Visual field

“ We learn, therefore, to react to a visual shape in a manner corresponding to how it would appear if it were in the centre of the field of vision, provided we do not immediately focus153 upon it, which is what we naturally do when anything visible interests us. ”

“ A network of associations is thus formed, whereby the sensation of each retinal point is connected with all the others in a manner which is that of points in a plane. Every visible point becomes thus a point in a field, and has a felt radiation of lines of possible motion about it. Our notion of visual space has this origin, since the manifold of retinal impressions is distributed in a manner which serves as the type and exemplar of what we mean by a surface. ”

“ What chiefly concerns us here is the fact that the pictures on our retinas are flat, of two dimensions, the same as the canvas on which we paint. If you examine these visual pictures without any prejudice, as one may with a camera obscura, you will see that they are composed of masses of colour in infinite variety and complexity, of different shapes and gradations, and with many varieties of edges; giving to the eye the illusion of nature with actual depths and distances, although one knows all the time that it is a flat table on which one is looking. ”

“ Instead of fixing my eye immovably upon one point, I deliberately run it over the leading lines of the view, and then trace lines upon the paper that produce the same effect upon my eye as those in Nature have done. My sketch will at best be imperfect, but its accuracy will be in proportion to the care with which I have examined the outlines in the landscape. In observing an object, we do not then look fixedly at it—we run the eye over it. Let us see what this means.
The retina is not in all parts equally sensitive to light, and the whole of a visual image is not distinctly perceived at once. ”

“ It is to be observed that there are two stages in the process by which this secernment is brought about—First, the stage in which the visual phenomena are associated with things different from the organ of vision, the very existence of which is as yet unknown. Let us suppose, then, the function of sight to be in operation. We behold a visible object—a particular colour. Let the touch now come into play. We feel a tangible object—say a book. ”

“ If we compare the picture on the glass screen of the camera when the camera is out of focus with the visual impressions of the mind when the eye is out of focus, there will be found to be a great difference between them. ”

“ IV Perspective of a Point, Visual Rays, &c.
We perceive objects by means of the visual rays, which are imaginary straight lines drawn from the eye to the various points of the thing we are looking at. As those rays proceed from the pupil of the eye, which is a circular opening, they form themselves into a cone called the Optic Cone, the base of which increases in proportion to its distance from the eye, so that the larger the view which we wish to take in, the farther must we be removed from it. ”

“ The great sweep of vision, a sweep covering a hundred subjects perhaps, is obtained by turning the eyes up or down or sideways. But to be true—that is, to see one picture at a time—the eye should be fixed like the lens of a camera, the limit of the picture being the range of the eye and no more. A departure from this rule not only confuses your perspective but crowds a number of points of interest into the square of your canvas, when there is really only one centre point before you in nature ”

“ To a large extent we are misled by the impressive prominence of the visual data. In vision we are presented with a system of inter-related and simultaneously occurring sensations which we find by experience to be the sure and certain indicators of the potent obstructions which our activity encounters. For this reason we habitually make use of the visual sign as the guide and instrument of our exertional activity, and this habitual use leads us to regard the visual presentation as the essential form of Reality. ”

“ So, on the authority of repeated experience, whatever is unsuitable is gradually excluded, and only those eye movements are retained which further the proper convergence of the two retinal images.”104 Of course, the power of vision is by no means completely developed at five months, though the technique of the function, so to speak, is by that time essentially perfected. Now begin the real tasks of visual practice: acquiring familiarity with external objects, imprinting the visual images on the mind, and widening the scope of association. ”

“ This central spot is a most important factor in the study of sight and art. For though the field of vision of the two eyes is more than 180° laterally, and 120° vertically, yet the field of distinct vision is but a fraction of this field, as we can all prove for ourselves. ”

“ The character, direction and curve of all lines in a canvas change with every shifting of the point of visual contact. Since the unity of a picture is different from every focus, all the lines consequently assume a slightly different direction every time our eye shifts from one spot to another. ”

“ Thus, for example, the natural instinct of the cultivated eye to look for a well-marked contour, as well as for a central element of repose, in a design, may be regarded as the result of ingrained habits, determined by the conditions of a distinct visual grasp and recognition of objects in every-day life. So the desire of the eye for proportion seems to be an outgrowth of a habit of attending to relative magnitude, a habit that is clearly connected with the paramount importance of identifying objects at different distances from the eye ”

“ The young student is the spectator on the other side of it, the dotted lines drawn from the corners of the cube to the eye of the spectator are the visual rays, and the points on the transparent picture plane where these visual rays pass through it indicate the perspective position 8 of those points on the picture. To find these points is the main object or duty of linear perspective.
Fig. 3.
Perspective up to a certain point is a pure science, not depending upon the accidents of vision, but upon the exact laws of reasoning. ”

“ When the general direction only of the object can be given and it is sought for, the whole landscape in that direction to the horizon should be divided into sections by imaginary lines, the limits of these sections being bounded between visible landmarks through which the bounding lines are supposed to pass. Each section should be scrutinized little by little until the glass has been passed over every spot. Such search will seldom fail to be successful. ”

“ The eye automatically, and almost instantaneously, adjusts itself for near or distant objects and for varying intensities of light, and has, moreover, a field of view of nearly 180 degrees—almost a complete half-circle. ”

“ If we limit visual phenomena as data to mere variations of kind and distinctness in color, we cannot account for such cases as the appearance and inversion of mathematical perspective, binocular localisation and translocation, and the distinct effect of the monocular parallax of motion, qualities which are dimensional in their nature. ”

“ We continue our experiments until our test happens to be applied to one particular phenomenon which lies, if not directly, yet virtually, within the sphere of vision. We remove or veil this small visual phenomenon, and a totally different effect is produced from those that took place when any of the other visual phenomena were removed or veiled. ”

“ The only reminder of the top view is the eye or point of vision, the picture plane that falls together for the sake of convenience with the horizon of the end view and the ray that determines the measurement point M, which is, in this suppressed reproduction, absolutely necessary, in order to find the apparent position of the real corners behind the picture plane. ”

“ 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. ”

“ If there were such a medium, acting as a mirror to nature or life, and retaining the impression, it must be universal as the supposed æther of the scientist; and how could impressions on this medium intelligible to us be focussed as the vibrations of light are through the needlepoint of the eye to record a single view-point? In our visions of the memory of nature we see undistorted figures. If we could imagine the whole body to be sensitive to light, as is that single point in the brain on which the optic nerves converge, what kind of vision would we have? ”

“ By so doing the range of the picture is increased and its space seems to take in more than its limits presupposed: If an isolated tree standing against a mass of trees, by opening the sky through that mass or by creating attraction of color or form therein, the vision is led to the far side of the object to be moved, which is thereby crowded out of its position in the balancing scheme.
An object upon a surface may frequently give place to a dark or light variation of the surface itself which becomes an equivalent of attraction. ”

“ When I look out of my window, the image of a very large tract falls upon my retina. I see at once a multitude of houses, and the infinitude of objects that go to make up the picture, and apparently I see every thing distinctly, but this is really far from being the case. If I look suddenly out at a landscape that I have never seen before, and fix my gaze upon a church-spire for a few moments, the image of the landscape falls immovably upon the retina ”

“ All that is properly perceived by the visual faculty amounts to no more than colours with their variations and different proportions of light and shade; but the perpetual mutability and fleetingness of those immediate objects of sight render them incapable of being managed after the manner of geometrical figures, nor is it in any degree useful that they should. ”

“ Thus we see that an image is formed on both eyes, and we consequently see the object, whatever it may be, repeated twice. By degrees, however, the eyes gain the power of converging their axes on objects at different distances, so that they fall on similar portions of each retina, and so convey a single impression to the brain. ”

“ In that case the image of the object is thrown upon what is called a photographic plate. Our own eyes act as lenses, and throw an image of whatever we look at, not upon a photographic plate, but upon a sensitive surface called the retina. ”

“ As an object becomes perceptible to us, by means of the image thereof which is formed on the retina, it will, therefore, be seen in that direction, in which the rays enter the eye to form the image, and will always be found in the line, in which the axis of a pencil of rays flowing from it enters the eye. ”

“ In photography, the operator, when he has found that a perfect image is thrown upon the ground glass, which represents the retina of the eye, substitutes for it a sensitive surface, on which the rays are projected, and which, by chemical means, produce a permanent instead of a fleeting object.
Examples of other lenses may be found in Nature. She, moreover, can perform a task which man has never even attempted, namely, the change of form in a lens according to the duty which it has to do. ”

“ We should find that by such undeveloped visual organs, the shadows traversing the rudimentary retina would be merely distinguished into those of the stationary objects which the creature passed during its own movements, and those of the moving objects which came near the creature while it was at rest; and that so the extremely general classification of visible things into stationary and moving, would be the earliest formed. ”

“ By the properties of light, the external scene produces, through the transparent parts of the eye, an image or picture exactly resembling the reality, upon the back part of the retina: and each point which we see, is seen in the direction of a line passing from its image on the retina, through the centre of the pupil of the eye. ”

“ 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. ”

“ Following upon the visual images are a swarm of others varying from picture to picture: tactile images giving the appearance of texture to surfaces, muscular images giving hardness, stiffness, softness, flexibility and so on to the volumes imagined—the lightness and insubstantiality of muslin, the solidity and fixity of rock being matters of the intervention of images due originally to the sensations we have received in the past from these materials. ”

“ The white wall or frame of white paper to receive the picture of objects, is a fair representation of the retina of the eye, on which all the images of external objects are depicted. Such are some of the general points of resemblance between the apparatus connected with the dark chamber, and the organ of vision; but the human eye is an organ of such exquisite construction, and composed of such a number and variety of delicate parts, that it cannot be adequately represented by any artificial contrivance. ”