considered this as anything but an eccentric opinion, worthy to pair
off with that other belief that color is primitively seen without
extent, had it not been for the remarkable Essay on Tactile and Visual
Space by M. Ch. Dunan, which appeared in the Revue Philosophique for
1888. This author quotes[206] three very competent witnesses, all
officials in institutions for the blind [it does not appear from the
text that more than one of them was blind himself], who say that blind
people _only live in time_. M. Dunan himself does not share exactly
this belief, but he insists that the blind man's and the seeing
man's representation of space have _absolutely naught_ in common,
and that we are deceived into believing that what they mean by space
is analogous to what we mean, by the fact that so many of them are
but semi-blind and still think in visual terms, and from the farther
fact that they all _talk_ in visual terms just like ourselves. But
on examining M. Dunan's reasons one finds that they all rest on the
groundless logical assumption that the perception of a geometrical
form which we get with our eyes, and that which a blind man gets with
his fingers, must either be absolutely identical or absolutely unlike.
They cannot be similar in diversity, "for they are simple notions,
and it is of the essence of such to enter the mind or leave it all at
once, so that one who has a simple notion at all, possesses it in all
its completeness.... Therefore, since it is impossible that the blind
should have of the forms in question ideas _completely identical_
with our seeing ones, it follows that their ideas must be _radically
different from and wholly irreducible to our own_."[207] Hereupon M.
Dunan has no difficulty in finding a blind man who still preserves a
crude sensation of diffused light, and who says when questioned that
_this light has no extent_. Having 'no extent' appears, however, on
farther questioning, to signify merely not enveloping any particular
tactile objects, nor being located within their outline; so that
(allowing for latitude of expression) the result tallies perfectly
with our own view. A relatively stagnant retinal sensation of diffused
light, not varying when different objects are handled, would naturally
remain an object quite apart. If the word 'extent' were habitually used
to denote tactile extent, this sensation, having no tactile associates
whatever, would naturally have 'extent' denied of it. And yet all the
while it would be _analogous_ to the tactile sensations in having
the quality of bigness. Of course it would have no _other_ tactile
qualities, just as the tactile objects have no other optical qualities
than bigness. All sorts of analogies obtain between the spheres of
sensibility. Why are 'sweet' and 'soft' used so synonymously in most
languages? and why are both these adjectives applied to objects of so
many sensible kinds? Rough sounds, heavy smells, hard lights, cold
colors, are other examples. Nor does it follow from such analogies as
these that the sensations compared need be composite and have some
of their parts identical. We saw in Chapter XIII that likeness and
difference are an elementary relation, not to be resolved in every
case into a mixture of absolute identity and absolute heterogeneity of
content (cf. Vol. I, pp. 492-3).
I conclude, then, that although in its more superficial determinations
the blind man's space is very different from our space, yet a deep
analogy remains between the two. 'Big' and 'little,' 'far' and
'near,' are similar contents of consciousness in both of us. But the
_measure_ of the bigness and the farness is very different in him and
in ourselves. He, for example, can have no notion of what we mean by
objects appearing smaller as they move away, because he must always
conceive of them as of their constant tactile size. Nor, whatever
analogy the two extensions involve, should we expect that a blind man
receiving sight for the first time should recognize his new-given
optical objects by their familiar tactile names. Molyneux wrote to
Locke:
"Suppose a man born blind, and now adult, and taught by his touch to
distinguish between a cube and a sphere,... so as to tell, when he
felt one and the other, which is the cube, which the sphere. Suppose
then the cube and sphere placed on a table and the blind man to be
made to see; query, whether by his sight, before he touched them, he
could now distinguish and tell which is the globe, which the cube?"
This has remained in literature as 'Molyneux's query.' Molyneux
answered 'No.' And Locke says:[208]
"I agree with this thinking gentleman whom I am proud to call my
friend, and am of opinion that the blind man at first sight would not
be able to say which was the globe, which the cube, whilst he only saw
them; though he could unerringly name them by his touch and certainly
distinguish them by the difference of their figures felt."
This opinion has not lacked experimental confirmation. From
Chesselden's case downwards, patients operated for congenital cataract
have been unable to name at first the things they saw. "So, Puss, I
shall know you another time," said Chesselden's patient, after catching
the cat, looking at her steadfastly, and setting her down. Some of
this incapacity is unquestionably due to general mental confusion at
the new experience, and to the excessively unfavorable conditions for
perception which an eye with its lens just extirpated affords. That the
analogy of inner nature between the retinal and tactile sensations goes
beyond mere extensity is proved by the cases where the patients were
the most intelligent, as in the young man operated on by Dr. Franz,
who named circular, triangular, and quadrangular figures at first
sight.[209]
VISUAL SPACE.
It is when we come to analyze minutely the conditions of _visual_
perception that difficulties arise which have made psychologists appeal
to new and _quasi_-mythical mental powers. But I firmly believe that
even here exact investigation will yield the same verdict as in the
cases studied hitherto. This subject will close our survey of the
facts; and if it give the result I foretell, we shall be in the best of
positions for a few final pages of critically historical review.
If a common person is asked how he is enabled to see things as they
are, he will simply reply, by opening his eyes and looking. This
innocent answer has, however, long since been impossible for science.
There are various paradoxes and irregularities about _what_ we appear
to perceive under seemingly identical optical conditions, which
immediately raise questions. To say nothing now of the time-honored
conundrums of why we see upright with an inverted retinal picture,
and why we do not see double; and to leave aside the whole field of
color-contrasts and ambiguities, as not directly relevant to the
space-problem,--it is certain that the same retinal image makes
us see quite differently-sized and differently-shaped objects at
different times, and it is equally certain that the same ocular
movement varies in its perceptive import. It ought to be possible,
were the act of perception completely and _simply_ intelligible, to
assign for every distinct judgment of size, shape, and position a
distinct optical modification of some kind as its occasion. And the
connection between the two ought to be so constant that, given the
same modification, we should always have the same judgment. But if
we study the facts closely _we soon find no such constant connection
between either judgment and retinal modification, or judgment and
muscular modification, to exist._ The judgment seems to result from the
combination of retinal, muscular and intellectual factors with each
other; and any one of them may occasionally overpower the rest in a way
which seems to leave the matter subject to no simple law.
The scientific study of the subject, if we omit Descartes, began with
Berkeley, and the particular perception he analyzed in his New Theory
of Vision was that of distance or depth. Starting with the physical
assumption that a difference in the distance of a point can make no
difference in the nature of its retinal image, since "distance being
a line directed endwise to the eye, it projects only one point in the
fund of the eye--which point remains invariably the same, whether
the distance be longer or shorter," he concluded that distance could
not possibly be a visual sensation, but must be an intellectual
'suggestion' from 'custom' of some non-visual experience. According
to Berkeley this experience was tactile. His whole treatment of the
subject was excessively vague,--no shame to him, as a breaker of fresh
ground,--but as it has been adopted and enthusiastically hugged in all
its vagueness by nearly the whole line of British psychologists who
have succeeded him, it will be well for us to begin our study of vision
by refuting his notion that depth cannot possibly be perceived in terms
of purely visual feeling.
_The Third Dimension._
Berkeleyans unanimously assume that no retinal sensation can
primitively be of volume; if it be of extension at all (which they
are barely disposed to admit), it can be only of two-, not of three-,
dimensional extension. At the beginning of the present chapter we
denied this, and adduced facts to show that all objects of sensation
are voluminous in three dimensions (cf. p. 136 ff.). It is impossible
to lie on one's back on a hill, to let the empty abyss of blue fill
one's whole visual field, and to sink deeper and deeper into the merely
sensational mode of consciousness regarding it, without feeling that
an indeterminate, palpitating, circling depth is as indefeasibly one
of its attributes as its breadth. We may artificially exaggerate this
sensation of depth. Rise and look from the hill-top at the distant
view; represent to yourself as vividly as possible the distance of the
uttermost horizon; and then _with inverted head_ look at the same.
There will be a startling increase in the perspective, a most sensible
recession of the maximum distance; and as you raise the head you can
actually see the horizon-line again draw near.[210]
Mind, I say nothing as yet about our estimate of the 'real' amount
of this depth or distance. I only want to confirm its existence as
a natural and inevitable optical consort of the two other optical
dimensions. The field of view is always a _volume-unit_. Whatever be
supposed to be its absolute and 'real' size, the relative sizes of its
dimensions are functions of each other. Indeed, it happens perhaps most
often that the breadth- and height-feeling take their absolute measure
from the depth-feeling. If we plunge our head into a wash-basin, the
felt nearness of the bottom makes us feel the lateral expanse to be
small. If, on the contrary, we are on a mountain-top, the distance of
the horizon carries with it in our judgment a proportionate height
and length in the mountain-chains that bound it to our view. But as
aforesaid, let us not consider the question of absolute size now,--it
must later be taken up in a thorough way. Let us confine ourselves to
the way in which the three dimensions which are seen, get their values
fixed _relatively to each other_.
Reid, in his Inquiry into the Human Mind, has a section 'Of the
Geometry of Visibles,' in which he assumes to trace what the
perceptions would be of a race of 'Idomenians' reduced to the sole
sense of sight. Agreeing with Berkeley that sight alone can give
no knowledge of the third dimension, he humorously deduces various
ingenious absurdities in their interpretations of the material
appearances before their eyes.
Now I firmly believe, on the contrary, that one of Reid's Idomenians
would frame precisely the same conception of the external world that we
do, if he had our intellectual powers.[211] Even were his very eyeballs
fixed and not movable like ours, that would only retard, not frustrate,
his education. For the _same object_, by alternately covering in its
lateral movements different parts of his retina, would determine the
mutual equivalencies of the first two dimensions of the field of view;
and by exciting the physiological cause of his perception of depth in
various degrees, it would establish a scale of equivalency between the
first two and the third.
First of all, one of the sensations given by the object is chosen to
represent its 'real' size and shape, in accordance with the principles
laid down on pp. 178 and 179. _One sensation measures the 'thing'
present, and the 'thing' then measures the other sensations._ The
peripheral parts of the retina are equated with the central by
receiving the image of the same object. This needs no elucidation
in case the object does not change its distance or its front. But
suppose, to take a more complicated case, that the object is a stick,
seen first in its whole length, and then rotated round one of its
ends; let this fixed end be the one near the eye. In this movement the
stick's image will grow progressively shorter; its farther end will
appear less and less separated laterally from its fixed near end; soon
it will be screened by the latter, and then reappear on the opposite
side, and finally on that side resume its original length. Suppose this
movement to become a familiar experience; the mind will presumably
react upon it after its usual fashion (which is that of unifying all
data which it is in any way possible to unify), and consider it the
movement of a constant object rather than the transformation of a
fluctuating one. Now, the _sensation of depth_ which it receives during
the experience is awakened more by the far than by the near end of
the object. But how much depth? What shall measure its amount? Why,
at the moment the far end is ready to be eclipsed, the difference of
its distance from the near end's distance must be judged equal to the
stick's whole length; but that length has already been judged equal
to a certain optical sensation of breadth. _Thus we find that given
amounts of the visual depth-feeling become signs of fixed amounts
of the visual breadth-feeling. The measurement of distance is, as
Berkeley truly said, a result of suggestion and experience. But visual
experience alone is adequate to produce it, and this he erroneously
denied._
Suppose a colonel in front of his regiment at dress-parade, and suppose
he walks at right angles towards the midmost man of the line. As he
advances, and surveys the line in either direction, he looks more and
more _down_ it and less and less _at_ it, until, when abreast of the
midmost man, he feels the end men to be _most_ distant; then when the
line casts hardly any lateral image on his retina at all, what distance
shall he judge to be that of the end men? Why, half the length of the
regiment as it was originally seen, of course; but this length was a
moment ago a retinal object spread out laterally before his sight.
He has now merely equated a retinal depth-feeling with a retinal
breadth-feeling. If the regiment moved, and the colonel stood still,
the result would be the same. In such ways as these a creature endowed
with eyes alone could hardly fail of measuring out all three dimensions
of the space he inhabited. And we ourselves, I think, although we _may_
often 'realize' distance in locomotor terms (as Berkeley says we must
always do), yet do so no less often in terms of our retinal map, and
always in this way the more spontaneously. Were this not so, the three
visual dimensions could not possibly feel to us as homogeneous as they
do, nor as commensurable _inter se_.
_Let us then admit distance to be at least as genuinely optical a
content of consciousness as either height or breadth. The question
immediately returns, Can any of them be said in any strictness to be
optical sensations?_ We have contended all along for the affirmative
reply to this question, but must now cope with difficulties greater
than any that have assailed us hitherto.
_Helmholtz and Reid on Sensations_.
A sensation is, as we have seen in Chapter XVII, the mental affection
that follows most immediately upon the stimulation of the sense-tract.
Its antecedent is directly physical, no psychic links, no acts of
memory, inference, or association intervening. Accordingly, if we
suppose the nexus between neural process in the sense-organ, on the one
hand, and conscious affection, on the other, to be by nature uniform,
_the same process ought always to give the same sensation;_ and
conversely, _if what seems to be a sensation varies whilst the process
in the sense-organ remains unchanged, the reason is presumably that
it is really not a sensation but a higher mental product, whereof the
variations depend on events occurring in the system of higher cerebral
centres._
Now the _size_ of the field of view varies enormously in all three
dimensions, without our being able to assign with any definiteness the
process in the visual tract on which the variation depends. We just
saw how impossible such assignment was in the case where turning down
the head produces the enlargement. In general, the maximum feeling of
depth or distance seems to take the lead in determining the apparent
magnitude of the whole field, and the two other dimensions seem to
follow. If, to use the former instance, I look close into a wash-basin,
the lateral extent of the field shrinks proportionately to its
nearness. If I look from a mountain, the things seen are vast in height
and breadth, in proportion to the farness of the horizon. But _when we
ask what changes in the eye determine how great this maximum feeling of
depth or distance_ (which is undoubtedly felt as a unitary vastness)
_shall be, we find ourselves unable to point to any one of them as
being its absolutely regular concomitant._ Convergence, accommodation,
double and disparate images, differences in the parallactic
displacement when we move our head, faintness of tint, dimness of
outline, and smallness of the retinal image of objects named and known,
are all processes that have _something to do_ with the perception of
'far' and of 'near'; but the effect of each and any one of them in
determining such a perception at one moment may at another moment be
reversed by the presence of some other sensible quality in the object,
that makes us, evidently by reminding us of past experience, judge it
to be at a different distance and of another shape. If we paint the
inside of a pasteboard-mask like the outside, and look at it with one
eye, the accommodation- and parallax-feelings are there, but fail to
make us see it hollow, as it is. Our mental knowledge of the fact that
human faces are always convex overpowers them, and we directly perceive
the nose to be nearer to us than the cheek instead of farther of.
The other organic tokens of farness and nearness are proved by similar
experiments (of which we shall ere long speak more in detail) to have
an equally fluctuating import. They lose all their value whenever the
collateral circumstances favor a strong intellectual conviction that
the object presented to the gaze is _improbable_--cannot be either
_what_ or _where_ they would make us perceive it to be.
Now the query immediately arises: _Can the feelings of these processes
in the eye, since they are so easily neutralized and reversed by
intellectual suggestions, ever have been direct sensations of distance
at all_? Ought we not rather to assume, since the distances which
we see _in spite_ of them are conclusions from past experience,
that the distances which we see _by means_ of them are equally
such conclusions? Ought we not, in short, to say unhesitatingly
that distance must be an intellectual and not a sensible content of
consciousness? and that each of these eye-feelings serves as a mere
signal to awaken this content, our intellect being so framed that
sometimes it notices one signal more readily and sometimes another?
Reid long ago (Inquiry, c. vi. sec. 17) said:
"It may be taken for a general rule that things which are produced by
custom may be undone or changed by disuse or by contrary custom. On
the other hand, it is a strong argument that an effect is not owing to
custom, but to the constitution of nature, when a contrary custom is
found neither to change nor to weaken it."
More briefly, a way of seeing things that can be unlearned was
presumably learned, and only what we cannot unlearn is instinctive.
This seems to be Helmholtz's view, for he confirms Reid's maxim by
saying in emphatic print:
"No elements in our perception can be sensational which may be
overcome or reversed by factors of demonstrably experimental origin.
Whatever can be overcome by suggestions of experience must be regarded
as itself a product of experience and custom. If we follow this rule
it will appear that only _qualities_ are sensational, whilst almost
all _spatial_ attributes are results of habit and experience."[212]
This passage of Helmholtz's has obtained, it seems to me, an almost
deplorable celebrity. The reader will please observe its very radical
import. Not only would he, and does he, for the reasons we have just
been ourselves considering, deny distance to be an optical sensation;
but, extending the same method of criticism to judgments of size,
shape, and direction, and finding no single retinal or muscular process
in the eyes to be indissolubly linked with any one of these, he goes
so far as to say that all optical space-perceptions whatsoever must
have an intellectual origin, and a content that no items of visual
sensibility can account for.[213]
As Wundt and others agree with Helmholtz here, and as their
conclusions, if true, are irreconcilable with all the sensationalism
which I have been teaching hitherto, it clearly devolves upon me to
defend my position against this new attack. But as this chapter on
Space is already so overgrown with episodes and details, I think it
best to reserve the refutation of their general principle for the
next chapter, and simply to assume at this point its untenability.
This has of course an arrogant look; but if the reader will bear with
me for not very many pages more, I shall hope to appease his mind.
Meanwhile I affirm confidently that _the same outer objects actually_
FEEL _different to us according as our brain reacts on them in one
way or another by making us perceive them as this or as that sort of
thing._ So true is this that one may well, with Stumpf,[214] reverse
Helmholtz's query, and ask: "What would become of our sense-perceptions
in case experience were _not_ able so to transform them?" Stumpf adds:
"All wrong perceptions that depend on peculiarities in the organs
are more or less perfectly corrected by the influence of imagination
following the guidance of experience."
If, therefore, among the facts of optical space-perception (which we
must now proceed to consider in more detail) we find instances of an
identical organic eye-process, giving us different perceptions at
different times, in consequence of different collateral circumstances
suggesting different objective facts to our imagination, we must
not hastily conclude, with the school of Helmholtz and Wundt, that
the organic eye-process pure and simple, without the collateral
circumstances, is incapable of giving us any sensation of a spatial
kind at all. We must rather seek to discover _by what means_ the
circumstances can so have transformed a space-sensation, which, but for
their presence, would probably have been felt in its natural purity.
And I may as well say now in advance that we shall find the means
to be nothing more or less than association--_the suggestion to the
mind of optical objects not actually present,_ but more habitually
associated with the 'collateral circumstances' than the sensation which
they now displace and being imagined now with a quasi-hallucinatory
strength. But before this conclusion emerges, it will be necessary to
have reviewed the most important facts of optical space-perception,
in relation to the organic conditions on which they depend. Readers
acquainted with German optics will excuse what is already familiar to
them in the following section.[215]
Let us begin the long and rather tedious inquiry by the most important
case. Physiologists have long sought for a simple law by which to
connect the seen direction and distance of objects with the retinal
impressions they produce. Two principal theories have been held of
this matter, the 'theory of identical points,' and the 'theory of
projection,'--each incompatible with the other, and each beyond certain
limits becoming inconsistent with the facts.
_The Theory of Identical Points_.
[Illustration: FIG. 54.]
This theory starts from the truth that on both retinæ an impression on
the upper half makes us perceive an object as below, on the lower half
as above, the horizon; and on the right half an object to the left, on
the left half one to the right, of the median line. Thus each quadrant
of one retina corresponds as a whole to the _similar_ quadrant of the
other; and within two similar quadrants, _al_ and _ar_ for example,
there should, if the correspondence were consistently carried out, be
geometrically similar points which, if impressed at the same time by
light emitted from the same object, should cause that object to appear
in the same direction to either eye. Experiment verifies this surmise.
If we look at the starry vault with parallel eyes, the stars all seem
single; and the laws of perspective show that under the circumstances
the parallel light-rays coming from each star must impinge on points
within either retina which _are_ geometrically similar to each other.
The same result may be more artificially obtained. If we take two
exactly similar pictures, smaller, or at least no larger, than those on
an ordinary stereoscopic slide, and if we look at them as stereoscopic
slides are looked at, that is, at one with each eye (a median partition
confining the view of either eye to the picture opposite it), we
shall see but one flat picture, all of whose parts appear sharp and
single.[216] Identical points being impressed, both eyes see their
object in the same direction, and the two objects consequently coalesce
into one.
The same thing may be shown in still another way. With fixed head
converge the eyes upon some conspicuous objective point behind a pane
of glass; then close either eye alternately and make a little ink-mark
on the glass, 'covering' the object as seen by the eye which is
momentarily open. On looking now with both eyes the ink-marks will seem
single, and in the same direction as the objective point. Conversely,
let the eyes converge on a single ink-spot on the glass, and then by
alternate shutting of them let it be noted what objects behind the
glass the spot covers to the right and left eye respectively. Now with
both eyes open, both these objects and the spot will appear in the same
place, one or other of the three becoming more distinct according to
the fluctuations of retinal attention.[217]
Now what is the direction of this common place? The only way of
defining the direction of an object is by _pointing to it_. Most
people, if asked to look at an object over the horizontal edge of a
sheet of paper which conceals their hand and arm, and then to point
their finger at it (raising the hand gradually so that at last a
finger-tip will appear above the sheet of paper), are found to place
the finger not between either eye and the object, but between the
latter and the root of the nose, and this whether both eyes or either
alone be used. Hering and Helmholtz express this by saying that we
judge of the direction of objects as they would appear to an imaginary
cyclopean eye, situated between our two real eyes, and with its optical
axis bisecting the angle of convergence of the latter. Our two retinæ
act, according to Hering, as if they were superposed in the place of
this imaginary double-eye; we see by the corresponding points of each,
situated far asunder as they really are, just as we _should_ see if
they were superposed and could both be excited together.
The judgment of objective singleness and that of identical direction
seem to hang necessarily together. And that of identical direction
seems to carry with it the necessity of a common origin, between the
eyes or elsewhere, from which all the directions felt may seem to be
estimated. This is why the cyclopean eye is really a fundamental part
of the formulation of the theory of identical retinal points, and why
Hering, the greatest champion of this theory, lays so much stress upon
it.
_It is an immediate consequence of the law of identical_ _projection
of images on geometrically similar points that images which fall upon
geometrically_ DISPARATE _points of the two retinæ should be projected
in_ DISPARATE _directions, and that their objects should consequently
appear in_ TWO _places, or_ LOOK DOUBLE. Take the parallel rays from
a star falling upon two eyes which converge upon a near object, O,
instead of being parallel, as in the previously instanced case. If SL
and SR in Fig. 55 be the parallel rays, each of them will fall upon the
nasal half of the retina which it strikes.
[Illustration: FIG. 55.]
But the two nasal halves are disparate, geometrically _symmetrical_,
not geometrically _similar_. The image on the left one will therefore
appear as if lying in a direction leftward of the cyclopean eye's
line of sight; the image of the right one will appear far to the
right of the same direction. The star will, in short, be seen
double,--'homonymously' double.
Conversely, if the star be looked at directly with parallel axes, O
will be seen double, because its images will affect the outer or cheek
halves of the two retinæ, instead of one outer and one nasal half. The
position of the images will here be reversed from that of the previous
case. The right eye's image will now appear to the left, the left
eye's to the right--the double images will be 'heteronymous.'
The same reasoning and the same result ought to apply where the
object's place with respect to the direction of the two optic axes is
such as to make its images fall not on non-similar retinal halves, but
on non-similar parts of similar halves. Here, of course, the directions
of projection will be less widely disparate than in the other case, and
the double images will appear to lie less widely apart.
Careful experiments made by many observers according to the so-called
haploscopic method confirm this law, and show that _corresponding
points, of single visual direction,_ exist upon the two retinæ. For the
detail of these one must consult the special treatises.
Note now an important consequence. If we take a stationary object
and allow the eyes to vary their direction and convergence, a purely
geometrical study will show that there will be some positions in which
its two images impress corresponding retinal points, but more in which
they impress disparate points. The former constitute the so-called
horopter, and their discovery has been attended with great mathematical
difficulty. Objects or parts of objects which lie in the eyes' horopter
at any given time cannot appear double. _Objects lying out of the
horopter would seem, if the theory of identical points were strictly
true, necessarily and always to appear double._
Here comes the first great conflict of the identity-theory with
experience. Were the theory true, we ought all to have an intuitive
knowledge of the horopter as the line of distinctest vision. Objects
placed elsewhere ought to seem, if not actually double, at least
blurred. And yet no living man makes any such distinction between the
parts of his field of vision. To most of us the whole field appears
single, and it is only by rare accident or by special education that
we ever catch a glimpse of a double image. In 1838, Wheatstone, in his
truly classical memoir on binocular vision and the stereoscope,[218]
showed that the disparateness of the points on which the two images
of an object fall does not within certain limits affect its seen
singleness at all, but rather the _distance_ at which it shall appear.
Wheatstone made an observation, moreover, which subsequently became the
bone of much hot contention, in which he strove to show that not only
might disparate images fuse, but images on corresponding or identical
points might be seen double.[219]
I am unfortunately prevented by the weakness of my own eyes from
experimenting enough to form a decided personal opinion on the matter.
It seems to me, however, that the balance of evidence is against the
Wheatstonian interpretation, and that disparate points may fuse,
without identical points for that reason ever giving double images. The
two questions, "Can we see single with disparate points?" and "Can we
see double with identical points?" although at the first blush they may
appear, as to Helmholtz they appear, to be but two modes of expressing
the same inquiry, are in reality distinct. The first may quite well be
answered affirmatively and the second negatively.
Add to this that the experiment quoted from Helmholtz above by no
means always succeeds, but that many individuals place their finger
between the object and _one_ of their eyes, oftenest the right;[220]
finally, observe that the identity-theory, with its Cyclopean starting
point for all lines of direction, gives by itself no ground for the
_distance_ on any line at which an object shall appear, and has to
be helped out in this respect by subsidiary hypotheses, which, in
the hands of Hering and others, have become so complex as easily to
fall a prey to critical attacks; and it will soon seem as if _the
law of identical seen directions by corresponding points, although a
simple formula for expressing concisely many fundamental phenomena,
is by no means an adequate account of the whole matter of retinal
perception._[221]
_The Projection-Theory_.
Does the theory of projection fare any better? This theory admits that
each eye sees the object in a different direction from the other,
along the line, namely, passing from the object through the middle of
the pupil to the retina. A point directly fixated is thus seen on the
optical axes of both eyes. There is only one point, however, which
these two optical axes have in common, and that is the point to which
they converge. Everything directly looked at is seen at this point, and
is thus seen both single and at its proper distance. It is easy to show
the incompatibility of this theory with the theory of identity. Take an
objective point (like O in Fig. 50, when the star is looked at) casting
its images R' and L' on geometrically dissimilar parts of the two
retinæ and affecting the outer half of each eye. On the identity-theory
it ought necessarily to appear double, whilst on the projection-theory
there is no reason whatever why it should not appear single, provided
only it be located by the judgment on each line of visible direction,
neither nearer nor farther than its point of intersection with the
other line.
_Every point in the field of view ought, in truth, if the
projection-theory were uniformly valid, to appear single,_ entirely
irrespective of the varying positions of the eyes, for from every
point of space two lines of visible direction pass to the two retinæ;
and at the intersection of these lines, or just where the point is,
there, according to the theory, it should appear. _The objection to
this theory is thus precisely the reverse of the objection to the
identity-theory. If the latter ruled, we ought to see most things
double all the time. If the projection-theory ruled, we ought never to
see anything double. As a matter of fact we get too few double images
for the identity-theory, and too many for the projection-theory._
The partisans of the projection-theory, beginning with Aguilonius, have
always explained double images as the result of an erroneous judgment
of the _distance_ of the object, the images of the latter being
projected by the imagination along the two lines of visible direction
either nearer or farther than the point of intersection of the latter.
A diagram will make this clear.
[Illustration: FIG. 56.]
Let O be the point looked at, M an object farther, and N an object
nearer, than it. Then M and N will send the lines of visible direction
MM and NN to the two retinæ. If N be judged as far as O, it must
necessarily lie where the two lines of visible direction NN intersect
the plane of the arrow, or in two places, at N' and at N''. If M be
judged as near as O, it must for the same reason form two images at M'
and M''.
It is, as a matter of fact, true that we often misjudge the distance in
the way alleged. If the reader will hold his forefingers, one beyond
the other, in the median line, and fixate them alternately, he will see
the one not looked at, double; and he will also notice that it appears
nearer to the plane of the one looked at, whichever the latter may be,
than it really is. Its changes of apparent size, as the convergence
of the eyes alter, also prove the change of apparent distance. The
distance at which the axes converge seems, in fact, to exert a sort of
attraction upon objects situated elsewhere. Being the distance of which
we are most acutely sensible, it invades, so to speak, the whole field
of our perception. If two half-dollars be laid on the table an inch
or two apart, and the eyes fixate steadily the point of a pen held in
the median line at varying distances between the coins and the face,
there will come a distance at which the pen stands between the left
half-dollar and the right eye, and the right half-dollar and the left
eye. The two half-dollars will then coalesce into one; and this one
will show its apparent approach to the pen-point by seeming suddenly
much reduced in size.[222]
Yet, in spite of this tendency to inaccuracy, we are never actually
mistaken about the half-dollar being behind the pen-point. It may not
seem far enough off, but still it is farther than the point. In general
it may be said that where the objects are known to us, no such illusion
of distance occurs in any one as the theory would require. And in some
observers, Hering for example, it seems hardly to occur at all. If I
look into infinite distance and get my finger in double images, they do
not seem infinitely far off. To make objects at different distances
seem equidistant, careful precautions must be taken to have them alike
in appearance, and to exclude all outward reasons for ascribing to the
one a different location from that ascribed to the other. Thus Donders
tries to prove the law of projection by taking two similar electric
sparks, one behind the other on a dark ground, one seen double; or
an iron rod placed so near to the eyes that its double images seem
as broad as that of a fixated stove-pipe, the top and bottom of the
objects being cut off by screens, so as to prevent all suggestions of
perspective, etc. The three objects in each experiment seem in the same
plane.[223]
Add to this the impossibility, recognized by _all_ observers, of ever
seeing double with the _fovea_, and the fact that authorities as able
as those quoted in the note on Wheatstone's observation deny that
they can see double then with identical points, and we are forced to
conclude that _the projection-theory, like its predecessor, breaks
down. Neither formulates exactly or exhaustively a law for all our
perceptions._
_Ambiguity of Retinal Impressions_.
[Illustration: FIG. 57.]
_What does each theory try to do? To make of seen location a fixed
function of retinal impression. Other facts may be brought forward
to show how far from fixed are the perceptive functions of retinal
impressions._ We alluded a while ago to the extraordinary ambiguity of
the retinal image as a revealer of magnitude. Produce an after-image of
the sun and look at your finger-tip: it will be smaller than your nail.
Project it on the table, and it will be as big as a strawberry; on the
wall, as large as a plate; on yonder mountain, bigger than a house. And
yet it is an unchanged retinal impression. Prepare a sheet with the
figures shown in Fig. 57 strongly marked upon it, and get by direct
fixation a distinct after-image of each.
[Illustration: FIGS. 58 & 59.]
Project the after-image of the cross upon the upper left-hand part of
the wall, it will appear as in Fig. 58; on the upper right-hand it will
appear as in Fig. 59. The circle similarly projected will be distorted
into two different ellipses. If the two parallel lines be projected
upon the ceiling or floor far in front, the farther ends will diverge;
and if the three parallel lines be thrown on the same surfaces, the
upper pair will seem farther apart than the lower.
[Illustration: FIG. 60.]
[Illustration: FIG. 61.]
Adding certain lines to others has the same distorting effect. In
what is known as Zöllner's pattern (Fig. 60), the long parallels tip
towards each other the moment we draw the short slanting lines over
them yet their retinal images are the same they always were. A similar
distortion of parallels appears in Fig 61.
[Illustration: FIG. 62.]
[Illustration: FIG. 63.]
Drawing a square inside the circle (Fig. 62) gives to the outline of
the latter an indented appearance where the square's corners touch
it. Drawing the radii inside of one of the right angles in the same
figure makes it seem larger than the other. In Fig. 63, the retinal
image of the space between the extreme dots is in all three lines the
same, yet it seems much larger the moment it is filled up with other
dots.
In the stereoscope certain pairs of lines which look single under
ordinary circumstances immediately seem double when we add certain
other lines to them.[224]
_Ambiguous Import of Eye-movements_.
These facts show the indeterminateness of the space-import of various
_retinal impressions_. Take now the _eye's movements_, and we find a
similar vacillation. When we follow a moving object with our gaze, the
motion is 'voluntary'; when our eyes oscillate to and fro after we have
made ourselves dizzy by spinning around, it is 'reflex'; and when the
eyeball is pushed with the finger, it is 'passive.' Now, in all three
of these cases we get a feeling from the movement as it effects itself.
But the objective perceptions to which the feeling assists us are by no
means the same. In the first case we may see a stationary field of view
with one moving object in it; in the second, the total field swimming
more or less steadily in one direction; in the third, a sudden jump or
twist of the same total field.
_The feelings of convergence_ of the eyeballs permit of the same
ambiguous interpretation. When objects are near we converge strongly
upon them in order to see them; when far, we set our optic axes
parallel. But the exact degree of convergence fails to be felt; or
rather, being felt, fails to tell us the absolute distance of the
object we are regarding. Wheatstone arranged his stereoscope in such
a way that the size of the retinal images might change without the
convergence altering; or conversely, the convergence might change
without the retinal image altering. Under these circumstances, he
says,[225] the object seemed to approach or recede in the first
case, without altering its size, in the second, to change its size
without altering its distance--just the reverse of what might have
been expected. Wheatstone adds, however, that 'fixing the attention'
converted each of these perceptions into its opposite. The same
perplexity occurs in looking through prismatic glasses, which alter the
eyes' convergence. We cannot decide whether the object has come nearer,
or grown larger, or both, or neither; and our judgment vacillates in
the most surprising way. We may even make our eyes diverge, and the
object will none the less appear at a finite distance. When we look
through the stereoscope, the picture seems at no determinate distance.
These and other facts have led Helmholtz to deny that the feeling of
convergence has any very exact value as a distance-measurer.[226]
With _the feelings of accommodation_ it is very much the same. Donders
has shown[227] that the apparent magnifying power of spectacles of
moderate convexity hardly depends at all upon their enlargement of the
retinal image, but rather on the relaxation they permit of the muscle
of accommodation. This suggests an object farther off, and consequently
a much larger one, since its retinal size rather increases than
diminishes. But in this case the same vacillation of judgment as in the
previously mentioned case of convergence takes place. The recession
made the object seem larger, but the apparent growth in size of the
object now makes it look as if it came nearer instead of receding. The
effect thus contradicts its own cause. Everyone is conscious, on first
putting on a pair of spectacles, of a doubt whether the field of view
draws near or retreats.[228]
There is still _another deception, occurring in persons who have had
one eye-muscle suddenly paralyzed._ This deception has led Wundt to
affirm that the eyeball-feeling proper, the incoming sensation of
effected rotation, tells us only of the direction of our eye-movements,
but not of their whole extent.[229] For this reason, and because
not only Wundt, but many other authors, think the phenomena in
these partial paralyses demonstrate the existence of a feeling of
innervation, a feeling of the outgoing nervous current, opposed to
every afferent sensation whatever, it seems proper to note the facts
with a certain degree of detail.
Suppose a man wakes up some morning with the external rectus muscle
of his right eye half paralyzed, what will be the result? He will be
enabled only with great effort to rotate the eye so as to look at
objects lying far off to the right. Something in the effort he makes
will make him feel as if the object lay much farther to the right than
it really is. If the left and sound eye be closed, and he be asked to
touch rapidly with his finger an object situated towards his right, he
will point the finger to the right of it. The current explanation of
the 'something' in the effort which causes this deception is that it
is the sensation of the outgoing discharge from the nervous centres,
the 'feeling of innervation,' to use Wundt's expression, requisite for
bringing the open eye with its weakened muscle to bear upon the object
to be touched. If that object be situated 20 degrees to the right, the
patient has now to innervate as powerfully to turn the eye those 20
degrees as formerly he did to turn the eye 30 degrees. He consequently
believes as before that he _has_ turned it 30 degrees; until, by a
newly-acquired custom, he learns the altered spatial import of all the
discharges his brain makes into his right abducens nerve. The 'feeling
of innervation,' maintained to exist by this and other observations,
plays an immense part in the space-theories of certain philosophers,
especially Wundt. I shall elsewhere try to show that the observations
by no means warrant the conclusions drawn from them, and that the
feeling in question is probably a wholly fictitious entity.[230]
Meanwhile it suffices to point out that even those who set most store
by it are compelled, by the readiness with which the translocation
of the field of view becomes corrected and further errors avoided,
to admit that the precise space-import of _the supposed sensation of
outgoing energy is as ambiguous and indeterminate as that of any other
of the eye-feelings we have considered hitherto._
* * * * *
I have now given what no one will call an understatement of the facts
and arguments by which it is sought to banish the credit of directly
revealing space from each and every kind of eye-sensation taken by
itself. The reader will confess that they make a very plausible show,
and most likely wonder whether my own theory of the matter can rally
from their damaging evidence. But the case is far from being hopeless;
and the introduction of a discrimination hitherto unmade will, if I
mistake not, easily vindicate the view adopted in these pages, whilst
at the same time it makes ungrudging allowance for all the ambiguity
and illusion on which so much stress is laid by the advocates of the
intellectualist-theory.
_The Choice of the Visual Reality._
We _have_ native and fixed optical space-sensations; _but experience
leads us to select certain ones from among them to be the exclusive
bearers of reality: the rest become mere signs and suggesters of
these._ The factor of _selection_, on which we have already laid so
much stress, here as elsewhere is the solving word of the enigma. If
Helmholtz, Wundt, and the rest, with an ambiguous retinal sensation
before them, meaning now one size and distance, and now another, had
not contented themselves with merely saying:--The size and distance
are not this sensation, they are something beyond it which it merely
calls up, and whose own birthplace is afar--in 'synthesis' (Wundt) or
in 'experience' (Helmholtz) as the case may be; if they had gone on
definitely to ask and definitely to answer the question, What are the
size and distance in their proper selves? they would not only have
escaped the present deplorable vagueness of their space-theories, but
they would have seen that the objective spatial attributes 'signified'
are simply and solely _certain other optical sensations now absent_,
but which the present sensations suggest.
What, for example, is the slant-legged cross which we think we see on
the wall when we project the rectangular after-image high up towards
our right or left (Figs. 58 and 59)? Is it not in very sooth a retinal
sensation itself? An imagined sensation, not a felt one, it is true,
but none the less essentially and originally sensational or retinal
for that,--the sensation, namely, which we should receive if a 'real'
slant-legged cross stood on the wall _in front of us_ and threw its
image on our eye. That image is not the one our retina now holds.
Our retina now holds the image which a cross of square shape throws
when in front, but which a cross of the slant-legged pattern _would_
throw, provided it were actually on the wall in the distant place at
which we look. Call this actual retinal image the 'square' image. The
square image is then one of the innumerable images the slant-legged
cross can throw. Why should another one, and that an absent one, of
those innumerable images be picked out to represent exclusively the
slant-legged cross's 'true' shape? Why should that absent and imagined
slant-legged image displace the present and felt square image from our
mind? Why, when the objective cross gives us so many shapes, as it
varies its position, should we think we feel the true shape only when
the cross is directly in front? And when that question is answered,
how can the absent and represented feeling of a slant-legged figure so
successfully intrude itself into the place of a presented square one?
Before answering either question, let us be doubly sure about our
facts, and see how true it is that _in our dealings with objects we
always do pick out one of the visual images they yield, to constitute
the real form or size._
The matter of size has been already touched upon, so that no more need
be said of it here. As regards shape, almost all the retinal shapes
that objects throw are perspective 'distortions.' Square table-tops
constantly present two acute and two obtuse angles; circles drawn on
our wall-papers, our carpets, or on sheets of paper, usually show
like ellipses; parallels approach as they recede; human bodies are
foreshortened; and the transitions from one to another of these
altering forms are infinite and continual. Out of the flux, however,
one phase always stands prominent. It is the form the object has when
we see it easiest and best: and that is when our eyes and the object
both are in what may be called _the normal position_. In this position
our head is upright and our optic axes either parallel or symmetrically
convergent; the plane of the object is perpendicular to the visual
plane; and if the object is one containing many lines it is turned so
as to make them, as far as possible, either parallel or perpendicular
to the visual plane. In this situation it is that we compare all shapes
with each other; here every exact measurement and decision is made.[231]
_It is very easy to see why the normal situation should have this
extraordinary pre-eminence._ First, it is the position in which we
easiest hold anything we are examining in our hands; second, it is a
turning-point between all right- and all left-hand perspective views
of a given object; third, it is the only position in which symmetrical
figures seem symmetrical and equal angles seem equal; fourth, it is
often that starting-point of movements from which the eye is least
troubled by axial rotations, by which _superposition_[232] of the
retinal images of different lines and different parts of the same line
is easiest produced, and consequently by which the eye can make the
best comparative measurements in its sweeps. All these merits single
the normal position out to be chosen. No other point of view offers
so many æsthetic and practical advantages. Here we believe we see
the object as it _is_; elsewhere, only as it seems. Experience and
custom soon teach us, however, that the seeming appearance passes into
the real one by continuous gradations. They teach us, moreover, that
seeming and being may be strangely interchanged. Now a real circle may
slide into a seeming ellipse; now an ellipse may, by sliding in the
same direction, become a seeming circle; now a rectangular cross grows
slant-legged; now a slant-legged one grows rectangular.
Almost any form in oblique vision may be thus a derivative of almost
any other in 'primary' vision; and we must learn, when we get one of
the former appearances, to translate it into the appropriate one of
the latter class; we must learn of what optical 'reality' it is one
of the optical signs. Having learned this, we do but obey that law
of economy or simplification which dominates our whole psychic life,
when we attend exclusively to the 'reality' and ignore as much as our
consciousness will let us the 'sign' by which we came to apprehend it.
The signs of each probable real thing being multiple and the thing
itself one and fixed, we gain the same mental relief by abandoning the
former for the latter that we do when we abandon mental images, with
all their fluctuating characters, for the definite and unchangeable
_names_ which they suggest. The selection of the several 'normal'
appearances from out of the jungle of our optical experiences, to
serve as the real sights of which we shall think, is psychologically a
parallel phenomenon to the habit of thinking in words, and has a like
use. Both are substitutions of terms few and fixed for terms manifold
and vague.
_Sensations which we Ignore._
This service of sensations as mere signs, to be ignored when they have
evoked the other sensations which are their significates, was noticed
first by Berkeley and remarked in many passages, as the following:
"Signs, being little considered in themselves, or for their own sake,
but only in their relative capacity and for the sake of those things
whereof they are signs, it comes to pass that the mind overlooks them,
so as to carry its attention immediately on to the things signified ...
which in truth and strictness are not _seen_, but only _suggested_ and
_apprehended_ by means of the proper objects of sight which alone are
seen." (Divine Visual Language, § 12.)
Berkeley of course erred in supposing that the thing suggested was not
even _originally_ an object of sight, as the sign now is which calls it
up. Reid expressed Berkeley's principle in yet clearer language:
"The visible appearances of objects are intended by nature only as
signs or indications, and the mind passes instantly to the things
signified, without making the least reflection upon the sign, or even
perceiving that there is any such thing.... The mind has acquired a
confirmed and inveterate habit of inattention to them (the signs). For
they no sooner appear than, quick as lightning, the thing signified
succeeds and engrosses all our regard. They have no name in language;
and although we are conscious of them when they pass through the mind,
yet their passage is so quick and so familiar that it is absolutely
unheeded; nor do they leave any footsteps of themselves, either in the
memory or imagination." (Inquiry, chap. v. §§ 2, 3.)
If we review the facts we shall find every grade of non-attention
between the extreme form of overlooking mentioned by Reid (or forms
even more extreme still) and complete conscious perception of the
sensation present. Sometimes it is literally impossible to become aware
of the latter. Sometimes a little artifice or effort easily leads us to
discern it together, or in alternation, with the 'object' it reveals.
Sometimes the present sensation is held to _be_ the object or to
reproduce its features in undistorted shape, and _then_, of course, it
receives the mind's full glare.
The deepest inattention is to subjective optical sensations, strictly
so called, or those which are not signs of outer objects at all.
Helmholtz's treatment of these phenomena, _muscæ volitantes_, negative
after-images, double images, etc., is very satisfactory. He says:
"We only attend with any ease and exactness to our sensations in
so far forth as they can be utilized for the knowledge of outward
things; and we are accustomed to neglect all those portions of them
which have no significance as regards the external world. So much is
this the case that for the most part special artifices and practice
are required for the observation of these latter more subjective
feelings. Although it might seem that nothing should be easier than
to be conscious of one's own sensations, experience nevertheless
shows that often enough either a special talent like that showed in
eminent degree by Purkinje, or accident or theoretic speculation,
are necessary conditions for the discovery of subjective phenomena.
Thus, for example, the blind spot on the retina was discovered by
Mariotte by the theoretic way; similarly by me the existence of
'summation'-tones in acoustics. In the majority of cases accident is
what first led observers whose attention was especially exercised
on subjective phenomena to discover this one or that; only where
the subjective appearances are so intense that they interfere with
the perception of objects are they noticed by all men alike. But
if they have once been discovered it is for the most part easy for
subsequent observers who place themselves in proper conditions and
bend their attention in the right direction to perceive them. But
in many cases--for example, in the phenomena of the blind spot,
in the discrimination of over-tones and combination-tones from the
ground-tone of musical sounds, etc.--such a strain of the attention
is required, even with appropriate instrumental aids, that most
persons fail. The very after-images of bright objects are by most
men perceived only under exceptionally favorable conditions, and it
takes steady practice to see the fainter images of this kind. It
is a commonly recurring experience that persons smitten with some
eye-disease which impairs vision suddenly remark for the first time
the _muscæ volitantes_ which all through life their vitreous humor
has contained, but which they now firmly believe to have arisen since
their malady; the truth being that the latter has only made them
more observant of all their visual sensations. There are also cases
where one eye has gradually grown blind, and the patient lived for
an indefinite time without knowing it, until, through the accidental
closure of the healthy eye alone, the blindness of the other was
brought to attention.
"Most people, when first made aware of binocular double images, are
uncommonly astonished that they should never have noticed them before,
although all through their life they had been in the habit of seeing
singly only those few objects which were about equally distant with
the point of fixation, and the rest, those nearer and farther, which
constitute the great majority, had always been double.
"We must then _learn_ to turn our attention to our particular
sensations, and we learn this commonly only for such sensations
as are means of cognition of the outer world. Only so far as they
serve this end have our sensations any importance for us in ordinary
life. Subjective feelings are mostly interesting only to scientific
investigators; were they remarked in the ordinary use of the
senses, they could only cause disturbance. Whilst, therefore, we
reach an extraordinary degree of firmness and security in objective
observation, we not only do not reach this where subjective phenomena
are concerned, but we actually attain in a high degree the faculty
of overlooking these altogether, and keeping ourselves independent
of their influence in judging of objects, even in cases where their
strength might lead them easily to attract our attention." (Physiol.
Optik, pp. 431-2.)
Even where the sensation is not merely subjective, as in the cases of
which Helmholtz speaks, but is a sign of something outward, we are also
liable, as Reid says, to overlook its intrinsic quality and attend
exclusively to the image of the 'thing' it suggests. But here everyone
_can_ easily notice the sensation itself if he will. Usually we see
a sheet of paper as uniformly white, although a part of it may be in
shadow. But we can in an instant, if we please, notice the shadow as
local color. A man walking towards us does not usually seem to alter
his size; but we can, by setting our attention in a peculiar way
make him appear to do so. The whole education of the artist consists
in his learning to see the presented signs as well as the represented
things. No matter what the field of view _means_, he sees it also
as it _feels_--that is, as a collection of patches of color bounded
by lines--the whole forming an optical diagram of whose intrinsic
proportions one who is not an artist has hardly a conscious inkling.
The ordinary man's attention passes _over_ them to their import; the
artist's turns back and dwells _upon_ them for their own sake. 'Don't
draw the thing as it _is_, but as it _looks_!' is the endless advice
of every teacher to his pupil; forgetting that what it 'is' is what
it would also 'look,' provided it were placed in what we have called
the 'normal' situation for vision. In this situation the sensation as
'sign' and the sensation as 'object' coalesce into one, and there is no
contrast between them.
_Sensations which seem Suppressed._
But a great difficulty has been made of certain peculiar cases which
we must now turn to consider. They are _cases in which a present
sensation, whose existence is supposed to be proved by its outward
conditions being there, seems absolutely suppressed or changed by the
image of the 'thing' it suggests._
This matter carries us back to what was said on p. 218. The passage
there quoted from Helmholtz refers to these cases. He thinks they
conclusively disprove the original and intrinsic spatiality of any of
our retinal sensations; for if such a one, actually present, had an
immanent and essential space-determination of its own, that might well
be added to and overlaid or even momentarily eclipsed by suggestions of
its signification, but how could it possibly be altered or completely
_suppressed_ thereby? Of actually present sensations, he says, being
_suppressed_ by suggestions of experience--
"We have not a single well-attested example. In all those illusions
which are provoked by _sensations_ in the absence of their usually
exciting objects, the mistake never vanishes by the better
understanding of the object really present, and by insight into the
cause of deception. Phosphenes provoked by pressure on the eyeball,
by traction on the entrance of the optic nerve, after-images, etc.,
remain projected into their apparent place in the field of vision,
just as the image projected from a mirror's surface continues to
be seen _behind_ the mirror, although we _know_ that to all these
appearances no outward reality corresponds. True enough, we can remove
our attention, and keep it removed, from sensations that have no
reference to the outer world, those, e.g., of the weaker after-images,
and of entoptic objects, etc.... But what would become of our
perceptions at all if we had the power not only of ignoring, but of
_transforming into their opposites_, any part of them that differed
from that outward experience, the image of which, as that of a present
reality, accompanies them in the mind?"[233]
And again:
"On the analogy of all other experience, we should expect that the
conquered feelings would persist to our perception, even if only in
the shape of recognized illusions. But this is not the case. One
does not see how the assumption of originally spatial sensations can
explain our optical cognitions, when in the last resort those who
believe in these very sensations find themselves obliged to assume
that they are _overcome_ by our better judgment, based on experience."
These words, coming from such a quarter, necessarily carry great
weight. But the authority even of a Helmholtz ought not to shake one's
critical composure. And the moment one abandons abstract generalities
and comes to close quarters with the particulars, I think one easily
sees that no such conclusions as those we have quoted follow from the
latter. But profitably to conduct the discussion _we must divide the
alleged instances into groups._
* * * * *
(a) With Helmholtz, _color-perception_ is equally with space-perception
an intellectual affair. The so-called simultaneous color-contrast, by
which one color modifies another alongside of which it is said, is
explained by him as an unconscious inference. In Chapter XVII we
discussed the color-contrast problem; the principles which applied to
its solution will prove also applicable to part of the present problem.
In my opinion, Hering has definitively proved that, when one color is
laid beside another, it modifies the sensation of the latter, not by
virtue of any mere mental suggestion, as Helmholtz would have it, but
by actually exciting a new nerve-process, to which the modified feeling
of color immediately corresponds. The explanation is physiological,
not psychological. The transformation of the original color by the
inducing color is due to the disappearance of the physiological
conditions under which the first color was produced, and to the
induction, under the new conditions, of a genuine new sensation, with
which the 'suggestions of experience' have naught to do.
[Illustration: FIG. 64.]
That processes in the visual apparatus propagate themselves laterally,
if one may so express it, is also shown by the _phenomena of contrast
which occur after looking upon motions_ of various kinds. Here are a
few examples. If, over the rail of a moving vessel, we look at the
water rushing along the side, and then transfer our gaze to the deck,
a band of planks will appear to us, moving in the opposite direction
to that in which, a moment previously, we had been seeing the water
move, whilst on either side of this band another band of planks will
move as the water did. Looking at a waterfall, or at the road from out
of a car-window in a moving tram, produces the same illusion, which may
be easily verified in the laboratory by a simple piece of apparatus.
A board with a window five or six inches wide and of any convenient
length is supported upright on two feet. On the back side of the board,
above and below the window, are two rollers, one of which is provided
with a crank. An endless band of any figured stuff is passed over these
rollers (one of which can be so adjusted on its bearings as to keep the
stuff always taut and not liable to slip), and the surface of the front
board is also covered with stuff or paper of a nature to catch the
eye. Turning the crank now sets the central band in continuous motion,
whilst the margins of the field remain really at rest, but after a
while appear moving in the contrary way. Stopping the crank results in
an illusory appearance of motion in reverse directions all over the
field.
A disk with an Archimedean spiral drawn upon it, whirled round on an
ordinary rotating machine, produces still more startling effects.
[Illustration: FIG. 65.]
"If the revolution is in the direction in which the spiral line
approaches the centre of the disk the entire surface of the latter
seems to expand during revolution and to contract after it has ceased;
and _vice versâ_ if the movement of revolution is in the opposite
direction. If in the former case the eyes of the observers are turned
from the rotating disk towards any familiar object--e.g. the face of a
friend--the latter seems to contract or recede in a somewhat striking
manner, and to expand or approach after the opposite motion of the
spiral."[234]
[Illustration: FIG. 66.]
An elementary form of these motor illusions seems to be the one
described by Helmholtz on pp. 568-571 of his Optik. The motion of
anything in the field of vision along an acute angle towards a straight
line sensibly distorts that line. Thus in Fig. 66: Let AB be a line
drawn on paper, CDE the tracing made over this line by the point of a
compass steadily followed by the eye, as it moves. As the compass-point
passes from C to D, the line appears to move downwards; as it passes
from D to E, the line appears to move upwards; at the same time the
whole line seems to incline itself in the direction FG during the first
half of the compass's movement; and in the direction HI during its last
half; the change from one inclination to another being quite distinct
as the compass-point passes over D.
Any line across which we draw a pencil-point appears to be animated by
a rapid movement of its own towards the pencil-point. This apparent
movement of both of two things in relative motion to each other, even
when one of them is absolutely still, reminds us of the instances
quoted from Vierordt on page 188, and seems to take us back to a
primitive stage of perception, in which the discriminations we now
make when we feel a movement have not yet been made. If we draw the
point of a pencil through 'Zöllner's pattern' (Fig. 60, p. 232), and
follow it with the eye, the whole figure becomes the scene of the most
singular apparent unrest, of which Helmholtz has very carefully noted
the conditions. The illusion of Zöllner's figure vanishes entirely,
or almost so, with most people, if they steadily look at one point
of it with an unmoving eye; and the same is the case with many other
illusions.
_Now all these facts taken together seem to show_--vaguely it is true,
but certainly--_that present excitements and after-effects of former
excitements may alter the result of processes occurring simultaneously
at a distance from them in the retina_ or other portions of the
apparatus for optical sensation. In the cases last considered, the
moving eye, as it sweeps the fovea over certain parts of the figure,
seems thereby to determine a modification in the feeling which the
_other_ parts confer, which modification is the figure's 'distortion.'
It is true that this statement explains nothing. It only keeps the
cases to which it applies from being explained spuriously. _The
spurious account of these illusions is that they are intellectual, not
sensational, that they are secondary, not primary, mental facts._ The
distorted figure is said to be one which the mind is led to _imagine_,
by falsely drawing an unconscious inference from certain premises of
which it is not distinctly aware. And the imagined figure is supposed
to be strong enough to suppress the perception of whatever real
sensations there may be. But Helmholtz, Wundt, Delbœuf, Zöllner, and
all the advocates of unconscious inference are at variance with each
other when it comes to the question what these unconscious premises and
inferences may be.
[Illustration: FIG. 67.]
[Illustration: FIG. 68.]
That small angles look proportionally larger than larger ones is, in
brief, the fundamental illusion to which almost all authors would
reduce the peculiarity of Fig. 67, as of Figs. 60, 61, 62 (p. 232).
This peculiarity of small angles is by Wundt treated as the
case of a filled space seeming larger than an empty one, as in Fig.
68; and this, according to both Delbœuf and Wundt, is owing to the
fact that more muscular innervation is needed for the eye to traverse
a filled space than an empty one, because the points and lines in
the filled space inevitably arrest and constrain the eye, and this
makes us feel as if it were doing more work, i.e. traversing a longer
distance.[235] When, however, we recollect that muscular movements
are positively proved to have _no_ share in the waterfall and
revolving-spiral illusions, and that it is hard to see how Wundt's and
Delbœuf's particular form of muscle-explanation can possibly apply to
the compass-point illusion considered a moment ago, we must conclude
that these writers have probably exaggerated, to say the least, the
reach of their muscle-explanation in the case of the subdivided angles
and lines. Never do we get such strong muscular feelings as when,
against the course of nature, we oblige our eyes to be still; but
fixing the eyes on one point of the figure, so far from making that
part of the latter seem larger, dispels, in most persons, the illusion
of these diagrams altogether.
As for Helmholtz, he invokes, to explain the enlargement of small
angles,[236] what he calls a '_law of contrast_' between directions
and distances of lines, analogous to that between colors and
intensities of light. Lines cutting another line make the latter seem
more inclined away from them than it really is. Moreover, clearly
recognizable magnitudes appear greater than equal magnitudes which
we but vaguely apprehend. But this is surely a sensationalistic law,
a native function of our seeing-apparatus. Quite as little as the
negative after-image of the revolving spiral could such contrast be
deduced from any association of ideas or recall of past objects. The
principle of contrast is criticised by Wundt,[237] who says that by it
small spaces ought to appear to us smaller, and not larger, than they
really are. Helmholtz might have retorted (had not the retort been as
fatal to the uniformity of his own principle as to Wundt's) that if
the muscle-explanation were true, it ought not to give rise to just
the opposite illusions in the skin. We saw on p. 141 that subdivided
spaces appear shorter than empty ones upon the skin. To the instances
there given add this: Divide a line on paper into equal halves,
puncture the extremities, and make punctures all along one of the
halves; then, with the finger-tip on the opposite side of the paper,
follow the line of punctures; the empty half will seem much longer than
the punctured half. This seems to bring things back to unanalyzable
laws, by reason of which our feeling of size is determined differently
in the skin and in the retina, even when the objective conditions are
the same. Hering's explanation of Zöllner's figure is to be found in
Hermann's Handb. d. Physiologie, iii. 1. p. 579. Lipps[238] gives
another reason why lines cutting another line make the latter seem to
bend away from them more than is really the case. If, he says, we draw
(Fig. 69) the line _pm_ upon the line _ab_, and follow the latter with
our eye, we shall, on reaching the point _m_, tend for a moment to slip
off _ab_ and to follow _mp_, without distinctly realizing that we are
not still on the main line. This makes us feel as if the remainder _mb_
of the main line were bent a little away from its original direction.
The illusion is apparent in the shape of a seeming approach of the
ends _b, b,_ of the two main lines. This to my mind would be a more
satisfactory explanation of this class of illusions than any of those
given by previous authors, were it not again for what happens in the
skin.
[Illustration: FIG. 69.]
_Considering all the circumstances, I feel justified in discarding
his entire batch of illusions as irrelevant to our present inquiry._
Whatever they may prove, they do not prove that our visual percepts of
form and movement may not be sensations strictly so called. They much
more probably fall into line with the phenomena of irradiation and of
color-contrast, and with Vierordt's primitive illusions of movement.
They show us, if anything, a realm of sensations in which our habitual
experience has not yet made traces, and which persist in spite of
our better knowledge, _un_suggestive of those other space-sensations
which we all the time know from extrinsic evidence to constitute
the real space-determinations of the diagram. Very likely, if these
sensations were as frequent and as practically important as they
now are insignificant and rare, we should end by substituting their
significates--the real space-values of the diagrams--for them. These
latter we should then seem to see directly, and the illusions would
disappear like that of the size of a tooth-socket when the tooth has
been out a week.
* * * * *
_(b) Another batch of cases which we may discard is that of double
images_. A thoroughgoing anti-sensationalist ought to deny all native
tendency to see double images when disparate retinal points are
stimulated, because, he should say, most people never get them, but
_see_ all things single which experience has led them to believe to
_be_ single. "Can a doubleness, so easily neutralized by our knowledge,
ever be a datum of sensation at all?" such an anti-sensationalist might
ask.
[Illustration: FIG. 70.]
To which the answer is that it _is_ a datum of sensation, but a datum
which, like many other data, must first be _discriminated_. As a
rule, no sensible qualities are discriminated without a motive.[239]
And those that later we learn to discriminate were originally felt
confused. As well pretend that a voice, or an odor, which we have
learned to pick out, is no sensation now. One may easily acquire
skill in discriminating double images, though, as Hering somewhere
says, it is an art of which one cannot become master in one year or
in two. For masters like Hering himself, or Le Conte, the ordinary
stereoscopic diagrams are of little use. Instead of combining into
one solid appearance, they simply cross each other with their doubled
lines. Volkmann has shown a great variety of ways in which the addition
of secondary lines, differing in the two fields, helps us to see
the primary lines double. The effect is analogous to that shown in
the cases which we despatched a moment ago, where given lines have
their space-value changed by the addition of new lines, without our
being able to say why, except that a certain mutual adhesion of
the lines and modification of the resultant feeling takes place by
psychophysiological laws. Thus, if in Fig. 70, _l_ and _r_ be crossed
by an horizontal line at the same level, and viewed stereoscopically,
they appear as a single pair of lines, _s_, in space. But if the
horizontal be at different levels, as in _l', r',_ three lines appear,
as in _s'_'.[240]
Let us then say no more about double images. All that the facts prove
is what Volkmann says,[241] that, although there may be sets of retinal
fibres so organized as to give an impression of two separate spots,
yet the excitement of other retinal fibres may inhibit the effect
of the first excitement, and prevent us from actually making the
discrimination. Still farther retinal processes may, however, bring the
doubleness to the eye of attention; and, once there, it is as genuine a
sensation as any that our life affords.[242]
* * * * *
_(c) These groups of illusions being eliminated,_ either as cases of
defective discrimination, or as changes of one space-sensation into
another when the total retinal process changes, _there remain but two
other groups to puzzle us._ The first is that of the after-images
distorted by projection on to oblique planes; the second relates to the
instability of our judgments of relative distance and size by the eye,
and includes especially what are known as pseudoscopic illusions.
The phenomena of the first group were described on page 232. A.
W. Volkmann has studied them with his accustomed clearness and
care.[243] Even an imaginarily inclined wall, in a picture, will, if an
after-image be thrown upon it, distort the shape thereof, and make us
_see_ a form of which our after-image would be the natural projection
on the retina, were that form laid upon the wall. Thus a signboard
is painted in perspective on a screen, and the eye, after steadily
looking at a rectangular cross, is turned to the painted signboard. The
after-image appears as an oblique-legged cross upon the signboard. It
is the converse phenomenon of a perspective drawing like Fig. 71, in
which really oblique-legged figures are seen as rectangular crosses.
[Illustration: FIG. 71.]
[Illustration: FIG. 72.]
[Illustration: FIG. 73.]
The unstable judgments of relative distance and size were also
mentioned on pp. 231-2. Whatever the size may be of the retinal
image which an object makes, the object is seen as of its own normal
size. A man moving towards us is not sensibly perceived to _grow_, for
example; and my finger, of which a single joint may more than conceal
him from my view, is nevertheless seen as a much smaller object than
the man. As for distances, it is often possible to make the farther
part of an object seem near and the nearer part far. A human profile in
intaglio, looked at steadily with one eye, or even both, soon appears
irresistibly as a bas-relief. The inside of a common pasteboard mask,
painted like the outside, and viewed with one eye in a direct light,
also looks convex instead of hollow. So strong is the illusion, after
long fixation, that a friend who painted such a mask for me told
me it soon became difficult to see how to apply the brush. Bend a
visiting-card across the middle, so that its halves form an angle of
90º more or less; set it upright on the table, as in Fig. 72, and view
it with one eye. You can make it appear either as if it opened towards
you or away from you. In the former case, the angle _ab_ lies upon
the table, _b_ being nearer to you than _a_; in the latter case _ab_
seems vertical to the table--as indeed it really is--with _a_ nearer
to you than _b_.[244] Again, look, with either one or two eyes, at
the opening of a wine-glass or tumbler (Fig. 73), held either above or
below the eye's level. The retinal image of the opening is an oval,
but we can see the oval in either of two ways,--as if it were the
perspective view of a circle whose edge _b_ were farther from us than
its edge _a_ (in which case we should seem to be looking down on the
circle), or as if its edge _a_ were the more distant edge (in which
case we should be looking up at it through the _b_ side of the glass).
As the manner of seeing the edge changes, the glass itself alters its
form in space and looks straight or seems bent towards or from the
eye,[245] according as the latter is placed beneath or above it.
[Illustration: FIG. 74.]
[Illustration: FIG. 75.]
[Illustration: FIG. 76.]
Plane diagrams also can be conceived as solids, and that in more than
one way. Figs. 74, 75, 76, for example, are ambiguous perspective
projections, and may each of them remind us of two different natural
objects. Whichever of these objects we conceive clearly at the moment
of looking at the figure, we seem to _see_ in all its solidity before
us. A little practice will enable us to flap the figures, so to speak,
backwards and forwards from one object to the other at will. We need
only attend to one of the angles represented, and imagine it either
solid or hollow--pulled towards us out of the plane of the paper, or
pushed back behind the same--and the whole figure obeys the cue and is
instantaneously transformed beneath our gaze.[246]
The peculiarity of all these cases is the ambiguity of the perception
to which the fixed retinal impression gives rise. With our retina
excited in exactly the same way, whether by after-image, mask or
diagram, we _see_ now this object and now that, as if the retinal image
_per se_ had no essential space-import. Surely if form and length were
originally retinal sensations, retinal rectangles ought not to become
acute or obtuse, and lines ought not to alter their relative lengths
as they do. If _relief_ were an optical feeling, it ought not to flap
to and fro, with every optical condition unchanged. Here, if anywhere,
the deniers of space-sensation ought to be able to make their final
stand.[247]
It must be confessed that their plea is plausible at first sight.
But it is one thing to throw out retinal sensibility altogether as
a space-yielding function the moment we find an ambiguity in its
deliverances, and another thing to examine candidly the conditions
which may have brought the ambiguity about. The former way is cheap,
wholesale, shallow; the latter difficult and complicated, but full of
instruction in the end. Let us try it for ourselves.
* * * * *
In the case of the diagrams 72, 73, 74, 75, 76, the real object, lines
meeting or crossing each other on a plane, is replaced by an _imagined
solid which we describe as seen. Really it is not seen but only so
vividly conceived as to approach a vision of reality._ We feel all the
while, however, that the solid suggested is not solidly there. _The
reason_ why one solid may seem more easily suggested than another, and
_why it is easier in general to perceive the diagram solid than flat,
seems due to probability._[248] Those lines have countless times in our
past experience been drawn on our retina by solids for once that we
have seen them flat on paper. And hundreds of times we have looked down
upon the upper surface of parallelopipeds, stairs and glasses, for once
that we have looked upwards at their bottom--hence we see the solids
easiest as if from above.
Habit or probability seems also to govern the illusion of the intaglio
profile, and of the hollow mask. We have _never_ seen a human face
except in relief--hence the case with which the present sensation is
overpowered. Hence, too, the obstinacy with which human faces and
forms, and other extremely familiar convex objects, refuse to appear
hollow when viewed through Wheatstone's pseudoscope. Our perception
seems wedded to certain total ways of seeing certain objects. The
moment the object is suggested at all, it takes possession of the mind
in the fulness of its stereotyped habitual form. This explains the
suddenness of the transformations when the perceptions change. The
object shoots back and forth completely from this to that familiar
thing, and doubtful, indeterminate, and composite things are excluded,
apparently because we are _unused_ to their existence.
When we turn from the diagrams to the actual folded visiting-card
and to the real glass, the imagined form seems fully as real as the
correct one. The card flaps over; the glass rim tilts this way or
that, as if some inward spring suddenly became released in our eye. In
these changes the actual retinal image receives different _complements
from the mind_. But the remarkable thing is that the complement and
the image combine so completely that the twain are one flesh, as it
were, and cannot be discriminated in the result. If the complement
be, as we have called it (on pp. 237-8), a set of imaginary absent
eye-sensations, they seem no whit less vividly there than the sensation
which the eye now receives from without.
The case of the after-images distorted by projection upon an oblique
plane is even more strange, for the imagined perspective figure, lying
in the plane, seems less to combine with the one a moment previously
seen by the eye than to suppress it and take its place.[249] The point
needing explanation, then, in all this, is how it comes to pass that,
when imagined sensations are usually so inferior in vivacity to real
ones, they should in these few experiences prove to be almost or quite
their match.
The mystery is solved when we note the class to which all these
experiences belong. They are 'perceptions' of definite 'things,'
definitely situated in tridimensional space. The mind uniformly uses
its sensations to _identify things by_. The sensation is invariably
apperceived by the idea, name, or 'normal' aspect (p. 238) of the
_thing_. The peculiarity of the _optical_ signs of things is their
extraordinary mutability. A 'thing' which we follow with the eye,
never doubting of its physical identity, will change its retinal image
incessantly. A cross, a ring, waved about in the air, will pass through
every conceivable angular and elliptical form. All the while, however,
as we look at them, we hold fast to the perception of their 'real'
shape, by mentally combining the pictures momentarily received with the
notion of peculiar positions in space. It is not the cross and ring
pure and simple which we perceive, but the cross _so held_, the ring
_so held_. From the day of our birth we have sought every hour of our
lives to _correct_ the apparent form of things, and translate it into
the real form by keeping note of the way they are placed or held. In no
other class of sensations does this incessant correction occur. What
wonder, then, that the notion 'so placed' should invincibly exert its
habitual corrective effect, even when the object with which it combines
is only an after-image, and make us perceive the latter under a changed
but more 'real' form? The 'real' form is also a sensation conjured up
by memory; but it is one so _probable_, so _habitually_ conjured up
when we have just this combination of optical experiences, that it
partakes of the invincible freshness of reality, and seems to break
through that law which elsewhere condemns reproductive processes to
being so much fainter than sensations.
Once more, _these cases form an extreme. Somewhere, in the list of our
imaginations of absent feelings, there must be found the vividest of
all. These optical reproductions of real form are the vividest of all._
It is foolish to reason from cases lower in the scale, to prove that
the scale can contain no such extreme cases as these; and particularly
foolish since we can definitely see why these imaginations ought to be
more vivid than any others, whenever they recall the forms of habitual
and probable things. These latter, by incessantly repeated presence and
reproduction, will plough deep grooves in the nervous system. There
will be developed, to correspond to them, paths of least resistance,
of unstable equilibrium, liable to become active in their totality
when any point is touched off. Even when the objective stimulus is
imperfect, we shall still _see_ the full convexity of a human face, the
correct inclination of an angle or sweep of a curve, or the distance
of two lines. Our mind will be like a polyhedron, whose facets are the
attitudes of perception in which it can most easily rest. These are
worn upon it by _habitual_ objects, and from one of these it can pass
only by tumbling over into another.[250]
Hering has well accounted for the sensationally vivid character of
these habitually reproduced forms. He says, after reminding us that
every visual sensation is correlated to a physical process in the
nervous apparatus:
"If this psycho-physical process is aroused, as usually happens, by
light-rays impinging on the retina, its form depends not only on the
nature of these rays, but on the constitution of the entire nervous
apparatus which is connected with the organ of vision, and on the
_state_ in which it finds itself. The same stimulus may excite widely
different sensations according to this state.
"The constitution of the nervous apparatus depends naturally in part
upon innate predisposition; but the _ensemble_ of effects wrought by
stimuli upon it in the course of life, whether these come through the
eyes or from elsewhere, is a co-factor of its development. To express
it otherwise, involuntary and voluntary experience and exercise assist
in determining the material structure of the nervous organ of vision,
and hence the ways in which it may react on a retinal image as an
outward stimulus. That experience and exercise should be possible at
all in vision is a consequence of the reproductive power, or memory,
of its nerve-substance. Every particular activity of the organ makes
it more suited to a repetition of the _same_; ever slighter touches
are required to make the repetition occur. The organ habituates itself
to the repeated activity....
"Suppose now that, in the first experience of a complex sensation
produced by a particular retinal image, certain portions were made
the special objects of attention. In a repetition of the sensible
experience it will happen that notwithstanding the identity of the
outward stimulus these portions will be more easily and strongly
reproduced; and when this happens a hundred times the inequality with
which the various constituents of the complex sensation appeal to
consciousness grows ever greater.
"Now in the present state of our knowledge we cannot assert that
in both the first and the last occurrence of the retinal image
in question the same _pure sensation_ is provoked, but that the
mind _interprets_ it differently the last time in consequence of
experience; for the only _given_ things we know are on the one hand
the retinal image which is both times the same, and on the other the
mental percept which is both times different; of a third thing, such
as a pure sensation, interpolated between image and percept, we know
nothing. We ought, therefore, if we wish to avoid hypotheses, simply
to say that the nervous apparatus reacts the last time differently
from the first, and gives us in consequence a different group of
sensations.
"But not only by repetition of the same retinal image, but by that of
similar ones, will the law obtain. Portions of the image common to
the successive experiences will awaken, as it were, a stronger echo
in the nervous apparatus than other portions. Hence it results that
_reproduction is usually elective_: the more strongly reverberating
parts of the picture yield stronger feelings than the rest. This
may result in the latter being quite overlooked and, as it were,
eliminated from perception. It may even come to pass that instead of
these parts eliminated by election a feeling of entirely different
elements comes to consciousness-elements not objectively contained
in the stimulus. A group of sensations, namely, for which a strong
tendency to reproduction has become, by frequent repetition, ingrained
in the nervous system will easily revive as a _whole_ when, not its
whole retinal image, but only an essential part thereof, returns.
In this case we get some sensations to which no adequate stimulus
exists in the retinal image, and which owe their being solely to the
reproductive power of the nervous apparatus. This is _complementary
(ergänzende) reproduction_.
"Thus a few points and disconnected strokes are sufficient to make
us see a human face, and without specially directed attention we
fail to note that we see much that really is not drawn on the paper.
Attention will show that the outlines were deficient in spots where
we thought them complete.... The portions of the percept supplied by
complementary reproduction depend, however, just as much as its other
portions, on the reaction of the nervous apparatus upon the retinal
image, indirect though this reaction may, in the case of the supplied
portions, be. And so long as they are present, we have a perfect
right to call them sensations, for they differ in no wise from such
sensations as correspond to an actual stimulus in the retina. Often,
however, they are not persistent; many of them may be expelled by more
close observation, but this is not proved to be the case with all....
In vision with one eye ... the distribution of parts within the third
dimension is essentially the work of this complementary reproduction,
i.e. of former experience.... When a certain way of localizing a
particular group of sensations has become with us a second nature,
our better knowledge, our judgment, our logic, are of no avail....
Things actually diverse may give similar or almost identical retinal
images; e.g., an object extended in three dimensions, and its
flat perspective picture. In such cases it often depends on small
accidents, and especially on our will, whether the one or the other
group of sensations shall be excited.... We can see a relief hollow,
as a mould, or _vice versâ_; for a relief illuminated from the left
can look just like its mould illuminated from the right. Reflecting
upon this, one may infer from the direction of the shadows that one
has a relief before one, and the idea of the relief will guide the
nerve-processes into the right path, so that _the feeling_ of the
relief is suddenly aroused.... Whenever the retinal image is of such a
nature that two diverse modes of reaction on the part of the nervous
apparatus are, so to speak, equally, or nearly equally, imminent, it
must depend on small accidents whether the one or the other reaction
is realized. In these cases our previous knowledge often has a
decisive effect, and helps the correct perception to victory. The
bare idea of the right object is itself a feeble reproduction which
with the help of the proper retinal picture develops into clear and
lively sensation. But if there be not already in the nervous apparatus
a disposition to the production of that percept which our judgment
tells us is right, our knowledge strives in vain to conjure up the
feeling of it; we then know that we see something to which no reality
corresponds, but we see it all the same."[251]
[Illustration: FIG. 77.]
_Note that no object not probable, no object which we are not
incessantly practised in reproducing, can acquire this vividness in
imagination._ Objective corners are ever changing their angles to the
eyes, spaces their apparent size, lines their distance. But by no
transmutation of position in space does an objective straight line
appear bent, and only in one position out of an infinity does a broken
line look straight. Accordingly, it is impossible by projecting the
after-image of a straight line upon two surfaces which make a solid
angle with each other to give the line itself a sensible 'kink.' Look
with it at the corner of your room: the after-image, which may overlap
all three surfaces of the corner, still continues straight. Volkmann
constructed a complicated surface of projection like that drawn in Fig.
77, but he found it impossible so to throw a straight after-image upon
it as to alter its visible form.
One of the situations in which we oftenest see things is spread out on
the ground before us. We are incessantly drilled in making allowance
for _this_ perspective, and reducing things to their real form in spite
of optical foreshortening. Hence if the preceding explanations are
true, we ought to find this habit inveterate. The _lower_ half of the
retina, which habitually sees the _farther_ half of things spread out
on the ground, ought to have acquired a habit of enlarging its pictures
by imagination, so as to make them more than equal to those which fall
on the upper retinal surface; and this habit ought to be hard to escape
from, even when both halves of the object are equidistant from the eye,
as in a vertical line on paper. Delbœuf has found, accordingly, that if
we try to bisect such a line we place the point of division about of
its length too high.[252]
[Illustration: FIG. 78.]
Similarly, a square cross, or a square, drawn on paper, should look
higher than it is broad. And that this is actually the case, the reader
may verify by a glance at Fig. 78. For analogous reasons the upper
and lower halves of the letter S, or of the figure 8, hardly seem to
differ. But when turned upside down, the upper half looks much the
larger.[253]
[Illustration: FIG. 79.]
Hering has tried to explain our exaggeration of small angles in the
same way. We have more to do with right angles than with any others:
right angles, in fact, have an altogether unique sort of interest for
the human mind. Nature almost never begets them, but we think space by
means of them and put them everywhere. Consequently obtuse and acute
ones, liable always to be the images of right ones foreshortened,
particularly easily revive right ones in memory. It is hard to look
at such figures as _a, b, c,_ in Fig. 79, without seeing them in
perspective, as approximations, at least, to foreshortened rectangular
forms.[254]
At the same time the genuine sensational form of the lines before us
can, in all the cases of distortion by suggested perspective, be felt
correctly by a mind able to abstract from the notion of perspective
altogether. Individuals differ in this abstracting power. Artistic
training improves it, so that after a little while errors in vertical
bisection, in estimating height relatively to breadth, etc., become
impossible. In other words, we learn to take the optical sensation
before us _pure_.[255]
_We may then sum up our study of illusions by saying that they in no
wise undermine our view that every spatial determination of things is
originally given in the shape of a sensation of the eyes._ They only
show how very potent certain _imagined_ sensations of the eyes may
become.
These sensations, so far as they bring definite forms to the mind,
appear to be retinal exclusively. The movements of the eyeballs play
a great part in educating our perception, it is true; but they have
nothing to do with _constituting_ any one feeling of form. Their
function is limited to _exciting_ the various feelings of form, by
tracing retinal streaks; and to _comparing_ them, and _measuring_ them
off against each other, by applying different parts of the retinal
surface to the same objective thing. Helmholtz's analysis of the
facts of our '_measurement of the field of view_' is, bating a lapse
or two, masterly, and seems to prove that the movements of the eye
have had some part in bringing our sense of retinal equivalencies
about--_equivalencies_, mind, of different retinal forms and sizes, not
forms and sizes themselves. _Superposition_ is the way in which the
eye-movements accomplish this result. An object traces the line AB on
a peripheral tract of the retina. Quickly we move the eye so that the
same object traces the line _ab_ on a central tract. Forthwith, to our
mind, AB and _ab_ are judged equivalent. But, as Helmholtz admits, the
equivalence-judgment is independent of the way in which we may feel the
form and length of the several retinal pictures themselves:
"The retina is like a pair of compasses, whose points we apply in
succession to the ends of several lines to see whether they agree
or not in length. All we need know meanwhile about the compasses
is that the distance of their points remains unchanged. What that
distance is, and what is the shape of the compasses, is a matter of no
account."[256]
_Measurement implies a stuff to measure. Retinal sensations give the
stuff; objective things form the yardstick; motion does the measuring
operation;_ which can, of course, be well performed only where it is
possible to make the same object fall on many retinal tracts. This is
practically impossible where the tracts make a wide angle with each
other. But there are certain directions in the field of view, certain
retinal lines, along which it is particularly easy to make the image
of an object slide. The object then becomes a 'ruler' for these lines,
as Helmholtz puts it,[257] making them seem straight throughout if the
object looked straight to us in that part of them at which it was most
distinctly seen.
But all this need of superposition shows how devoid of exact
space-import the feelings of movement are _per se_. As we compare the
space-value of two retinal tracts by superposing them successively upon
the same objective line, so we also have to compare the space-value
of objective angles and lines by superposing them on the same retinal
tract. Neither procedure would be required if our eye-movements were
apprehended immediately, by pure muscular feeling or innervation,
for example, as distinct lengths and directions in space. To compare
retinal tracts, it would then suffice simply to notice how it feels to
move _any_ image over them. And two objective lines could be compared
as well by moving different retinal tracts along them as by laying them
along the same. It would be as easy to compare non-parallel figures
as it now is to judge of those which are parallel.[258] Those which
it took the same amount of movement to traverse would be equal, in
whatever direction the movement occurred.
GENERAL SUMMARY.
With this we may end our long and, I fear to many readers, tediously
minute survey. The facts of vision form a jungle of intricacy; and
those who penetrate deeply into physiological optics will be more
struck by our omissions than by our abundance of detail. But for
students who may have lost sight of the forest for the trees, I
will recapitulate briefly the points of our whole argument from the
beginning, and then proceed to a short historical survey, which will
set them in relief.
All our sensations are positively and inexplicably extensive wholes.
The sensations contributing to space-_perception_ seem exclusively to
be the surface of skin, retina, and joints. 'Muscular' feelings play no
appreciable part in the generation of our feelings of form, direction,
etc.
The total bigness of a cutaneous or retinal feeling soon becomes
subdivided by discriminative attention.
_Movements_ assist this discrimination by reason of the peculiarly
exciting quality of the sensations which stimuli moving over surfaces
arouse.
Subdivisions, once discriminated, acquire definite relations of
position towards each other within the total space. These 'relations'
are themselves feelings of the subdivisions that intervene. When these
subdivisions are not the seat of stimuli, the relations are only
reproduced in imaginary form.
The various sense-spaces are, in the first instance, incoherent with
each other; and primitively both they and their subdivisions are but
vaguely comparable in point of bulk and form.
The _education_ of our space-perception consists largely of two
processes--reducing the various sense-feelings to a common _measure_,
and _adding them together_ into the single all-including space of the
real world.
Both the measuring and the adding are performed by the aid of _things_.
The imagined aggregate of positions occupied by all the actual or
possible, moving or stationary, things which we know, is our notion of
'real' space--a very incomplete and vague conception in all minds.
The _measuring_ of our space-feelings against each other mainly comes
about through the successive arousal of different ones by the same
_thing_, by our selection of certain ones as feelings of its _real_
size and shape, and by the degradation of others to the status of being
merely _signs_ of these.
For the successive application of the same thing to different
space-giving surfaces motion is indispensable, and hence plays a great
part in our space-education, especially in that of the eye. Abstractly
considered, the motion of the object over the sensitive surface would
educate us quite as well as that of the surface over the object. But
the self-mobility of the organ carrying the surface _accelerates_
immensely the result.
In completely educated space-perception, the present sensation is
usually just what Helmholtz (Physiol. Optik, p. 797) calls it, 'a sign,
the interpretation of whose meaning is left to the understanding.' But
the understanding is exclusively reproductive and never productive in
the process; and its function is limited to the recall of previous
space-sensations with which the present one has been associated and
which may be judged more real than it.
Finally, this reproduction may in the case of certain visual forms be
as vivid, or almost so, as actual sensation is.
The third dimension forms an original element of all our
space-sensations. In the eye it is subdivided by various
discriminations. The more distant subdivisions are often shut out
altogether, and, in being suppressed, have the effect of diminishing
the absolute space-value of the total field of view.[259]
HISTORICAL.
Let us now close with a brief historical survey. The first achievement
of note in the study of space-perception was Berkeley's theory of
vision. This undertook to establish two points, first that _distance_
was not a visual but a tactile form of consciousness, suggested by
visual signs; secondly, that there is no one quality or 'idea' common
to the sensations of touch and sight, such that prior to experience one
might possibly anticipate from the look of an object anything about its
felt size, shape, or position, or from the touch of it anything about
its look.
In other words, that primitively chaotic or semi-chaotic condition of
our various sense-spaces which we have demonstrated, was established
for good by Berkeley; and he bequeathed to psychology the problem of
describing the manner in which the deliverances are harmonized so as
all to refer to one and the same extended world.
His disciples in Great Britain have solved this problem after
Berkeley's own fashion, and to a great extent as we have done
ourselves, by the ideas of the various senses suggesting each other in
consequence of Association. But, either because they were intoxicated
with the principle of association, or because in the number of details
they lost their general bearings, they have forgotten, as a rule, to
state _under what sensible form the primitive spatial experiences are
found_ which later became associated with so many other sensible signs.
Heedless of their master Locke's precept, that the mind can frame unto
itself no one new simple idea, they seem for the most part to be trying
to _explain the extensive quality itself_, account for it, and evolve
it, by the mere association together of feelings which originally
possessed it not. They first evaporate the nature of extension by
making it tantamount to mere 'coexistence,' and then they explain
coexistence as being the same thing as _succession_, provided it be
an extremely rapid or a reversible succession. Space-perception thus
emerges without being anywhere postulated. The only things postulated
are unextended feelings and time. Says Thomas Brown (lecture xxiii.):
"I am inclined to reverse exactly the process commonly supposed; and
instead of deriving the measure of time from extension, to derive the
knowledge and original measure of extension from time." Brown and both
the Mills think that retinal sensations, colors, in their primitive
condition, are felt with no extension and that the latter merely
becomes inseparably associated with them. John Mill says: "Whatever may
be the retinal impression conveyed by a line which bounds two colors, I
see no ground for thinking that by the eye alone we could acquire the
conception of what we now mean when we say that one of the colors is
outside [beside] the other."[260]
Whence does the extension come which gets so inseparably associated
with these non-extended colored sensations? From the 'sweep and
movements' of the _eye_--from muscular feelings. But, as Prof. Bain
says, if movement-feelings give us any property of things, "it would
seem to be not space, but time."[261] And John Mill says that "the idea
of space is, at bottom, one of time."[262] Space, then, is not to be
found in any elementary sensation, but, in Bain's words, "as a quality,
it has no other origin and no other meaning than the _association_ of
these different [non-spatial] motor and sensitive effects."[263]
This phrase is mystical-sounding enough to one who understands
association as _producing_ nothing, but only as knitting together
things already produced in separate ways. The truth is that the English
Associationist school, in trying to show how much their principle can
accomplish, have altogether overshot the mark and espoused a kind
of theory in respect to space-perception which the general tenor of
their philosophy should lead them to abhor. Really there are but three
possible kinds of theory concerning space. Either (1) there is no
spatial _quality_ of sensation at all, and space is a mere symbol of
succession; or (2) there is an _extensive quality given_ immediately
in certain particular sensations; or, finally, (3) there is a _quality
produced_ out of the inward resources of the mind, to envelop
sensations which, as given originally, are not spatial, but which, on
being cast into the spatial form, become united and orderly. This last
is the Kantian view. Stumpf admirably designates it as the 'psychic
stimulus' theory, the crude sensations being considered as goads to the
mind to put forth its slumbering power.
Brown, the Mills, and Bain, amid these possibilities, seem to have gone
astray like lost sheep. With the 'mental chemistry' of which the Mills
speak--precisely the same thing as the 'psychical synthesis' of Wundt,
which, as we shall soon see, is a principle expressly intended to do
what Association can never perform--they hold the third view, but again
in other places imply the first. And, between the impossibility of
getting from mere association anything not contained in the sensations
associated and the dislike to allow spontaneous mental productivity,
they flounder in a dismal dilemma. Mr. Sully joins them there in what
I must call a vague and vacillating way. Mr. Spencer of course is
bound to pretend to 'evolve' all mental qualities out of antecedents
different from themselves, so that we need perhaps not wonder at his
refusal to accord the spatial quality to any of the several elementary
sensations out of which our space-perception grows. Thus (Psychology,
ii. 168, 172, 218):
"No idea of extension can arise from a _simultaneous_ excitation" of a
multitude of nerve-terminations like those of the skin or the retina,
since this would imply a "knowledge of their relative positions"--that
is, "a pre-existent idea of a special extension, which is absurd." "No
relation between _successive_ states of consciousness gives in itself
any idea of extension." "The muscular sensations accompanying motion
are quite distinct from the notions of space and time associated with
them."
Mr. Spencer none the less inveighs vociferously against the Kantian
position that space is produced by the mind's own resources. And yet
he nowhere denies space to be a specific affection of consciousness
different from time!
Such incoherency is pitiful. The fact is that, at bottom, all these
authors are really 'psychical stimulists,' or Kantists. The space they
speak of is a super-sensational mental product. This position appears
to me thoroughly mythological. But let us see how it is held by those
who know more definitely what they mean. Schopenhauer expresses the
Kantian view with more vigor and clearness than anyone else. He says: