Psychology: Briefer Course

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.

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.

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.

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.

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.

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.

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.

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):

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.