Principles of optics. Since the writing of which there hath been published
in the TRANSACTIONS [Numb. 187. P. 323] another paper relating to the same
affair by the celebrated Dr. Wallis, wherein he attempts to account for
that phenomenon which, though it seems not to contain anything new or
different from what had been said before by others, I shall nevertheless
consider in this place.

76. His opinion, in short, is this; we judge not of the magnitude of an
object by the visual angle alone, but by the visual angle in conjunction
with the distance. Hence, though the angle remain the same, or even
become less, yet if withal the distance seem to have been increased, the
object shall appear greater. Now, one way whereby we estimate the
distance of anything is by the number and extent of the intermediate
objects: when therefore the moon is seen in the horizon, the variety of
fields, houses, etc., together with the large prospect of the wide
extended land or sea that lies between the eye and the utmost limb of the
horizon, suggest unto the mind the idea of greater distance, and
consequently magnify the appearance. And this, according to Dr. Wallis,
is the true account of the extraordinary largeness attributed by the mind
to the horizontal moon at a time when the angle subtended by its diameter
is not one jot greater than it used to be.

77. With reference to this opinion, not to repeat what hath been already
said concerning distance, I shall only observe, FIRST, that if the
prospect of interjacent objects be that which suggests the idea of
farther distance, and this idea of farther distance be the cause that
brings into the mind the idea of greater magnitude, it should hence
follow that if one looked at the horizontal moon from behind a wall, it
would appear no bigger than ordinary. For in that case the wall
interposing cuts off all that prospect of sea and land, etc. which might
otherwise increase the apparent distance, and thereby the apparent
magnitude of the moon. Nor will it suffice to say the memory doth even
then suggest all that extent of land, etc., which lies within the
horizon; which suggestion occasions a sudden judgment of sense that the
moon is farther off and larger than usual. For ask any man who, from such
a station beholding the horizontal moon, shall think her greater than
usual, whether he hath at that time in his mind any idea of the
intermediate objects, or long tract of land that lies between his eye and
the extreme edge of the horizon? And whether it be that idea which is the
cause of his making the aforementioned judgment? He will, I suppose,
reply in the negative, and declare the horizontal moon shall appear
greater than the meridional, though he never thinks of all or any of
those things that lie between him and it. SECONDLY, it seems impossible
by this hypothesis to account for the moon's appearing in the very same
situation at one time greater than at another; which nevertheless has
been shown to be very agreeable to the principles we have laid down, and
receives a most easy and natural explication from them. For the further
clearing' up of this point it is to be observed that what we immediately
and properly see are only lights and colours in sundry situations and
shades and degrees of faintness and clearness, confusion and
distinctness. All which visible objects are only in the mind, nor do they
suggest ought external, whether distance or magnitude, otherwise than by
habitual connexion as words do things. We are also to remark that, beside
the straining of the eyes, and beside the vivid and faint, the distinct
and confused appearances (which, bearing some proportion to lines and
angles, have been substituted instead of them in the foregoing part of
this treatise), there are other means which suggest both distance and
magnitude; particularly the situation of visible points of objects, as
upper or lower; the one suggesting a farther distance and greater
magnitude, the other a nearer distance and lesser magnitude: all which is
an effect only of custom and experience; there being really nothing
intermediate in the line of distance between the uppermost and lowermost,
which are both equidistant, or rather at no distance from the eye, as
there is also nothing in upper or lower, which by necessary connexion
should suggest greater or lesser magnitude. Now, as these customary,
experimental means of suggesting distance do likewise suggest magnitude,
so they suggest the one as immediately as the other. I say they do not
(VIDE sect. 53) first suggest distance, and then leave the mind from
thence to infer or compute magnitude, jut suggest magnitude as
immediately and directly as they suggest distance.

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