Depth perception

“ 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 [Pg 213] 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 ”

“ Nevertheless we know that the mental picture of external objects loses nothing essential in focal sharpness through binocular vision but, on the other hand, the two unlike retinal pictures combine, as it were, in the mind to form a new idea—the concept of depth. ”

“ We will now investigate the conditions necessary for the perception of depth.
If the head is held still only one portion of space can be observed stereoscopically. The single eye, when moved, sur- veys, including indirect vision, a field which measures 180° in a horizontal direction, and 135 in a vertical direction. The two fields overlap and a smaller conical space is formed, with the nose as vertex (B V S in fig. 2) , in which both eyes can see simultaneously; and outside this space stereoscopic vision is impossible. The shape and size of this space are very differ- ent in men and animals. ”

“ It is surely as little possible for us, when we open our eyes on some wide and magnificent landscape, to separate the colour, as a mere visual sensation, from the field, the mountain, the forest, the stream, the sky, as to separate it from the half inch, or inch of our retina, of the perception of which we have no consciousness in any case ”

“ We are there in the midst of a three-dimensional world, and the movements of the persons or of the animals or even of the lifeless things, like the streaming of the water in the brook or the movements of the leaves in the wind, strongly maintain our immediate impression of depth. Many secondary features characteristic of the motion picture may help. For instance, by a well-known optical illusion the feeling of depth is strengthened if the foreground is at rest and the background moving. ”

“ A certain depth beneath the surface is most favorable to it,—a dim, midway region of twilight and calm, remote alike from the stagnant obscurity of mere sensation and from the agitated surface of day, the dry light of the intellect. ”

“ If you was to be carried up to any of these bodies at this vast distance from us, you would still discover others as much above you, as the stars that you see here are above the earth. Were you to go up or down, east or west, north or south, you would find the same height without any top, and the same depth without any bottom. ”

“ Owing to our former surroundings upon the surface of the earth, and to your inheritance of a so-called instinctive education, you would naturally suppose that we are now on the brink of an impassable chasm. This sphere of material vacuity extends beneath us to a depth that I am sure you will be astonished to learn is over six thousand miles. We may now look straight into the earth cavity, and this streaming light is the reflected purity of the space below. ”

“ Here in an external phenomenon we meet the same weakening in the light's tendency to expand that we recognized in the shortening of our experience of depth on looking through a dense medium. Obviously, we expect the externally observable narrowing of the light-cone and the subjectively experienced change of optical depth to show the same ratio. ”

“ And thus the nature of the world stands firm With never blows from outward, nor can be Nowhere disparted—since all height and depth Have always inward to the centre pressed (If thou art ready to believe that aught Itself can rest upon itself ) ; or that The ponderous bodies which be under earth Do all press upwards and do come to rest Upon the earth, in some way upside down, Like to those images of things we see At present through the waters. ”

“ 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. ”

“ I cannot close my eyes to the fact, that a world-view which ascribes to itself illimitable depth must needs lead by the circuitous route of a mere appearance of such depth to a certain superficiality. ”

“ If the earth is a distinct structure standing in and upon the waters of the “great deep,” it follows that, unless it can be shown that something else sustains the waters, that the depth is fathomless. ”

“ Thus, partly by the fusion of the successive impressions on the retina, and partly from the muscular sensations caused by repeated accommodations and possibly of ocular movements, we obtain a notion of depth in space, even with the use of only one eye. ”

“ Our eyes have become accustomed by the information received through other channels, especially the sense of touch, to interpret perspective in combination with certain shadings as depth so that even the one-eyed man sees things not as two dimensional pictures but as three dimensional corporeal forms. ”

“ From these experiments and observations, it will be seen that the conversion of form, excepting in the normal case, depends upon various causes, which are influential only under particular conditions, such as the depth of the hollow or the height of the relief, the distance of the object, the sharpness of vision, the use of one or both eyes, the inversion of the shadow, the nature of the object, and the means used by the mind itself to produce the illusion. ”

“ Another curious change, which comes from the possession of this sight, is that the solid ground upon which the man walks becomes to a certain extent transparent to him, so that he is able to see down into it to a considerable depth, much as we can now see into fairly clear water. ”

“ We feel immediately the depth of things. It is as if we were looking at a small plastic model of the landscape and in spite of our objective knowledge cannot recognize the flat pictures in the solid forms which we perceive. It cannot be otherwise, because whenever in practical life we see an object, a vase on our table, as a solid body, we get the impression of its plastic character first of all by seeing it with our two eyes from two different points of view. The perspective in which our right eye sees the things on our table is different from the perspective for the left eye. ”

“ Wherever I move, whether by rays or waves of light, from the farthest star to the nearest leaf with its complexity of vein and tint, there comes to that pin-point of space, the eye, a multitudinous vision. If every pin-point of external space is dense yet not blind with immensity, what more miracle of subtlety, of ethereal delicacy, could be affirmed of matter and be denied because it strains belief? ”

“ Height, size, and worldly greatness, though in themselves they have no resemblance, produce emotions in the mind that have a resemblance; and, led by this resemblance, we naturally express worldly greatness by height or size. One feels a certain uneasiness in looking down to a great depth: and hence depth is made to express any thing disagreeable by excess ”

“ But when the eye of the mind is permitted to wander through space and into the depths of the ocean, and is able to form a conception of Infinity and of Omnipotence, then it knows no danger; it is elevated—it comprehends itself. ”

“ The difficulty is that our physical senses only perceive the surface of our surroundings, and that we have hitherto been looking at the Woof of Nature as though it were the glass of a window covered with patterns, smudges, flies, &c., comprising all that we call physical phenomena and which, when analysed in terms of Time and Space, produce the appearance of succession and motion. It requires a keener perception, unbounded by these limitations, to look through the glass at the Reality which is beyond. ”

“ But more than this, these sky spots in a dark background are air holes. They enable us to breathe in the picture, giving a decided sense of atmosphere. When well subordinated they offer no distraction to the subject, but give to the picture a depth. When no other object is introduced, a gradation is serviceable. Much may be thus suggested and besides the depth and air properties thus introduced, such variety of surface excites visual motion. The eye always follows the course of light from the shadow. ”

“ Clearer air gives a greater sense of the upward depth of space. At night this is especially noticeable. The stars are not pasted on black canvas, all one level surface. There is depth in the outer sky. ”

“ The more you study that boulder, the more colour you will see in it; and if all this unexpected colour exists in one simple rock, to leave the charms of varying form unconsidered, what must there not exist on the whole wide moor? Look for yourself and see. After your eye has had its first lesson in the art of seeing and searching out the beautiful, it will naturally, unconsciously almost, begin to look for it everywhere—and expect it! ”

“ The great quantity of peculiar forms and of soft bodies existing in the innumerable carapaces, accompanied by the observation of the number of unknowns, increasing with the depth—these are the arguments which seem to me to hold firmly to the opinion of stationary life at the bottom of the deep sea. ”

“ The only way in which the impression of depth can really be gotten in a picture is by using the familiar method of the stereoscope, where two pictures are taken simultaneously by two cameras, placed a little farther apart than the two eyes ”

“ Day by day light and shadow fall in unbroken succession on the sensitive surface of his mind, and gradually an elementary order discovers itself in the regularity of these recurring impressions. Form, colour, distance, size, relativity of position are felt rather than seen, and the dim and confused mass of sensations discovers something trustworthy and stable behind. Nature is now simple appearance ”

“ Perhaps the most important of the images which come in to give depth, volume, solidity to the partly imagined and partly perceived ‘picture-space’ are those which are relicts of eye movements, kinæsthetic images of the convergence of the eyes and accommodation of the lenses according to the distance of the object contemplated. When, as it seems, we look past an object in a picture to some more distant object, seeming in so doing to change the focus of our eyes, we do not as a rule actually make any change. ”

“ On gazing down into its depths, when not a breath of wind sweeps over its surface, when its face is like a polished mirror, we find the water almost as transparent as the air we breathe, yet the keenest optics can penetrate but a few fathoms below the surface. ”

“ Only one point of the surface of any external object is at any one instant in the direct line of vision, yet so rapidly does the eye pass from point to point, that we seem to perceive at a glance the whole surface. ”

“ No matter how vast may be the depth which our instruments have sounded, there is yet a beyond of infinite extent. Imagine a mighty globe described in space, a globe of such stupendous dimensions that it shall include the sun and his system, all the stars and nebulæ, and even all the objects which our finite capacities can imagine. ”

“ There are always heights beyond heights, depths below depths, further recesses to penetrate, fresh things to find out. And nowhere is this more clearly demonstrated than when we come to the study of human nature itself. ”

“ At such a moment the mind is often affected with a sense of vastness all around. The eternal river, eluding the utmost range of vision, like the dimly visible destiny of man, has faded away into the unknown. ”

“ Others have emphasized that this strong feeling of depth touches them most when persons in the foreground stand with a far distant landscape as background—much more than when they are seen in a room. Psychologically this is not surprising either. If the scene were a real room, every detail in it would appear differently to the two eyes. In the room on the screen both eyes receive the same impression, and the result is that the consciousness of depth is inhibited. But when a far distant landscape is the only background, the impression from the picture and life is indeed the same. ”

“ If, notwithstanding, we think the moon bigger than the pea, and the tree than the pencil, the reason is that, apart from our being well acquainted with the tree, our judgment is a combination of the size of the image on the retina and the distance of the object. Now, as consciousness is for the most part founded on experience, so just and correct perspective sight is in the main something we acquire. ”

“ When you are upon the land and you look at [Pg 71] something in the distance your eyesight is stretched from point to point by intervening objects, while on the ocean your vision instead of being stretched out collapses as it were because there is a monotonous level between your eye and the object. ”

“ We notice that it required a separate intention of the eye, a more free and abstracted vision, to see the reflected trees and the sky, than to see the river bottom merely; and so are there manifold visions in the direction of every object, and even the most opaque reflect the heavens from their surface. Some men have their eyes naturally intended to the one and some to the other object. ”