to prove the existence of this blind spot. Close the right
ye and look steadily with the left at the cross in Fig. 6,




Fio. 6.

holding the book vertically in front of the face, and mov-
insr 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 measure-
ment tliat this blind spot lies where the optic nerve enters.

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 pass-
ing 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 ro-
fAted so as to cause the two images of any object whicli
catches the attention to fall on the two foveit, as the s])ot8
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 jx-ripherally lying o])JHct does
catch our attention, IIh- tu ruing of the eyes being only



32



FSTCEOLOO y.



another name for such rotation of the eyeballs as will
bring the foveas under the object's image.



Ciliary^ muscle //"*
relaxed, y y^ /«>




Ciltary muscle
contracted.



Accommodation. — The focussing or sharpening of the
image is performed by a special apparatus. In every cam-
era, the farther the object is from the eye the farther for-
ward, 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 re-
cedes. 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 ren-
dering 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



SIQET. 33

the more active change, since it involves contraction of
the ciliary muscle. AVhen we look fai off, we simply let
our eyes ijo passive. We feel this difference in the effort
when we t-oinpare the two sensations of cliange.

Convergence accompanies accommodation. The two eyes
act as one organ ; that is, when an object catches the atten-
tion, both eyeballs turn so that its images may fall on the
fovea. 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 accommo-
dation, and to accommodate with parallel axes of vision.
These are accomplishments which the student of psycho-
logical optics will find most useful.

Single Vision by the two Retinae. — 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, wher'?as under no con-
ditions can we hear or smell double. The main conditions
of single vision can be simply expressed.

In the first place, impressions on the two fovea 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 foveie of
tiie two converging eyeballs will necessarily always a])-
pear 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 fovea*, the two will also ap])c;ir
as one, or (in common j)arlance) * their images will fuse.'
To verify this, let the reader stare fixedly before him
iUJ if through the paper at infinite distance, witli the
black spots in Fig. b in front of his respective eyes, llo



34 PSYCHOLOGY.

will then see the two black spots swim togethei, as ii 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 oppo-
site both eyes being seen in the same place. But in addi-
tion 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



Fig. 8.

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.*

If, instead of two identical spots, we use two different
figures, or two differently colored spots, as objects for the
two fovege 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 retinae round about the foveae,
a similar correspondence obtains. Any impression on the

* This vertical partition is introduced into stereoscopes, which
otherwise would give us three pictures instead of one.



SIGHT.



35



npper half of either retina makes us aee an object as below,
on tlie 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;





Fio. 9.



and within two similar quadrants, aJ and ar for example,
there should, if the correspondence were carried out in de-
tail, 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 verities 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 circum-
stances the parallel light-rays coming from each star must
impinge on points within either retina which are geometri-
cally 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 Hat picture, all of wiiose jjarts
appear single. 'Identical retinal points' being iinj)ressed,
both eyes see their object in the same direction, and the
two objects consequently coalesce into one.

Here again retinal rivalry occurs if the ])icturcs dilTer.
And it must bo noted that when the experiment is per-



36



PSTCHOLOG r.



formed 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.

Double Images. — New it is an immediate consequence of
the law of identical location of images falling on geometri-
cally similar points that images which fall upon geomet-
ricaUg disparate jyoinfs of the two retince should be seen
in DISPARATE directions, and that thei?' objects should




Fig. 10.

cnnseqnentig appear in two places, or look double.
Take the parallel rays from a star falling upon two eyes
which converge upon a near object, 0, instead of being
parallel as in the previously instanced case. The two
foveas will receive the images of 0, 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



610HT. 37

which it strikes. But the two nasal halves are disparate,
geoinetrieally symmetrical, not geometrically similar. The
star's image on tiie left eye will tlierefore appear as if lying
to tiie left of 0; its image on the right eye will appear to
the right of this point. The star will, in short, bn. seen
double — * homonvmouslv ' double.

Conversely, if the star be looked at directly with parallel
axes, any near object like will be seen double, because
its inuiires will allect the outer or cheek halves of the two
retina^ instead of one outer and one nasal half. The posi-
tion 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 sim-
ilar 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 tlie so-called iiaploscopic method confirm this law, and
show tliat corresponding poinis, of single visual direction,
exist upon the two retina?. 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 tiie innu-
merable double images which objects nearer and farther
than the point looked at must be constantly producing.
Tiie answer to this is that we have trained ourselves to
hahils of inaf/entioH in regard to double images. So far
aa things interest us we turn our foveit* upon them, and they
are necessarily seen single; so that if an obji-ct iinjiresses
disparat<; pcjints, that may b«! taken m proof that, it is su



38 P8TGH0L0GT.

unimportant for us that we needn't notice whetner 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.

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




Fig. 11,

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) :

The moment we attend to the tops of these lines,
however, our foveas tend to abandon the dots and
to move upwards, and in doing so, to converge fig. 12.

somewhat, following the lines,
which then appear coalescing at
the top as in Fig. 13.

If we think of the bottom,
the eyes descend and diverge,
and what we see is Fig. 14.
Running our eyes up and

FlG.ia Fig. 14. ^^^^ ^-^^ jj^^^g ^^^^^ ^^^^

converge and diverge just as they would were they running





sionr. 39

up uud down some single line whose top was nearer to us
than its bottom. Now, if the inclination of the lines be
moderate, we mav 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 bot-
tom — 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 foveas 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.*

The Perception of Distance. — When we look about us at
things, our eyes are incessantly moving, converging, diverg-
ing, 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 liy 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 beff)re one eye, while ♦he 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 weget 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 IfK)k at an object with two eyes the visual axes are converged
on it, and the nearer tbe object the greater the convergence. We have
a pretty accurate knowledge of the degree of muscular effort reijuired
to converge the eyes on all tolerably near points. When objects are



* The Himj)lest form of stereosc<ti)e is two tin tubes about one
and one-half inches calibre, dead l)lack inside and (for nonnal eyes)
ten inches long. Close ea<;h end with jtaper not too opacpn', <m which
an inch-long thick black line is drawn. The tubes can l)e looked
through, one by each eye, and held either parallel or with their farther
ends converging. When jtropcrly rotated, their images will show
every variety of fusion and non-fusion, and stereoscopic effect.



40 FSYCaOLOOY.

fartlier off, their apparent size, and the modifications their retina]
images experience by agrial perspective, come in to help. The rela-
tive 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 move-
ment we can tell which are farther and which nearer. " *

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 sigjis 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 dis-
tant, 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 conclu-
sions 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 reti-
nal 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 repre-
sented." f

Sensations of Color. — The system of colors is a very com-
plex thing. If one take any color, say green, one can pass

* Martin: The Human Body, p. 530.
+ Ibid



SIOHT. 41

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 signifi-
cance. 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 Fig. is.

pure color towards black as well as towards white. The
best way is to jjut black into the third dimension, beneath
the paper, e.g., as is shown perspectively in Fig. IG, 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 Wmidt <-alls a tri-dimensional continuum.

Color-mixture. — I'hysioldgically considered, the colors
have tills pcfiiliarity, that many pairs of them, when they
impress the retina together, produce the sensation of wliite.
The colors which do this are called complemenfarirs. Suc-h
are spectral red and green-ljlue, spectral yellow and indigo-
blue. Green and purple, again, are complementaries. All




42



PSTCHOLOGT.



the spectral colors added together also make white light,
such as we daily experience in the sunshine. Further^

more, 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.*
From all this it follows
that there is no particular
congruence between our sys-
tem of color-sensations and
the physical stimuli which ex-
cite them. Each color-feeling
is a 'specific energy' (p. 11)
which many different physical
causes may arouse. Helm-
holtz, Hering, and others have
sought to simplify the tangle
of the facts, by physiologi-
cal hypotheses which, differ-
ing much in detail, agree in
principle, since they all postulate a limited number of
elementary retinal processes to which, when excited singly,

* The ordinary mixing of pigments is not an addition, but rather, as
Helmholtz has shown, a subtraction, of lights. To add one color to
another we must either by appropriate glasses throw differently col-
ored beams upon the same reflecting surface; or we must let the eye
look at one color through an inclined plate of glass beneath which it
lies, whilst the upper surface ot the glass reflects into the same eye




Black
Fig. 16 (after Ziehen).



SIOHT. 43

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 com-
pounded — 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 tlie 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 con-
sult the pliysiological books.

The Duration of Luminous Sensations. — "This is greater
than that of the stimulus, a fact taken advantage of in
nuiking fireworks: an ascending rocket produces the sen-
sation 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 wliite 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, sucli as would
be produced if the white and ])lack were mixed and spread
evenly over the disk. In each revolution the eye gets as

another color placed alongside — the two lights then mix on the retina;
or. finally, we must let the difTerently colored lights fall in succession
Ujjon the retina, so fast that the second is tln-re In-fore the imj)n's-
sion made by the first has died away. This is hest done by I(K)kiiig
at a rapid! V rotating disk whose sectors are of the several colors to b«
mixed.



44 PSYGHOLOOY.

much light as if that were the case, and is unable to dis-
tinguish 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."* 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 condi-
tions, 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 alternatel_y excited and rested. How con-
stant 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 prac-
tice. If any small object is steadily ' fixed ' for twenty or thirty
seconds, it will be found that the whole field of vision becomes gray-