SIGHT.
=The Eye's Structure= is described in all the books on anatomy. I will
only mention the few points which concern the psychologist.[10] It is a
flattish sphere formed by a tough
[Illustration: FIG. 3.]
white membrane (the sclerotic), which encloses a nervous surface and
certain refracting media (lens and 'humors') which cast a picture of the
outer world thereon. It is in fact a little camera obscura, the
essential part of which is the sensitive plate.
[Illustration: FIG. 4.]
[Illustration:
FIG. 5.--Scheme of retinal fibres, after Küss. _Nop._ optic nerve;
_S_, sclerotic; _Ch_, choroid; _R_, retina; _P_, papilla (blind
spot); _F_, fovea.
]
=The retina= is what corresponds to this plate. The optic nerve pierces
the sclerotic shell and spreads its fibres radially in every direction
over its inside, forming a thin translucent film (see Fig. 3, _Ret._).
The fibres pass into a complicated apparatus of cells, granules, and
branches (Fig. 4), and finally end in the so-called rods and cones (Fig.
4,--9), which are the specific organs for taking up the influence of the
waves of light. Strange to say, these end-organs are not pointed forward
towards the light as it streams through the pupil, but backwards towards
the sclerotic membrane itself, so that the light-waves traverse the
translucent nerve-fibres, and the cellular and granular layers of the
retina, before they touch the rods and cones themselves. (See Fig. 5.)
=The Blind Spot.=--The optic nerve-fibres must thus be unimpressible by
light directly. The place where the nerve enters is in fact entirely
blind, because nothing but fibres exist there, the other layers of the
retina only beginning round about the entrance. Nothing is easier than
to prove the existence of this blind spot. Close the right eye and look
steadily with the left at the cross in Fig. 6, holding the book
vertically in front of the face, and moving it to and fro. It will be
found that at about a foot off the black disk disappears; but when the
page is nearer or farther, it is seen. During the experiment the gaze
must be kept fixed on the cross. It is easy to show by measurement that
this blind spot lies where the optic nerve enters.
[Illustration: FIG. 6.]
=The Fovea.=--Outside of the blind spot the sensibility of the retina
varies. It is greatest at the _fovea_, a little pit lying outwardly from
the entrance of the optic nerve, and round which the radiating
nerve-fibres bend without passing over it. The other layers also
disappear at the fovea, leaving the cones alone to represent the retina
there. The sensibility of the retina grows progressively less towards
its periphery, by means of which neither colors, shapes, nor number of
impressions can be well discriminated.
In the normal use of our two eyes, the eyeballs are rotated so as to
cause the two images of any object which catches the attention to fall
on the two foveæ, as the spots of acutest vision. This happens
involuntarily, as any one may observe. In fact, it is almost impossible
_not_ to 'turn the eyes,' the moment any peripherally lying object does
catch our attention, the turning of the eyes being only another name
for such rotation of the eyeballs as will bring the foveæ under the
object's image.
[Illustration: FIG. 7.]
=Accommodation.=--The _focussing_ or _sharpening_ of the image is
performed by a special apparatus. In every camera, the farther the
object is from the eye the farther forward, and the nearer the object is
to the eye the farther backward, is its image thrown. In photographers'
cameras the back is made to slide, and can be drawn away from the lens
when the object that casts the picture is near, and pushed forward when
it is far. The picture is thus kept always sharp. But no such change of
length is possible in the eyeball; and the same result is reached in
another way. The lens, namely, grows more convex when a near object is
looked at, and flatter when the object recedes. This change is due to
the antagonism of the circular 'ligament' in which the lens is
suspended, and the 'ciliary muscle.' The ligament, when the ciliary
muscle is at rest, assumes such a spread-out shape as to keep the lens
rather flat. But the lens is highly elastic; and it springs into the
more convex form which is natural to it whenever the ciliary muscle, by
contracting, causes the ligament to relax its pressure. The contraction
of the muscle, by thus rendering the lens more refractive, adapts the
eye for near objects ('accommodates' it for them, as we say); and its
relaxation, by rendering the lens less refractive, adapts the eye for
distant vision. Accommodation for the near is thus the more _active_
change, since it involves contraction of the ciliary muscle. When we
look far off, we simply let our eyes go passive. We feel this difference
in the effort when we compare the two sensations of change.
=Convergence accompanies accommodation.= The two eyes act as one organ;
that is, when an object catches the attention, both eyeballs turn so
that its images may fall on the foveæ. When the object is near, this
naturally requires them to turn inwards, or converge; and as
accommodation then also occurs, the two movements of convergence and
accommodation form a naturally associated couple, of which it is
difficult to execute either singly. Contraction of the pupil also
accompanies the accommodative act. When we come to stereoscopic vision,
it will appear that by much practice one can learn to converge with
relaxed accommodation, and to accommodate with parallel axes of vision.
These are accomplishments which the student of psychological optics will
find most useful.
=Single Vision by the two Retinæ.=--We hear single with two ears, and
smell single with two nostrils, and we also see single with two eyes.
The difference is that we also _can_ see double under certain
conditions, whereas under no conditions can we hear or smell double. The
main conditions of single vision can be simply expressed.
In the first place, impressions on the two foveæ always appear in the
same place. By no artifice can they be made to appear alongside of each
other. The result is that one object, casting its images on the foveæ of
the two converging eyeballs will necessarily always appear as what it
is, namely, one object. Furthermore, if the eyeballs, instead of
converging, are kept parallel, and two similar objects, one in front of
each, cast their respective images on the foveæ, the two will also
appear as one, or (in common parlance) 'their images will fuse.' To
verify this, let the reader stare fixedly before him as if through the
paper at infinite distance, with the black spots in Fig. 8 in front of
his respective eyes. He will then see the two black spots swim
together, as it were, and combine into one, which appears situated
between their original two positions and as if opposite the root of his
nose. This combined spot is the result of the spots opposite both eyes
being seen in the same place. But in addition to the combined spot, each
eye sees also the spot opposite the _other_ eye. To the right eye this
appears to the left of the combined spot, to the left eye it appears to
the right of it; so that what is seen is _three_ spots, of which the
middle one is seen by both eyes, and is flanked by two others, each seen
by one. That such are the facts can be tested by interposing some small
opaque object so as to cut off the vision of either of the spots in the
figure from the _other_ eye. A vertical partition in the median plane,
going from the paper to the nose, will effectually confine each eye's
vision to the spot in front of it, and then the single combined spot
will be all that appears.[11]
[Illustration: FIG. 8.]
If, instead of two identical spots, we use two different figures, or two
differently colored spots, as objects for the two foveæ to look at, they
still are seen in the _same place_; but since they cannot appear as a
single object, they appear there _alternately_ displacing each other
from the view. This is the phenomenon called _retinal rivalry_.
As regards the parts of the retinæ round about the foveæ, a similar
correspondence obtains. Any impression on the upper half of either
retina makes us see an object as below, on the lower half as above, the
horizon; and on the right half of either retina, an impression makes us
see 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 geometrically _similar_ quadrant of the other; and within two
similar quadrants, _al_ and _ar_ for example, there should, if the
correspondence were carried out in detail, 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. Similarly, a pair of
spectacles held an inch or so from the eyes seem like one large median
glass. Or we may make an experiment like that with the spots. If we take
two exactly similar pictures, no larger than those on an ordinary
stereoscopic slide, and if we look at one with each eye (a median
partition confining the view) we shall see but one flat picture, all of
whose parts appear single. 'Identical retinal points' being impressed,
both eyes see their object in the same direction, and the two objects
consequently coalesce into one.
[Illustration: FIG. 9.]
Here again retinal rivalry occurs if the pictures differ. And it must be
noted that when the experiment is performed for the first time the
combined picture is always far from sharp. This is due to the difficulty
mentioned on p. 33, of accommodating for anything as near as the surface
of the paper, whilst at the same time the convergence is relaxed so that
each eye sees the picture in front of itself.
[Illustration: FIG. 10.]
=Double Images.=--Now it is an immediate consequence of the law of
identical location of images falling on geometrically similar points
that _images which fall upon geometrically_ DISPARATE _points of the two
retinæ should be seen 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. The two foveæ will receive the images of _O_, which
therefore will look single. If then _SL_ and _SR_ in Fig. 10 be the
parallel rays, each of them will fall upon the nasal half of the retina
which it strikes. But the two nasal halves are disparate, geometrically
_symmetrical_, not geometrically _similar_. The star's image on the left
eye will therefore appear as if lying to the left of _O_; its image on
the right eye will appear to the right of this point. The star will, in
short, be seen double--'homonymously' double.
Conversely, if the star be looked at directly with parallel axes, any
near object like _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 positions seen
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.
=Vision of Solidity.=--This description of binocular vision follows what
is called the theory of identical points. On the whole it formulates the
facts correctly. The only odd thing is that we should be so little
troubled by the innumerable double images which objects nearer and
farther than the point looked at must be constantly producing. The
answer to this is that _we have trained ourselves to habits of
inattention_ in regard to double images. So far as things interest us we
turn our foveæ upon them, and they are necessarily seen single; so that
if an object impresses disparate points, that may be taken as proof that
it is so unimportant for us that we needn't notice whether it appears
in one place or in two. By long practice one may acquire great
expertness in detecting double images, though, as some one says, it is
an art which is not to be learned completely either in one year or in
two.
[Illustration: FIG. 11.]
Where the disparity of the images is but slight it is almost impossible
to see them as if double. They give rather the perception of a solid
object being there. To fix our ideas, take Fig. 11. Suppose we look at
the dots in the middle of the lines _a_ and _b_ just as we looked at the
spots in Fig. 8. We shall get the same result--i.e., they will coalesce
in the median line. But the entire lines will not coalesce, for, owing
to their inclination, their tops fall on the temporal, and their bottoms
on the nasal, retinal halves. What we see will be two lines crossed in
the middle, thus (Fig. 12):
[Illustration: FIG. 12.]
The moment we attend to the tops of these lines, however, our foveæ tend
to abandon the dots and to move upwards, and in doing so, to converge
somewhat, following the lines, which then appear coalescing at the top
as in Fig. 13.
[Illustration: FIG. 13.]
[Illustration: FIG. 14.]
If we think of the bottom, the eyes descend and diverge, and what we see
is Fig. 14.
Running our eyes up and down the lines makes them converge and diverge
just as they would were they running up and down some single line whose
top was nearer to us than its bottom. Now, if the inclination of the
lines be moderate, we may not see them double at all, but single
throughout their length, when we look at the dots. Under these
conditions their top does look nearer than their bottom--in other words,
we see them stereoscopically; and we see them so even when our eyes are
rigorously motionless. In other words, the slight disparity in the
bottom-ends which _would_ draw the foveæ divergently apart makes us see
those ends farther, the slight disparity in the top ends which _would_
draw them convergently together makes us see these ends nearer, than the
point at which we look. The disparities, in short, affect our perception
as the actual movements would.[12]
=The Perception of Distance.=--When we look about us at things, our eyes
are incessantly moving, converging, diverging, accommodating, relaxing,
and sweeping over the field. The field appears extended in three
dimensions, with some of its parts more distant and some more near.
"With one eye our perception of distance is very imperfect, as
illustrated by the common trick of holding a ring suspended by a
string in front of a person's face, and telling him to shut one eye
and pass a rod from one side through the ring. If a penholder be
held erect before one eye, while the other is closed, and an
attempt be made to touch it with a finger moved across towards it,
an error will nearly always be made. In such cases we get the only
clue from the amount of effort needed to 'accommodate' the eye to
see the object distinctly. When we use both eyes our perception of
distance is much better; when we look at an object with two eyes
the visual axes are converged on it, and the nearer the object the
greater the convergence. We have a pretty accurate knowledge of the
degree of muscular effort required to converge the eyes on all
tolerably near points. When objects are farther off, their
apparent size, and the modifications their retinal images
experience by aërial perspective, come in to help. The relative
distance of objects is easiest determined by moving the eyes; all
stationary objects then appear displaced in the opposite direction
(as for example when we look out of the window of a railway car)
and those nearest most rapidly; from the different apparent rates
of movement we can tell which are farther and which nearer."[13]
Subjectively considered, distance is an altogether peculiar content of
consciousness. Convergence, accommodation, binocular disparity, size,
degree of brightness, parallax, etc., all give us special feelings which
are _signs_ of the distance feeling, but not it. They simply suggest it
to us. The best way to get it strongly is to go upon some hill-top and
invert one's head. The horizon then looks very distant, and draws near
as the head erects itself again.
=The Perception of Size.=--"The dimensions of the retinal image determine
primarily the sensations on which conclusions as to size are based; and
the larger the visual angle the larger the retinal image: since the
visual angle depends on the distance of an object, the correct
perception of size depends largely upon a correct perception of
distance; having formed a judgment, conscious or unconscious, as to
that, we conclude as to size from the extent of the retinal region
affected. Most people have been surprised now and then to find that what
appeared a large bird in the clouds was only a small insect close to the
eye; the large apparent size being due to the previous incorrect
judgment as to the distance of the object. The presence of an object of
tolerably well-known height, as a man, also assists in forming
conceptions (by comparison) as to size; artists for this purpose
frequently introduce human figures to assist in giving an idea of the
size of other objects represented."[14]
=Sensations of Color.=--The system of colors is a very complex thing. If
one take any color, say green, one can pass away from it in more than
one direction, through a series of greens more and more yellowish, let
us say, towards yellow, or through another series more and more bluish
towards blue. The result would be that if we seek to plot out on paper
the various distinguishable tints, the arrangement cannot be that of a
line, but has to cover a surface. With the tints arranged on a surface
we can pass from any one of them to any other by various lines of
gradually changing intermediaries. Such an arrangement is represented in
Fig. 15. It is a merely classificatory diagram based on degrees of
difference simply felt, and has no physical significance. Black is a
color, but does not figure on the plane of the diagram. We cannot place
it anywhere alongside of the other colors because we need both to
represent the straight gradation from untinted white to black, and that
from each pure color towards black as well as towards white. The best
way is to put black into the third dimension, beneath the paper, _e.g._,
as is shown perspectively in Fig. 16, then all the transitions can be
schematically shown. One can pass straight from black to white, or one
can pass round by way of olive, green, and pale green; or one can change
from dark blue to yellow through green, or by way of sky-blue, white and
straw color; etc., etc. In any case the changes are continuous; and the
color system thus forms what Wundt calls a tri-dimensional continuum.
[Illustration: FIG. 15.]
=Color-mixture.=--Physiologically considered, the colors have this
peculiarity, that many pairs of them, when they impress the retina
together, produce the sensation of white. The colors which do this are
called _complementaries_. Such are spectral red and green-blue, spectral
yellow and indigo-blue. Green and purple, again, are complementaries.
All the spectral colors added together also make white light, such as
we daily experience in the sunshine. Furthermore, both homogeneous
ether-waves and heterogeneous ones may make us feel the same color, when
they fall on our retina. Thus yellow, which is a simple spectral color,
is also felt when green light is added to red; blue is felt when violet
and green lights are mixed. Purple, which is not a spectral color at
all, results when the waves either of red and of violet or those of blue
and of orange are superposed.[15]
[Illustration: FIG. 16 (after Ziehen).]
From all this it follows that there is no particular congruence between
our system of color-sensations and the physical stimuli which excite
them. Each color-feeling is a 'specific energy' (p. 11) which many
different physical causes may arouse. Helmholtz, Hering, and others have
sought to simplify the tangle of the facts, by physiological hypotheses
which, differing much in detail, agree in principle, since they all
postulate a limited number of elementary retinal processes to which,
when excited singly, certain 'fundamental' colors severally correspond.
When excited in combination, as they may be by the most various physical
stimuli, other colors, called 'secondary,' are felt. The secondary
color-sensations are often spoken of as if they were compounded of the
primary sensations. This is a great mistake. The _sensations as such_
are not compounded--yellow, for example, a secondary on Helmholtz's
theory, is as unique a quality of feeling as the primaries red and
green, which are said to 'compose' it. What are compounded are merely
the elementary retinal processes. These, according to their combination,
produce diverse results on the brain, and thence the secondary colors
result immediately in consciousness. The 'color-theories' are thus
physiological, not psychological, hypotheses, and for more information
concerning them the reader must consult the physiological books.
=The Duration of Luminous Sensations.=--"This is greater than that of the
stimulus, a fact taken advantage of in making fireworks: an ascending
rocket produces the sensation of a trail of light extending far behind
the position of the bright part of the rocket itself at the moment,
because the sensation aroused by it in a lower part of its course still
persists. So, shooting stars appear to have luminous tails behind them.
By rotating rapidly before the eye a disk with alternate white and black
sectors we get for each point of the retina alternate stimulation (due
to the passage of white sector) and rest (when a black sector is
passing). If the rotation be rapid enough the sensation aroused is that
of a uniform gray, such as would be produced if the white and black were
mixed and spread evenly over the disk. In each revolution the eye gets
as much light as if that were the case, and is unable to distinguish
that this light is made up of separate portions reaching it at
intervals: the stimulation due to each lasts until the next begins, and
so all are fused together. If one turns out suddenly the gas in a room
containing no other light, the image of the flame persists a short time
after the flame itself is extinguished."[16] If we open our eyes
instantaneously upon a scene, and then shroud them in complete darkness,
it will be as if we saw the scene in ghostly light through the dark
screen. We can read off details in it which were unnoticed whilst the
eyes were open. This is the primary positive after-image, so-called.
According to Helmholtz, one third of a second is the most favorable
length of exposure to the light for producing it.
=Negative after-images= are due to more complex conditions, in which
fatigue of the retina is usually supposed to play the chief part.
"The nervous visual apparatus is easily fatigued. Usually we do not
observe this because its restoration is also rapid, and in ordinary
life our eyes, when open, are never at rest; we move them to and
fro, so that parts of the retina receive light alternately from
brighter and darker objects, and are alternately excited and
rested. How constant and habitual the movement of the eyes is can
be readily observed by trying to 'fix' for a short time a small
spot without deviating the glance; to do so for even a few seconds
is impossible without practice. If any small object is steadily
'fixed' for twenty or thirty seconds, it will be found that the
whole field of vision becomes grayish and obscure, because the
parts of the retina receiving most light get fatigued, and arouse
no more sensation than those less fatigued and stimulated by light
from less illuminated objects. Or look steadily at a black object,
say a blot on a white page, for twenty seconds, and then turn the
eye on a white wall; the latter will seem dark gray, with a white
patch on it; an effect due to the greater excitability of the
retinal parts previously rested by the black, when compared with
the sensation aroused elsewhere by light from the white wall acting
on the previously stimulated parts of the visual surface. All
persons will recall many instances of such phenomena, which are
especially noticeable soon after rising in the morning. Similar
things may be noticed with colors; after looking at a red patch the
eye turned on a white wall sees a blue-green patch; the elements
causing red sensations having been fatigued, the white mixed light
from the wall now excites on that region of the retina only the
other primary color sensations. The blending of colors so as to
secure their greatest effect depends on this fact; red and green go
well together because each rests the parts of the visual apparatus
most excited by the other, and so each appears bright and vivid as
the eye wanders to and fro; while red and orange together, each
exciting and exhausting mainly the same visual elements, render
dull, or in popular phrase 'kill,' one another.
"If we fix steadily for thirty seconds a point between two white
squares about 4 mm. (⅙ inch) apart on a large black sheet, and then
close and cover our eyes, we get a negative after-image in which
are seen two dark squares on a brighter surface; this surface is
brighter close around the negative after-image of each square, and
brightest of all between them. This luminous boundary is called the
_corona_, and is explained usually as an effect of simultaneous
contrast; the dark after-image of the square it is said makes us
mentally err in judgment, and think the clear surface close to it
brighter than elsewhere; and it is brightest between the two dark
squares, just as a middle-sized man between two tall ones looks
shorter than if alongside one only. If, however, the after-image be
watched, it will often be noticed not only that the light band
between the squares is intensely white, much more so than the
normal idio-retinal light [see below], but, as the image fades
away, often the two dark after-images of the squares disappear
entirely with all of the corona, except that part between them
which is still seen as a bright band on a uniform grayish field.
Here there is no _contrast_ to produce the error of judgment; and
from this and other experiments Hering concludes that light acting
on one part of the retina produces inverse changes in all the rest,
and that this plays an important part in producing the phenomena of
contrasts. Similar phenomena may be observed with colored objects;
in their negative after-images each tint is represented by its
complementary, as black is by white in colorless vision."[17]
This is one of the facts referred to on p. 27 which have made Hering
reject the psychological explanation of simultaneous contrast.
=The Intensity of Luminous Objects.=--Black is an optical sensation. We
have no black except in the field of view; we do not, for instance, see
black out of our stomach or out of the palm of our hand. _Pure_ black
is, however, only an 'abstract idea,' for the retina itself (even in
complete objective darkness) seems to be always the seat of internal
changes which give some luminous sensation. This is what is meant by the
'idio-retinal light,' spoken of a few lines back. It plays its part in
the determination of all after-images with closed eyes. Any objective
luminous stimulus, to be perceived, must be strong enough to give a
sensible increment of sensation over and above the idio-retinal light.
As the objective stimulus increases the perception is of an intenser
luminosity; but the perception changes, as we saw on p. 18, more slowly
than the stimulus. The latest numerical determinations, by König and
Brodhun, were applied to six different colors and ran from an intensity
arbitrarily called 1 to one which was 100,000 times as great. From
intensity 2000 to 20,000 Weber's law held good; below and above this
range discriminative sensibility declined. The relative increment
discriminated here was the same for all colors of light, and lay
(according to the tables) between 1 and 2 per cent of the stimulus.
Previous observers have got different results.
A certain amount of luminous intensity must exist in an object for its
color to be discriminated at all. "In the dark all cats are gray." But
the colors rapidly become distincter as the light increases, first the
blues and last the reds and yellows, up to a certain point of intensity,
when they grow indistinct again through the fact that each takes a turn
towards white. At the highest bearable intensity of the light all colors
are lost in the blinding white dazzle. This again is usually spoken of
as a 'mixing' of the sensation white with the original color-sensation.
It is no mixing of two sensations, but the replacement of one sensation
by another, in consequence of a changed neural process.