Visual acuity

“ With the Snellen test card and trial lenses conclusions must be drawn from the patient's statements as to what he sees; but the patient often becomes so worried and confused during the examination that he does not know what he sees, or whether different glasses make his sight better, or worse; and, moreover, visual acuity is not reliable evidence of the state of the refraction. ”

“ In view of the prevailing theories about the relation of heredity to myopia, this picture is particularly interesting. A decline in visual acuity at the distance, however, is no more a peculiarity of civilization than is a similar decline at the near-point. ”

“ You ask how men have arrived at the knowledge of these things, seeing the mechanism in question is almost entirely removed from any direct investigation. First, then, the umbellar expansion of the optic nerve, the retina, enables us to make experiments; this retina being in such close contact with the eye, the optical part of the visual organ, that it is accessible to every sort of mechanical irritation. ”

“ The apparatus of rods, cones, and other structures are absolutely essential to enable light to give that stimulus to the optic nerve which, conveyed to the brain, is converted into visual sensations. So if this apparatus were absent in our organs of vision, in vain would the optic nerve proper be spread out over the interior of the eyeball: we should be no more able to see with such eyes than we are able to see with our hands.
We now invite the reader’s careful consideration to the diagram, Fig. 236, which is a section of the retina through the yellow spot. ”

“ Accommodation for near objects consists in a contraction of the circular ciliary muscle and an increase in the convexity of the anterior surface of the crystalline lens.
The light enters the eyeball through the pupil, falls upon the retina, which has often been compared to the sensitive plate of a camera, is received and transmitted by the optic nerve to the visual centers of the brain. The eyeball does not see. It is only a sensitive end-organ, which receives and transmits the impressions to the higher centers of sight. The act of vision is performed in the brain. ”

“ 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. ”

“ 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. ”

“ It can be taken as a fact, which the theory of the action of the eye implies, that for each external impression, that is, for each image produced upon the retina, the ends of the visual nerves, concerned in the conveyance of the impression to the mind, must be under a peculiar stress or in a vibratory state. ”

“ The blind spot does not indicate diseased eyes, but it simply marks the point where the optic nerve enters the eyeball, which point is not provided with the necessary visual end organs of the sight, known as rods and cones. ”

“ AS noted in the chapter on What Glasses Do to Us, the sight always improves when glasses are discarded, though this improvement may be so slight as not to be noticed. In a few unusual cases the patients, when freed from the handicap of a condition which compels them to keep their eyes continually under a strain, find out, in some way, how to avoid strain, and thus regain a greater or less degree of their normal visual power. ”

“ This strain, when it is habitual, leads to all sorts of abnormal conditions and is, in fact, at the bottom of most eye troubles, both functional and organic. The discomfort and pain may be absent, however, in the chronic condition, and it is an encouraging symptom when the patient begins to experience them.
When the eye possesses central fixation it not only possesses perfect sight, but it is perfectly at rest and can be used indefinitely without fatigue. It is open and quiet; no nervous movements are observable; and when it regards a point at the distance the visual axes are parallel. ”

“ 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. ”

“ The commonest visual defects are, first, inflammation of the cornea, or imperfections of the lens—the cornea is often so scarred as to make vision imperfect; second, myopia, or progressive shortsightedness, a condition in which the axis of the eye gradually grows longer. This lengthening is accompanied by stretching of the eyeball, and such children always run the risk of the inner and most important part of the wall of the eye, the retina or nerve layer, being torn away, and blindness resulting. ”

“ In early life, when we look at near objects, the crystalline lens automatically becomes thicker, more convex. This adjustment brings the rays to a focus on the retina, which is required for good vision. As we get old, the crystalline lens loses its power to change its adjustment for near objects, al though the eye may see at a distance as well as ever. The old person, therefore, must wear convex glasses when looking at near objects, as in reading and sewing.
Another visual defect of a different nature is known as partial color blindness. ”

“ On a similar principle, we should flood with diffused light the rooms in which numerous workmen are to be gathered, some of whom must be far from the windows. With a good light, or one which is equivalent to several million candles a yard off, we use in reading only a fraction of the cornea, and the contraction of the pupil has the effect of greatly diminishing the diameter of the circles of diffusion which are liable to produce in the retina faults of vision. Under these conditions, a badly formed eye may perform good service, and is subject to only a moderate degree of fatigue. ”

“ The sense of vision is excited by the influence of light on the retina, the special terminal organ connected with the optic nerve. By excitation of the retina, a change is induced in the optic nerve fibres, and is conveyed by these to the brain, the result being a luminous perception, or what we call a sensation of light or colour. ”

“ Even if the doctor says this girl is sound and strong, her early months of college labor should be carefully watched. Above all, her eyes should be seen to, because in my experience some unsuspected disorder of vision has been fruitful of headaches and overstrain of brain, nor is it enough to know that at the beginning her eyes are good. Extreme use often evolves practical evils from visual difficulties at first so slight as to need or seem to need no correction. ”

“ 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. ”

“ 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. ”

“ Behind the vitreous humour, and immediately opposite the pupil and lens, is the most delicate and important of all the membranes of the eye, the retina, which serves as a screen whereon are received the images of the objects around us. This membrane is an expansion of the optic nerve N leading from the brain, and lines the whole of the interior of the eye. ”

“ The demand for binocular fusion of the retinal images will be greater if both eyes are of equal value; on the contrary it will be less, if the retinal image or the visual acuteness of one eye is less perfect than that of the other. Varieties of weakness; when one eye always receives a clear retinal image, the other an indistinct one; lowering of the visual acuteness of one eye by nebulæ, astigmatism or any other cause. ”

“ 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 ”

“ The seat of vision is generally supposed to be the retina, though some philosophers regard the choroid coat as the sensitive tablet upon which the impression is made. We may trace the phenomena of vision up to this point, but no further. We know that a distinct image is formed upon one or other of the delicate coats of the eye, but the manner in which the sensation is conveyed to the brain is an inscrutable mystery. ”

“ EYE-STRAIN.—Eye-strain is commonly due to either astigmatism, nearsightedness, farsightedness, or weakness of the eye muscles. The farsighted eye is one in which parallel rays entering the eye, as from a distance, come to a focus behind the retina. The [Pg 22] retina is the sensitive area for receiving light impressions in the back of the eyeball. Sight is really a brain function; one sees with the brain, since the optic nerve endings in the back of the eye merely carry light impressions to the brain where they are properly interpreted. ”

“ 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. ”

“ Having established this, we have a basis for an understanding of the complete process of vision. We see that it is by no means solely the nerve part of the eye which is responsible for vision, as the spectator-physiology was bound to imagine. The very fact that the place where the optic nerve enters the eye is blind indicates that the function of mediating sight cannot be ascribed to the nerve alone. What we call 'seeing' is far more the result of an interplay between the retina carrying the nerves, and the choroid carrying the blood-vessels. ”

“ Unnatural impressions are made on the retina; flashes of strong light, or luminous sparks, appear to dart across the eyes; darkened spots are seen where none exist; gnats, flies, or other minute bodies, various in colour and brilliancy, seem to flutter before the face; or a single dark speck intercepts the vision. Usually the pupil is dilated and the iris insensible to the stimulus of light ”

“ In this case, while the retina, or optic nerve, was deprived of light, its excitability accumulated, or became more easily affected by light; for if a person goes out of one room, into another which has an equal degree of light, he will feel no effect. You may convince yourselves of this law by a very simple experiment—shut your eyes, and cover them for a minute or two with your hand, and endeavour not to think of the light, or of what you are doing ”

“ In the normal human eye, only the central parts of the retina are sensitive to colour, the peripheral parts are practically colour-blind. Anæmia of the retina, which may be produced by pressure on the eye-ball, will render the retina, first colour-blind and then insensitive to light. ”

“ 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. ”

“ Sudden fluctuations of light undoubtedly cause discomfort to many persons, but far from being injurious, I have found them, in all cases observed, to be actually beneficial. The pupil of the normal eye, when it has normal sight, does not change appreciably under the influence of changes of illumination; and persons with normal vision are not inconvenienced by such changes. ”

“ For, when we view any object with attention, we make the central point of it fall upon the central part of the retina. Farther, as the optic nerve sends off no branches, but is spent wholly upon the retina, this may perhaps contribute in some degree. And these considerations may a little help us to conceive, how the optic nerve, and corresponding region of the brain, may be the repository of such an immense variety of visible ideas, as they are in fact. ”