Vision: the structure of the eye (1936)

As soon as we get up, we start to use our eyes. Yet, how often do we give a thought to these delicate and complicated instruments so long as they are working to our satisfaction? From early morning until late at night, we make constant demands on them. At home, at work, and at play, they are continually adjusting themselves to changing conditions. Let's examine the eye in detail and see how it works. Here is a section of an eye seen from the side. The cornea is the transparent part of the strong outer covering. The iris controls the amount of light admitted to the eye by making the pupil or aperture of a light larger or smaller. The lens throws an image onto the sensitive screen known as the retina. The impressions on the retina are carried to the brain by the optic nerve. In this way, we see. In principle, the working of the eye is similar to that of the ordinary camera. The camera records the image of an object on a film or glass plate which is covered with a substance sensitive to light. The light rays from the object pass through the lens and form an image on the sensitive plate or film. In the same way, the lens of the eye forms an image on the sensitive surface of the retina. It is then transmitted by the optic nerve to the brain. But if the brain or optic nerve is damaged or affected by ill health, eyesight will be defected. Here is an inverted image of an object as it appears at the back of the camera. To make it sharp, a focusing device is fitted which corrects the distance between the lens and the plate. In the eye, focusing takes place automatically by means of muscles which alter the shape or curvature of the lens. This has the same effect as altering the distance between lens and retina. The pupil is surrounded by a colored band known as the iris which by adjusting itself to the varying intensity of light alters the size of the pupil. This too has its counterpart in the camera. The variable lens diaphragm controls the amount of light passing through the lens. The eye adjusts itself automatically to the quantity of light. Coming from the bright light of day to the inside of a cinema, the eye can at first see little or nothing of the surroundings. But after a time, it adjusts itself and becomes accustomed to the change. The iris has expanded so that more light is admitted to the eye. In modern life, the muscles of the eye which enable it to adapt itself to varying circumstances become strained. This may be on account of the conditions of work of a particular job. In the case of a watchmaker, powerful magnifiers or in oxy acetylene welding, the intense glare which cannot be born without a mask or goggle. Car driving where the eyes are glued on the road and where all objects are on the move is very tiring to the eye muscles. Even in more hundrum occupations, a great strain is put on them. By being kept adjusted in one position for long periods at a time, the eye becomes fatigued and loses its adaptability. It then becomes necessary to assist the tired muscles by screwing up the eyes or by taking the object away from the eye instead of adjusting the eye to the object. This is a symptom of long sight and the eyeball instead of being spherical is slightly short. A short eyeball means longsightedness and is a common defect. At first the distance between the lens and the retina can be corrected by making the lens thicker. But after a time when the eye muscles through strain become tired or lose their adaptability, the eye cannot focus properly. In long sight, the image is not sharp on the retina, but a little behind it. Glasses of the proper type bring the image forward onto the retina. Glasses may be required to correct other faults which strain may uncover, slightism, for example. This means that the eye can focus in one direction but not in another. Headaches and nervous disorders result until proper glasses give correction. In short sight, the eyeball is long. The image is sharp at a point in front of the retina and is therefore not clearly seen. To counteract this defect, it would be necessary to flatten the lens. The focusing muscles are unable to do this. The only adjustment they can make is to thicken the lens, which brings the image still further forward. A concave spectacle lens pushes the image back onto the retina. Strain or physical abnormality is not always the cause of defective vision. Here are the nerve and blood vessels at the back of the eye coming from the brain. Thus, the eye shares in the life and health of the body. Since the eye is closely connected with the general nervous system and the bloodstream, faulty eyesight is often an indication of poor bodily health. Whenever you have eye trouble, go and see an eye doctor. You may think you only need glasses. Go to him just the same. Only he can tell whether the brain, nerves, or blood system are at fault or are being affected. 29 people out of every hundred with defective vision require medical attention as well as glasses. And for six in every 100 such persons, medical attention alone is required. In everyday life, neglect may result in permanent damage to the eye or to the general health. See an eye doctor at the first indication of defective vision. The advice of an eye doctor saves sight.