Visual system

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Work built up on outline drawing to which has been added light and shade, colour, aerial perspective, &c., may eventually approximate to the perfect visual appearance. And inversely, representations approached from the point of view of pure vision, the mosaic of colour on the retina, if pushed far enough, may satisfy the mental perception of form with its touch associations. And of course the two points of view are intimately connected. You cannot put an accurate outline round an object without observing the shape it occupies in the field of vision. ”

“ But, if in place of looking at the solid object itself we look with the right and left eyes respectively at pictures of the object corresponding to those which would be formed by it on the retina of the two eyes if it were placed at a moderate distance in front of them, and these visual pictures brought into coincidence, the same conception of a solid form is generated in the mind just as if the object itself were there. ”

“ 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. ”

“ Through a pair of experimental lenses placed by his examiner before his hitherto undiscovered visual brain cells, the young student who has had poor vision and has never known it, may obtain, for the first time, a glimpse of the beauty in his surroundings. ”

“ As a matter of fact, we are constantly looking in different directions, and, as a rule, only noticing what, at the moment, is in the centre of our field of vision. Thus our visual world consists rather of a synthesis of things viewed directly in succession than of things seen simultaneously while the centre of the field is kept fixed. ”

“ Here, following an unusual position of the eyes, an object is seen double because it stimulates parts of the retinae ordinarily stimulated by two objects, and the modification has also its analogue in the visual sense. ”

“ In pursuance of this line of thought I will offer one additional illustration of the direct dependence of psychic perception upon what may be termed the structural aberration of the visual apparatus.
When external objects are viewed with one eye, held at rest, the image upon the retina is exactly similar to that upon the sensitive plate of the camera, it has length and breadth, but no depth, and it has no power of directly arousing in the mind a perception of the third dimension—projection. ”

“ For every individual observer, the position of the disc on the screen has been affected by each change in the direction of these visible lines. In every case, also, its location when these boundaries lay in a horizontal plane was intermediate between the other two. The importance of such relations in the objects of the visual field as factors in our ordinary determination of the subjective horizon is made evident by these experimental results. They become construction lines having assumed permanence in the world of visual-motor experience. ”

“ 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 ”

“ 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. ”

“ As soon as the eye opens, the visual act is effected. Just because the act is simple, the slightest negligence on the part of nature in the building of the infinitely complex machine would have made vision impossible. This contrast between the complexity of the organ and the unity of the function is what gives us pause. ”

“ Sight.—Vision is a distance sense; we can see afar off. The stimulus is chemical in its action; this means that the ether waves, on striking the retina, cause a chemical change which sets up the nerve current responsible for the sensation.
The eye, whose general structure is sufficiently described in all standard physiologies, consists of a visual apparatus designed to bring the images of objects to a92 clear focus on the retina at the fovea, or area of clearest vision, near the point of entrance of the optic nerve. ”

“ 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. ”

“ But the chief point remains that we see the whole picture with both eyes and not with only one, and that we are constantly reminded of the flatness of the picture because the two eyes receive identical impressions. And we may add an argument nearly related to it, namely, that the screen as such is an object of our perception and demands an adaptation of the eye and an independent localization. We are drawn into this conflict of perception even when we look into a mirror. ”

“ 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. ”

“ 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. ”

“ To be satisfactorily seen as a single composition or group they ought to lie, all of them, within a visual angle of thirty degrees. We may, within these limits, disregard [Pg 23] the fact that visual attractions lose their force as they are removed from the center of the field of vision. As equal attractions in the field of vision, the dots in any unsymmetrical arrangement may be brought into a balance by weighing the several attractions and indicating what I might call the center of equilibrium. This is best done by means of a symmetrical inclosure or frame. ”

“ The vision system has to cope with poor contrast, partial views of parts, an infinite number of stable states, variable incident and reflected lighting, shadows, geometric transformations of the image due to variable distance from camera lens to part, etc., a formidable problem in scene analysis. ”

“ 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? ”

“ For the most part there will be nothing beyond a confused idea of light and shade. The time of this momentary vision will be too short to enable the human retina to perceive or the human brain to register any definite impression of anything.
How is it with the photographic camera and lens, our artificial eye? We will suppose that everything is in readiness, that its retina or sensitive plate is in perfect condition, and that not a ray of light has yet entered within the darkened chamber. ”

“ For, as we have been told, “to look at anything means to place the eye in such a position that the image of the object falls on the small region of perfectly clear vision, ... and ... whatever we want to see, we look at, and see it accurately; what we do not look at, we do not, as a rule, care for at the moment, and so do not notice how imperfectly we see it.” Such is the case, as has been shown, for when we fix our sight on the principal object or motif of a picture, binocular vision represents clearly by direct vision only the parts of the picture delineated on the points of sight. ”

“ Were it not for the retina there would be no color; and when the sensory nerves of the retina are partially paralyzed or deficient, as in the case of the color-blind, nature appears to the eye in her true monochromatic garb.
The human eye resembles closely the photographic camera, both in structure and in its manner of functioning. ”

“ It is impossible that the eye should project from itself, by visual rays, the visual virtue, since, as soon as it opens, that front portion [of the eye] which would give rise to this emanation would have to go forth to the object and this it could not do without time. And this being so, it could not travel so high as the sun in a month's time when the eye wanted to see it. ”

“ Often, too, a light point visible to one eye will not be so to the other, this sometimes provoking the phenomenon of binocular vision, which asserts itself for a moment or two, when the diverse images coalesce again, though imperfectly, giving an impression of movement. For one gifted with imagination and the natural quality of visualizing brain-pictures, these shifting light-points and the more or less definite and repeated reflections of surrounding objects offer abundant material out of which to construct lifelike pictures apparently seen in the crystal. ”

“ Under ordinary conditions of vision then, there is thrown upon the center of the retina a more or less sharply defined image of objects the light from which has entered the pupil. In addition, the whole of the retina is illuminated by a diffuse reddish glow, due to light leaking through the white of the eye, a condition the parallel of which would completely subvert the efficiency of an artificial camera. Apparency, then, evolution has produced for us an optical instrument which is hopelessly defective. ”

“ CHAPTER XIV - IMAGINATION AS AN AID TO VISION
WE see very largely with the mind, and only partly with the eyes. The phenomena of vision depend upon the mind's interpretation of the impression upon the retina. What we see is not that impression, but our own interpretation of it. Our impressions of size, color, form and location can be demonstrated to depend upon the interpretation by the mind of the retinal picture. ”

“ When we look through the window and see an object such as a house before us, we do not habitually localize the sensation in a certain part of the retina, but we refer the object to a particular position more or less distant in the world around us. This projection of the object outwards in a right line from the eyes is really a marvellous process, though the wonder of it is lost in its familiarity. It is the outcome of the experience of hundreds of generations. And the experience is not gained through vision alone, but through this in combination with other senses and activities. ”

“ Get the form, as much as possible, by the motion of the hand, not looking at the needle-point. Use the eye only to select colour, and then to keep the work within certain bounds. The practice of following with the needle lines traced on the material with a pen or chalk is fatal to colour inspiration (not necessarily to colour knowledge) ; as the retina, fatigued by unnatural exercise, loses its sensitiveness to immediate colour-impulses. ”