An Essay Towards a New Theory of Vision
G >>
George Berkeley >> An Essay Towards a New Theory of Vision
Pages:
1 | 2 |
3 |
4 |
5 |
6 |
7
30. The ancient and received principle, which Dr. Barrow here mentions as
the main foundation of Tacquet's CATOPTRICS, is that: 'every visible point
seen by reflection from a speculum shall appear placed at the intersection
of the reflected ray, and the perpendicular of incidence:' which
intersection in the present case, happening to be behind the eye, it
greatly shakes the authority of that principle, whereon the
aforementioned author proceeds throughout his whole CATOPTRICS in
determining the apparent place of OBJECTS seen by reflection from any kind
of speculum.
31. Let us now see how this phenomenon agrees with our tenets. The eye
the nearer it is placed to the point B in the foregoing figures, the more
distinct is the appearance of the OBJECT; but as it recedes to O the
appearance grows more confused; and at P it sees the OBJECT yet more
confused; and so on till the eye being brought back to Z sees the OBJECT
in the greatest confusion of all. Wherefore by sect. 21 the OBJECT should
seem to approach the eye gradually as it recedes from the point B, that
is, at O it should (in consequence of the principle I have laid down in
the aforesaid section) seem nearer than it did at B, and at P nearer than
at 0, and at Q nearer than at P; and so on, till it quite vanishes at Z.
Which is the very matter of fact, as anyone that pleases may easily
satisfy himself by experiment.
32. This case is much the same as if we should suppose an Englishman to
meet a foreigner who used the same words with the English, but in a
direct contrary signification. The Englishman would not fail to make a
wrong judgment of the IDEAS annexed to those sounds in the mind of him
that used them. Just so, in the present case the OBJECT speaks (if I may
so say) with words that the eye is well acquainted with, that is,
confusions of appearance; but whereas heretofore the greater confusions
were always wont to signify nearer distances, they have in this case a
direct, contrary signification, being connected with the greater
distances. Whence it follows that the eye must unavoidably be mistaken,
since it will take the confusions in the sense it has been used to, which
is directly opposed to the true.
33. This phenomenon as it entirely subverts the opinion of those who will
have us judge of distance by lines and angles, on which supposition it is
altogether inexplicable, so it seems to me no small confirmation of the
truth of that principle whereby it is explained. But in order co a more
full explication of this point, and to show how far the hypothesis of the
mind's judging by the various divergency of rays may be of use in
determining the apparent place of an OBJECT, it will be necessary to
premise some few things, which are already well known to those who have
any skill in dioptrics.
34. FIRST, any radiating point is then distinctly seen when the rays
proceeding from it are, by the refractive power of the crystalline,
accurately reunited in the retina or fund of the eye: but if they are
reunited, either before they arrive at the retina, or after they have
passed it, then there is confused vision.
35. SECONDLY, suppose in the adjacent figures NP represent an eye duly
framed and retaining its natural figure. In Fig. 1 the rays falling
nearly parallel on the eye, are by the crystalline AB refracted, so as
their focus or point of union F falls exactly on the retina: but if the
rays fall sensibly diverging on the eye, as in Fig. 2, then their focus
falls beyond the retina: or if the rays are made to converge by the lens
QS before they come at the eye, as in Fig. 3, their focus F will fall
before the retina. In which two last cases it is evident from the
foregoing section that the appearance of the point Z is confused. And by
how much the greater is the convergency, or divergency, of the rays
falling on the pupil, by so much the farther will the point of their
reunion be from the retina, either before or behind it, and consequently
the point Z will appear by so much the more confused. And this, by the
bye, may show us the difference between confused and faint vision.
Confused vision is when the rays proceedings from each distinct point of
the OBJECT are not accurately recollected in one corresponding point on
the retina, but take up some space thereon, so that rays from different
points become mixed and confused together. This is opposed to a distinct
vision, and attends near objects. Faint vision is when by reason of the
distance of the object or grossness of the interjacent medium few rays
arrive from the object to the eye. This is opposed to vigorous or clear
vision, and attends remote objects. But to return.
36. The eye, or (to speak truly) the mind, perceiving only the confusion
itself, without ever considering the cause from which it proceeds, doth
constantly annex the same degree of distance to the same degree of
confusion. Whether that confusion be occasioned by converging or by
diverging rays, it matters not. Whence it follows that the eye viewing
the object Z through the glass QS (which by refraction causeth the rays
ZQ, ZS, etc., to converge) should judge it to be at such a nearness at
which if it were placed it would radiate on the eye with rays diverging
to that degree as would produce the same confusion which is now produced
by converging rays, i.e. would cover a portion of the retina equal to DC
(VID. Fig. 3 supra). But then this must be understood (to use Dr.
Barrow's phrase) SECLUSIS PRAENOTIONIBUS ET PRAEJUDICIIS, in case we
abstract from all other circumstances of vision, such as the figure,
size, faintness, etc. of the visible objects; all which do ordinarily
concur to form our idea of distance, the mind having by frequent
experience observed their several sorts or degrees to be conneted with
various distances.
37 It plainly follows from what hath been said that a person perfectly
purblind (i.e. that could not see an object distinctly but when placed
close to his eye) would not make the same wrong judgment that others do
in the forementioned case. For to him greater confusions constantly
suggesting greater distances, he must, as he recedes from the glass and
the object grows more confused, judge it to be at a farther distance,
contrary to what they do who have had the perception of the objects
growing more confused connected with the idea of approach.
38. Hence also it doth appear there may be good use of computation by
lines and angles in optics; not that the mind judgeth of distance
immediately by them, but because it judgeth by somewhat which is
connected with them, and to the determination whereof they may be
subservient. Thus the mind judging of the distance of an object by the
confusedness of its appearance, and this confusedness being greater or
lesser to the naked eye, according as the object is seen by rays more or
less diverging, it follows that a man may make use of the divergency of
the rays in computing the apparent distance, though not for its own sake,
yet on account of the confusion with which it is connected. But, so it
is, the confusion itself is entirely neglected by mathematicians as
having no necessary relation with distance, such as the greater or lesser
angles of divergency are conceived to have. And these (especially for
that they fall under mathematical computation) are alone regarded in
determining the apparent places of objects, as though they were the sole
and immediate cause of the judgments the mind makes of distance. Whereas,
in truth, they should not at all be regarded in themselves, or any
otherwise, than as they are supposed to be the cause of confused vision.
39. The not considering of this has been a fundamental and perplexing
oversight. For proof whereof we need go no farther than the case before
us. It having been observed that the most diverging rays brought into the
mind the idea of nearest distance, and that still, as the divergency
decreased, the distance increased: and it being thought the connexion
between the various degrees of divergency and distance was immediate;
this naturally leads one to conclude, from an ill-grounded analogy, that
converging rays shall make an object appear at an immense distance: and
that, as the convergency increases, the distance (if it were possible)
should do so likewise. That this was the cause of Dr. Barrow's mistake is
evident from his own words which we have quoted. Whereas had the learned
doctor observed that diverging and converging rays, how opposite soever
they may seem, do nevertheless agree in producing the same effect, to
wit, confusedness of vision, greater degrees whereof are produced
indifferently, either as the divergency or convergency and the rays
increaseth. And that it is by this effect, which is the same in both,
that either the divergency or convergency is perceived by the eye; I say,
had he but considered this, it is certain he would have made a quite
contrary judgment, and rightly concluded that those rays which fall on
the eye with greater degrees of convergency should make the object from
whence they proceed appear by so much the nearer. But it is plain it was
impossible for any man to attain to a right notion of this matter so long
as he had regard only to lines and angles, and did not apprehend the true
nature of vision, and how far it was of mathematical consideration.
40. Before we dismiss this subject, it is fit we take notice of a query
relating thereto, proposed by the ingenious Mr. Molyneux, is his TREATISE
OF DIOPTRICS,[Par. I. Prop. 31, Sect. 9.] where speaking of this
difficulty, he has these words: 'And so he (i.e. Dr. Barrow) leaves this
difficulty to the solution of others, which I (after so great an example)
shall do likewise; but with the resolution of the same admirable author of
not quitting the evident doarine which we have before laid down, for
determining the LOCUS OBJECTI, on account of being pressed by one
difficulty which seems inexplicable till a more intimate knowledge of the
visive faculty be obtained by mortals. In the meantime, I propose it to
the consideration of the ingenious, whether the LOCUS APPARENS of an
object placed as in this 9th section be not as much before the eye as the
distinct base is behind the eye!' To which query we may venture to answer
in the negative. For in the present case the rule for determining the
distance of the distinct base, or respective focus from the glass, is this:
as the difference between the distance of the object and focus is to the
focus or focal length, so the distance of the object from the glass is to
the distance of the respective focus or distinct base from the glass.
[Molyneux Dioptr., Par. I. Prop. 5.] Let us now suppose the object to be
placed at the distance of the focal length, and one half of the focal
length from the glass, and the eye close to the glass, hence it will follow
by the rule that the distance of the distinct base behind the eye is double
the true distance of the object before the eye. If therefore Mr. Molyneux's
conjecture held good, it would follow that the eye should see the object
twice as far off as it really is; and in other cases at three or four times
its due distance, or more. But this manifestly contradicts experience, the
object never appearing, at farthest, beyond its due distance. Whatever,
therefore, is built on this supposition (VID. COROL. I. PROP. 57, IBID.)
comes to the ground along with it.
41. From what hath been premised it is a manifest consequence that a man
born blind, being made to see, would, at first, have no idea of distance
by sight; the sun and stars, the remotest objects as well as the nearer,
would all seem to be in his eye, or rather in his mind. The objects
intromitted by sight would seem to him (as in truth they are) no other
than a new set of thoughts or sensations, each whereof is as near to him
as the perceptions of pain or pleasure, or the most inward passions of
his soul. For our judging objects provided by sight to be at any
distance, or without the mind, is (VID. sect. 28) entirely the effect of
experience, which one in those circumstances could not yet have attained
to.
42. It is indeed otherwise upon the common supposition that men judge of
distance by the angle of the optic axes, just as one in the dark, or a
blind-man by the angle comprehended by two sticks, one whereof he held in
each hand. For if this were true, it would follow that one blind from his
birth being made to see, should stand in need of no new experience in
order to perceive distance by sight. But that this is false has, I think,
been sufficiently demonstrated.
43. And perhaps upon a strict inquiry we shall not find that even those
who from their birth have grown up in a continued habit of seeing are
irrecoverably prejudiced on the other side, to wit, in thinking what they
see to be at a distance from them. For at this time it seems agreed on
all hands, by those who have had any thoughts of that matter, that
colours, which are the proper and immediate object of sight, are not
without the mind. But then it will be said, by sight we have also the
ideas of extension, and figure, and motion; all which may well be thought
without, and at some distance from the mind, though colour should not. In
answer to this I appeal to any man's experience, whether the visible
extension of any object doth not appear as near to him as the colour of
that object; nay, whether they do not both seem to be in the very same
place. Is not the extension we see coloured, and is it possible for us,
so much as in thought, to separate and abstract colour from extension?
Now, where the extension is there surely is the figure, and there the
motion too. I speak of those which are perceived by sight.
44. But for a fuller explication of this point, and to show that the
immediate objects of sight are not so much as the ideas or resemblances
of things placed at a distance, it is requisite that we look nearer into
the matter and carefully observe what is meant in common discourse, when
one says that which he sees is at a distance from him. Suppose, for
example, that looking at the moon I should say it were fifty or sixty
semidiameters of the earth distant from me. Let us see what moon this is
spoken of: it is plain it cannot be the visible moon, or anything like
the visible moon, or that which I see, which is only a round, luminous
plane of about thirty visible points in diameter. For in case I am
carried from the place where I stand directly towards the moon, it is
manifest the object varies, still as I go on; and by the time that I am
advanced fifty or sixty semidiameters of the earth, I shall be so far
from being near a small, round, luminous flat that I shall perceive
nothing like it; this object having long since disappeared, and if I
would recover it, it must be by going back to the earth from whence I set
out. Again, suppose I perceive by sight the faint and obscure idea of
something which I doubt whether it be a man, or a tree, or a tower, but
judge it to be at the distance of about a mile. It is plain I cannot mean
that what I see is a mile off, or that it is the image or likeness of
anything which is a mile off, since that every step I take towards it the
appearance alters, and from being obscure, small, and faint, grows clear,
large, and vigorous. And when I come to the mile's end, that which I saw
first is quite lost, neither do I find anything in the likeness of it.
45. In these and the like instances the truth of the matter stands thus:
having of a long time experienced certain ideas, perceivable by touch, as
distance, tangible figure, and solidity, to have been connected with
certain ideas of sight, I do upon perceiving these ideas of sight
forthwith conclude what tangible ideas are, by the wonted ordinary course
of Nature like to follow. Looking at an object I perceive a certain
visible figure and colour, with some degree of faintness and other
circumstances, which from what I have formerly observed, determine me to
think that if I advance forward so many paces or miles, I shall be
affected with such and such ideas of touch: so that in truth and
strictness of speech I neither see distance itself, nor anything that I
take to be at a distance. I say, neither distance nor things placed at a
distance are themselves, or their ideas, truly perceived by sight. This I
am persuaded of, as to what concerns myself: and I believe whoever will
look narrowly into his own thoughts and examine what he means by saying
he sees this or that thing at a distance, will agree with me that what he
sees only suggests to his understanding that after having passed a
certain distance, to be measured by the motion of his body, which is
perceivable by touch, he shall come to perceive such and such tangible
ideas which have been usually connected with such and such visible ideas.
But that one might be deceived by these suggestions of sense, and that
there is no necessary connexion between visible and tangible ideas
suggested by them, we need go no farther than the next looking-glass or
pictures to be convinced. Note that when I speak of tangible ideas, I
take the word idea for any the immediate object of sense or
understanding, in which large signification it is commonly used by the
moderns.
46. From what we have shown it is a manifest consequence that the ideas
of space, outness, and things placed at a distance are not, strictly
speaking, the object of sight; they are not otherwise perceived by the
eye than by the ear. Sitting in my study I hear a coach drive along the
street; I look through the casement and see it; I walk out and enter into
it; thus, common speech would incline one to think I heard, saw, and
touched the same thing, to wit, the coach. It is nevertheless certain,
the ideas intromitted by each sense are widely different and distinct
from each other; but having been observed constantly to go together, they
are spoken of as one and the same thing. By the variation of the noise I
perceive the different distances of the coach, and know that it
approaches before I look out. Thus by the ear I perceive distance, just
after the same manner as I do by the eye.
47. I do not nevertheless say I hear distance in like manner as I say
that I see it, the ideas perceived by hearing not being so apt to be
confounded with the ideas of touch as those of sight are. So likewise a
man is easily convinced that bodies and external things are not properly
the object of hearing; but only sounds, by the mediation whereof the idea
of this or that body or distance is suggested to his thoughts. But then
one is with more difficulty brought to discern the difference there is
betwixt the ideas of sight and touch: though it be certain a man no more
sees and feels the same thing than he hears and feels the same thing.
48. One reason of which seems to be this. It is thought a great absurdity
to imagine that one and the same thing should have any more than one
extension, and one figure. But the extension and figure of a body, being
let into the mind two ways, and that indifferently either by sight or
touch, it seems to follow that we see the same extension and the same
figure which we feel.
49. But if we take a close and accurate view of things, it must be
acknowledged that we never see and feel one and the same object. That
which is seen is one thing, and that which is felt is another. If the
visible figure and extension be not the same with the tangible figure and
extension, we are not to infer that one and the same thing has divers
extensions. The true consequence is that the objects of sight and touch
are two distinct things. It may perhaps require some thought rightly to
conceive this distinction. And the difficulty seems not a little
increased, because the combination of visible ideas hath constantly the
same name as the combination of tangible ideas wherewith it is connected:
which doth of necessity arise from the use and end of language.
50. In order therefore to treat accurately and unconfusedly of vision, we
must bear in mind that there are two sorts of objects apprehended by the
eye, the one primarily and immediately, the other secondarily and by
intervention of the former. Those of the first sort neither are, nor
appear to be, without the mind, or at any distance off; they may indeed
grow greater or smaller, more confused, or more clear, or more faint, but
they do not, cannot approach or recede from us. Whenever we say an object
is at a distance, whenever we say it draws near, or goes farther off, we
must always mean it of the latter sort, which properly belong to the
touch, and are not so truly perceived as suggested by the eye in like
manner as thoughts by the ear.
51. No sooner do we hear the words of a familiar language pronounced in
our ears, but the ideas corresponding thereto present themselves to our
minds: in the very same instant the sound and the meaning enter the
understanding: so closely are they united that it is not in our power to
keep out the one, except we exclude the other also. We even act in all
respects as if we heard the very thoughts themselves. So likewise the
secondary objects, or those which are only suggested by sight, do often
more strongly affect us, and are more regarded than the proper objects of
that sense; along with which they enter into the mind, and with which
they have a far more strict connexion, than ideas have with words. Hence
it is we find it so difficult to discriminate between the immediate and
mediate objects of sight, and are so prone to attribute to the former
what belongs only to the latter. They are, as it were, most closely
twisted, blended, and incorporated together. And the prejudice is
confirmed and riveted in our thoughts by a long tract of time, by the use
of language, and want of reflexion. However, I believe anyone that shall
attentively consider what we have already said, and shall say, upon this
subject before we have done (especially if he pursue it in his own
thoughts) may be able to deliver himself from that prejudice. Sure I am
it is worth some attention, to whoever would understand the true nature
of vision.
52. I have now done with distance, and proceed to show how it is that we
perceive by sight the magnitude of objects. It is the opinion of some
that we do it by angles, or by angles in conjunction with distance: but
neither angles nor distance being perceivable by sight, and the things we
see being in truth at no distance from us, it follows that as we have
shown lines and angles not to be the medium the mind makes use of in
apprehending the apparent place, so neither are they the medium whereby
it apprehends the apparent magnitude of objects.
53. It is well known that the same extension at a near distance shall
subtend a greater angle, and at a farther distance a lesser angle. And by
this principle (we are told) the mind estimates the magnitude of an
object, comparing the angle under which it is seen with its distance, and
thence inferring the magnitude thereof. What inclines men to this mistake
(beside the humour of making one see by geometry)is that the same
perceptions or ideas which suggest distance do also suggest magnitude.
But if we examine it we shall find they suggest the latter as immediately
as the former. I say, they do not first suggest distance, and then leave
it to the judgment to use that as a medium whereby to collect the
magnitude; but they have as close and immediate a connexion with the
magnitude as with the distance; and suggest magnitude as independently of
distance as they do distance independently of magnitude. All which will
be evident to whoever considers what hath been already said, and what
follows.
54. It hath been shown there are two sorts of objects apprehended by
sight; each whereof hath its distinct magnitude, or extension. The one,
properly tangible, i.e. to be perceived and measured by touch, and not
immediately falling under the sense of seeing: the other, properly and
immediately visible, by mediation of which the former is brought in view.
Each of these magnitudes are greater or lesser, according as they contain
in them more or fewer points, they being made up of points or minimums.
For, whatever may be said of extension in abtract, it is certain sensible
extension is not infinitely divisible. There is a MINIMUM TANGIBILE and a
MINIMUM VISIBILE, beyond which sense cannot perceive. This everyone's
experience will inform him.
55. The magnitude of the object which exists without the mind, and is at
a distance, continues always invariably the same: but the visible object
still changing as you approach to, or recede from, the tangible object,
it hath no fixed and determinate greatness. Whenever, therefore, we speak
of the magnitude of anything, for instance a tree or a house, we must
mean the tangible magnitude, otherwise there can be nothing steady and
free from ambiguity spoken of it. But though the tangible and visible
magnitude in truth belong to two distinct objects: I shall nevertheless
(especially since those objects are called by the same name, and are
observed to coexist), to avoid tediousness and singularity of speech,
sometimes speak of them as belonging to one and the same thing.
Pages:
1 | 2 |
3 |
4 |
5 |
6 |
7