Binocular vision

“ 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. ”

“ When clearness of vision in one eye is impaired, whether artificially or by disease, binocular vision is possible so long as the weak eye receives an impression of the object, however dim or indistinct the object may appear. ”

“ Binocular vision in man and in the higher animals is the last result of the gradual improvement of the most refined sense-organ, adapting it to meet the requirements of highly complex organisms. It cannot exist in some animals, such as birds and fishes, because they cannot converge their two optical axes upon a near point. When a chicken wishes to look intently at an object it turns its head and looks with one eye. ”

“ In the absence of any nerve lesions, central or other, interfering with their associated movements, the eyes continue throughout life to respond equally to the stimuli which cause these movements, even when, owing to a visual defect of the one eye, binocular vision has become impossible. ”

“ In monocular vision, on the contrary, the movement is uniformly upward and involves a progressive increase in error. When the illuminated point is exposed in an otherwise dark field the progression is preponderatingly downward in binocular vision and upward in vision with the single eye. The relation of these changes to phenomena of convergence, and the tendency to upward rotation in the eyeball has already been stated. ”

“ This final appearance of homogeneity may perhaps be facilitated by the fact that in binocular vision two points situated on the prolongation of the optical axis of one of the eyes, so that the near one hides the far one, are by the other eye seen laterally apart. Each eye has in fact a foreshortened lateral view of the other's line of sight. ”

“ Lesions which do not involve the centres may prevent the response to associated impulses in one eye alone by interfering with the functional activity of one or more of the nerves along which the stimuli are conveyed. Paralysis of oculo-motor nerves is thus a common cause of defects of association in the movements of the double eye. The great advantage of simultaneous binocular vision—viz. the appreciation of depth, or stereoscopic vision—is thus lost for some, or it may be all directions of fixation. ”

“ Frequently, however, even a slight impediment to binocular fusion, such as the application of a red glass to one eye, suffices to procure preponderance in the elastic tensions of the muscles, and to cause the fixed point to appear double. We can put a stop to binocular single vision still more surely by applying to one eye a prism with the base upwards or downwards. ”

“ By appropriate muscular training the eyes may be directed, each slightly outward, so that these lines meet behind the observer's head while the object, apparently duplicated, is seen still in front. The recognition of the subjective fusion of the two eyes into a Cyclopean, or central binocular eye, is a fundamental prerequisite for the explanation of vision in the stereoscope. ”

“ Binocular instruments should aid the natural spatial or stereoscopic vision, or make it possible if the eyes fail. If the objects be so far distant that the two perspectives formed by the naked eye are no more distinguished from each other, recourse may be had to binocular telescopes and range-finders; and if the objects be so small that, in order to observe details on them, we must bring our eyes so close to the objects that they cannot accommodate the images, recourse may be had to binocular microscopes and magnifying glasses. ”

“ With others, all efforts are in vain, it is impossible to render them conscious of the double images, notwithstanding that the presence of the binocular disturbance of vision proves that the habit of binocular fixation exists. ”

“ Contrast is occasioned, not by a false idea resulting from unconscious conclusions, but by the fact that the excitation of any portion of the retina—and the consequent sensation—depends not only on its own illumination, but on that of the rest of the retina as well.
"If this psycho-physical process is aroused, as usually happens, by light-rays impinging on the retina, its nature depends not only on the nature of these rays, but also on the constitution of the entire nervous apparatus which is connected with the organ of vision, and on the state in which it finds itself." ”

“ In this way an objective existence may appear to be given to the binocular eye by approaching a mirror until the nose touches the glass, and avoiding the convergence of visual lines that would otherwise be natural. A narrow face is seen, possessing but a single eye, that looks into the very depths of the observer's Cyclopean eye. ”

“ If we call these eyes, we must remember that in all probability they have no power of what we call vision—only a power of distinguishing light from dark. Where, however, there exists beneath the lens a so-called retina, that is, a layer of rod-like endings of a nerve, it might, at first sight, be thought that there, at any rate, we have true vision. But in all probability, in a great number of cases the retinal rods are simply for the purpose of rendering the organism sensitive, not only to the presence of light, but to its direction. ”

“ As the eyes of different persons are not the same distance apart, the first and most important point to observe in using the binocular is that each eye has a full and clear view of the object. This is easily tried by closing each eye alternately without moving the head, when it may be found that some adjustment is necessary by racking out the draw-tubes D, E, of the bodies by means of the small milled head near the eye-pieces ”

“ The new experiments show that, if a certain condition is fulfilled, the usual prolonged stare becomes unnecessary, a momentary glance sufficing to produce a strong but fugitive after-image. The condition is that the part of the retina where the image is to be formed, shall have been darkened immediately before excitation by the bright object. The retinal nerves, when in darkness, rapidly acquire a state of sensitiveness far exceeding the normal average in the light, but quickly diminishing again under the influence of illumination. ”

“ In all, or nearly all, perfect eyes the lens is flatter in front than behind, and this enables us to see things far off by bringing the rays from them exactly to a focus on the retina. But when we look at nearer things the rays require to be more bent or refracted, so without any conscious effort on our part this ciliary muscle contracts and allows the lens to bulge out slightly in front. Instantly we have a stronger magnifier, and the rays are brought to the right focus on the retina, so that a clear and full-size image of the near object is formed. ”

“ And now let us consider how it comes about that each elementary portion of the retina—at least in its central region—has the power of distinguishing so many hundreds of different hues. It is incredible that every little area of microscopic dimensions should be furnished with such a multitude of independent organs as would be necessary if each of the many colours met with in nature required a separate organ for its perception ”

“ 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. ”

“ The eye—which sees all objects reversed—retains the images for some time. This conclusion is proved by the results; because, the eye having gazed at light retains some impression of it. After looking (at it) there remain in the eye images of intense brightness, that make any less brilliant spot seem dark until the eye has lost the last trace of the impression of the stronger light. ”

“ The eyes of some persons are large, others small, and others of a moderate size—the last-mentioned are the best. And some eyes are projecting, some deep-set, and some moderate, and those which are deep-set have the most acute vision in all animals; the middle position is a sign of the best disposition. ”

“ Heaven lies about us in our infancy!
Shades of the prison-house begin to close
Upon the growing Boy,
But he beholds the light, and whence it flows,
He sees it in his joy;
The Youth, who daily farther from the east
Must travel, still is Nature's priest,
And by the vision splendid
Is on his way attended;
At length the Man perceives it die away,
And fade into the light of common day.
Earth fills her lap with pleasures of her own ”

“ Anaxagoras holds that sense perception comes to pass by means of opposites, for the like is unaffected by the like. He then essays to review each sense separately. Accordingly he maintains that seeing is due to the reflection a in the pupil, but that nothing is reflected in what is of like hue, but only in what is of a different hue. Now with most [creatures] this contrast of hue [with that of the pupil] occurs by day, but with some by night, and this is why the latter are keen of vision by night. But, in general, night the rather is of the eye's own hue. ”

“ Some great vision upward rises, Curving, bending, floating fair. Bands and sweeps, and hill and hollow Lead my fascinated eye; Some apocalypse will follow, Some new world of deity. Zoned unseen, and outward swelling, With new thoughts and wonders rife, Queenly majesty foretelling, See the expanding house of life! Sudden heaving, unforbidden Sighs eternal, still the same— Mounts of snow have summits hidden In the mists of uttered flame. But the spirit, dawning nearly Finds no speech for earnest pain; Finds a soundless sighing merely— Builds its stairs, and mounts again. ”

“ Our flesh shrinks from what it dreads and responds to the stimulus of what it desires by a purely reflex action of the nervous system. Our eyelid closes before we are aware that the fly is about to enter our eye. ”

“ THE BODY WHICH IS NEAREST TO THE LIGHT CASTS THE LARGEST SHADOW, AND WHY? If an object placed in front of a single light is very close to it you will see that it casts a very large shadow on the opposite wall, and the farther you remove the object from the light the smaller will the image of the shadow become. ”

“ This observation might be repeated with regard to all objects of the outer world which enter into relation with us. Whether the knowledge of them be of the common-place or of a scientific order matters little. Sensation is its limit, and [14] all objects are known to us by the sensations they produce in us, and are known to us solely in this manner. A landscape is nothing but a cluster of sensations. The outward form of a body is simply sensation; ”

“ He lived then before me; he lived as much as he had ever lived—a shadow insatiable of splendid appearances, of frightful realities; a shadow darker than the shadow of the night, and draped nobly in the folds of a gorgeous eloquence. The vision seemed to enter the house with me—the stretcher, the phantom-bearers, the wild crowd of obedient worshippers, the gloom of the forests, the glitter of the reach between the murky bends, the beat of the drum, regular and muffled like the beating of a heart—the heart of a conquering darkness. ”

“ I ask how far away the eye can discern a non-luminous body, as, for instance, a mountain. It will be very plainly visible if the sun is behind it; and could be seen at a greater or less distance according to the sun's place in the sky. ”

“ No small hole can so modify the convergence of rays of light as to prevent, at a long distance, the transmission of the true form of the luminous body causing them. ”

“ Kind words and loving deeds are as pebbles cast upon the great sea of humanity, the ever-widening circle of whose influence extends beyond the limited vision of him who projects them; and the eternal ages alone will reveal how many souls have been saved, and saved forever, as the grand result. How many girls and boys are watching every opportunity to share in this blessed work? ”

“ When making the analysis of matter we impliedly admitted two propositions: first, that sensation is the tertium quid which is interposed between the excitant of our sensory nerves and ourselves; secondly, that the aggregate of our sensations is all we can know of the outer world, so that it is correct to define this last as the collection of our present, past, and possible sensations. ”

“ For the eye is always in search of beauty, and if we do not gratify its desire for pleasure by a proportionate enlargement in these measures, and thus make compensation for ocular deception, a clumsy and awkward appearance will be presented to the beholder. ”

“ I believe it has required a long series of accepted observations for us to have arrived at this idea, now so natural in appearance, that the modifications produced within our nervous system are the only states of which we can have a direct consciousness; and as experimental demonstration is always limited, there can be no absolute certainty that things never happen otherwise, that we never go outside ourselves, and that neither our consciousness nor our nervous influx can exteriorise itself, shoot beyond our material ”