Eye (anatomy)

“ The left eye is the reflexion of the right. And the light-feeling retina of the left eye is a reflexion of the light-feeling retina of the right, in all its functions.
The lense of the eye, like a magic lantern, casts images of objects on the retina. And you may picture to yourself the light-feeling retina of the eye, with its countless nerves, as a hand with innumerable fingers, adapted to feeling light. The ends of the visual nerves, like our fingers, are endowed with varying degrees of sensitiveness. ”

“ The optic nerve is the channel by which the mind peruses the hand-writing of Nature on the retina, and through which it transfers to that material tablet its decisions and its creations. The eye is consequently the principal seat of the supernatural. ”

“ Let us now apply this to the optic nerve, which is the object of our present enquiry. In the first place, we know that no kind of action upon any part of the body except the eye and the nerve which belongs to it, can ever produce the sensation of light. ”

“ We regard the organ and its dispositions merely as a general condition regulating the apprehension of the visual sensations, and no more.
Thus, by attending to the two associations that occur,—the association (in place) of visual sensations with tangible bodies that are not the eye; and the association (in place) of vision with a small tangible body that is the eye—the eye regarded as the condition on which the apprehension of these sensations depends; by attending to these, we can understand how a protensive interval comes to be recognised between the organ and its objects. ”

“ We have only made one step: we have learnt how the optical arrangement of the eye renders it possible to separate the rays of light which come in from all parts of the field of vision, and to bring together again all those that have proceeded from a single point, so that they may produce their effect upon a single fibre of the optic nerve. ”

“ The End-organs and Their Response to Stimuli.—Thus the radiations of ether from the sun, our chief source of light, are so rapid that billions of them enter the eye in a second of time, and the retina is of such a nature that its nerve cells are thrown into activity by these waves; the impulse is carried over the optic nerve to the occipital lobe of the cortex, and the sensation of sight is the result. ”

“ The phenomena of ocular spectra and complementary colours experienced by every one forms a curious chapter in the history of those illusions which take their origin in the eye itself. Every one has noticed that after looking fixedly at a bright light or a striking colour for a few moments, the eye preserves an impression of the object for a certain time. A very light window looked at intently for several seconds will leave the impression of its cross-bars on the retina for several minutes, the colour of the image changing at every movement of the eye. ”

“ Aristotle, however, says that this fact is to be explained in another way: the iris of the eye shines like a phosphorescent substance; when the eye is struck, the sudden shock of the blow causes the eye as an object of vision to become separate from the eye as the organ of vision, and thus the eye for an instant sees itself! ”

“ The sensitive retina, spreading out, as it does, to form the back of the eyeball, is the nerve-coat of the eye; and from its centre a thick round bundle of nerve fibres, known as the optic nerve, runs back to the brain. ”

“ Having often observed in the dissections of men, as well as of brutes, that the optic nerve is not inserted exactly opposite to the pupil, that is, in the place where the picture of the objects upon which we look directly, is made: and that in man it is somewhat higher, and on the side towards the nose, he had the curiosity to examine the reason of this structure, by throwing the image of an object on this part of the eye. ”

“ Perhaps it may still be imagined, that although we are not made so as to see objects always in their true place, nor so as to see them precisely in the direction of the rays when they fall upon the cornea; yet we may be so made, as to see the object in the direction which the rays have when they fall upon the retina, after they have undergone all their refractions in the eye, that is, in the direction in which the rays pass from the crystalline to the retina. ”

“ If you will bear in mind the fact that the colors and values that are seen out of the corners of the eyes, are, on account of their very situation, able to affect only a very limited number of the sensitive nerves of the retina, you will understand that the force of their impact must be proportionately less than those which come to the eye from the full centre of vision ”

“ For the eye has every possible defect that can be found in an optical instrument, and even some which are peculiar to itself; but they are all so counteracted, that the inexactness of the image which results from their presence very little exceeds, under ordinary conditions of illumination, the limits which are set to the delicacy of sensation by the dimensions of the retinal cones.... ”

“ The optic nerve being seated at the bottom of the eye, has the images of surrounding objects painted upon it; it looks out through the different humors. The different membranes and humours which compose the eye, and serve for vision, are, the tunica conjunctiva, the tunica sclerotica, the cornea, the iris, the pupil, the choroides, and the retina. ”

“ 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. ”

“ For example, the very important optic nerve, which is necessary to vision, comes from each eye separately in a large bunch of fibres, and divides at the base of the brain, so that each eye sends impulses directly to the visual centres of both hemispheres. ”

“ For example, if the eye, adjusted to a dim light, is suddenly turned toward a brilliantly lighted object, the retina will receive too much light and will be shocked before the muscles controlling the iris can react to shut out the superabundance of light. ”

“ We answer, by teaching us to associate vision in general, or the abstract condition regulating our visual impressions, with the presence of the small tangible body we call the eye, and vision in particular, or the individual sensations of vision, (i.e. colours,) with the presence of immeasurably larger bodies revealed to us by touch, and tangibly external to the tangible eye. Sight, as we have said, does not inform us that its sensations are situated in the eye: it does not inform us that we have an eye at all. Neither does touch inform us that our visual sensations are located in the eye. ”

“ But the eye is the only sense-organ that has a bodily constitution peculiar to itself. It is fluid and cold, and does not exist from the first in the place which it occupies later in the same way as the other parts do, for they exist potentially to begin with and actually come into being later, but the eye is the purest part of the liquidity about the brain drained off through the passages which are visible running from them to the membrane round the brain. ”

“ According to our new theory, then, there are in the retina of the eye, where the pictures of external objects fall, three sets of nerves, adapted for the production of three separate, distinct sensations, which we call red, green, and violet. When, owing to any cause whatever, one of these sets of nerves is excited into action, the result is the corresponding sensation; if, for example, we act upon the last set by electricity, pressure, or by the luminous waves, the result will be the sensation of seeing violet light, even though not a ray of light of any kind has actually reached the eye. ”

“ Since some few years now we have learnt how to explain the obscurity of the interior of the eye, and by what optical contrivances we can overcome this darkness and look into the depths of the ocular globe ”

“ There is one fastened to the eyeball in two places, and so arranged that it will move the eye in any direction, as when we roll our eyes; and a sixth fastened to the under side of the eye to keep it steady when we do not need to move it. Then the eyelids are provided with suitable gearing, and it needs to be durable too, for it is said to be used thirty thousand times a day, in fact, every time we wink.
Not less wonderful is the construction of the eye itself. The optic nerve is the part of the eye which conveys visions to the mind. ”

“ We are so accustomed to merely play with children, that we are easily blinded to the full seriousness of such requests.
We cannot break off these reflections on the image of the retina without making mention of a remarkable spot in the background of the eye—the yellow spot marking the entrance of the optic nerve. All perception whatever is arrested within the bounds of this spot—it is a blind point in our field of view. ”

“ Here we enter a border region where the seeing process begins to pass over into a pathological condition.4 A 'secret' of the effect of light on the eye is here revealed which remains hidden in ordinary vision, for normally the different forces working together in the eye hold each other in balance, so that none is able to manifest separately. This equilibrium is disturbed, however, when we suddenly expose the eye to light while it is adapted to darkness. The light then acts on the eye in its usual way, but without the immediate counter-action which normally restores the balance. ”

“ The shapes reach the retina, but they are not received by a spontaneous, direct action of that organ. The white surface of the paper is reflected, but the letters are detected only by a discriminative effort of the optic nerves. This effort annoys the nerves, and, when long continued, exhausts their susceptibility. The human eye can not long sustain the broad glare of a white surface without injury. ”

“ The third, those who have become blind by some morbid action, though the mechanism of the eye has been little affected, and though no impediment has occurred but in regard to the action of the optic nerve. ”

“ The nerve which goes to the eye is called the optic nerve, and it is connected with each square by pretty rods and cones, which look like those in your own eye (Fig. 27) . The rods and cones are covered with coloring-matter or pigment, which turns red when it is boiled. ”

“ IV. Of direct ocular spectra.
A quantity of stimulus somewhat greater than natural excites the retina into spasmodic action, which ceases in a few seconds. A certain duration and energy of the stimulus of light and colours excites the perfect action of the retina in vision; for very quick motions are imperceptible to us, as well as very slow ones, as the whirling of a top, or the shadow on a sun-dial. So perfect darkness does not affect the eye at all; and excess of light produces pain, not vision. ”

“ The eye with normal vision, therefore, sees one part of everything it looks at best, and everything else worse, in proportion as it is removed from the point of maximum vision; and it is an invariable symptom of all abnormal conditions of the eyes, both functional and organic, that this central fixation is lost. ”

“ Hence, when the eye shifts normally no error of refraction is manifest. The more rapid the unconscious shifting of the eye, the better the vision; but if one tries to be conscious of a too rapid shift, a strain will be produced. ”

“ As the disc revolves, each hole in turn crosses the line EL. Thus the luminous hole H is successively covered and uncovered to the eye E; and if the eye moves, a succession of points on the retina is stimulated by the successive uncovering of the luminous spot. No fixation-points are provided for the eye, since such points, if bright enough to be of use in the otherwise dark room, might themselves produce confusing streaks, and also since an exact determination of the arc of eye-movement would be superfluous. ”