Just as soon as lessons begin, you get out your books; and a good share of the day in school you have a book before you, reading it or studying it or copying from it. It makes a great difference to your eyes how you hold the book and how the light falls. In reading, you should always hold your book so that the light falls upon the page from behind you, or from over one of your shoulders. In this way, the brightest light that comes into your eyes is not from the window, but from the page of your book.
If the light comes from a window in front of you, or if you sit in the evening with your face toward the lamp when you read, the light coming straight from the lamp or the window, as well as the light coming up from the pages of the book, pours into your eyes; and this dazzles and confuses your eyes, so that you can’t see plainly and comfortably and are very likely after a while to find that your head aches. At home, of course, you can seat yourself with your back to the light when you read; and usually at school your seats are so arranged that the light falls from behind you or from one side. If not, by turning a little in your seat, you can get the light from over your shoulder.
Notice how the light falls upon the blackboard. When the light comes from the windows behind you, or from one side, you can see what is written there quite plainly. But if the blackboard happens to be between two windows, and especially if this is the lightest side of the room, you will find that the light dazzles you so that you cannot see the writing clearly.
You must have noticed, too, that if, after you have been reading from the blackboard you look down again suddenly to the page of your book, for an instant you will not see the letters plainly. Then, almost before you have time to notice it, you feel a little change take place inside your eyes, and the print upon the page of your book becomes quite plain. This is because your eye has to change the shape of one of the parts inside it, called the lens, before you can see clearly the things that are near you. This change, which is called accommodation, is made by a little muscle of the eye; and if you keep your eyes working at close work, like reading or writing or fancy-work, too long at a time, or if your eyes need glasses to make them see clearly, and you haven’t them on, this little muscle becomes tired. Then the print of your book, or your writing, or the stitches you have taken begin to blur before your eyes. Your eyes begin to feel tired, and your head begins to ache. This is what we call eye strain.
Sometimes this eye strain upsets your appetite or your digestion and makes you sleepless and worried. The trouble may be caused by your own carelessness: you may have been reading too long, or in a poor light, or with the light shining right in your face instead of coming over your shoulder. But sometimes it is caused by the fact that your eyes are not just the right shape; and then the only way to relieve it is to have proper glasses, or spectacles, fitted, which will make up for this too flat or too round shape, or too large or too small size, of your eyes.
If you cannot see clearly what is written on the blackboard when the light falls upon it from behind you, or above; or if, in a good light, you cannot read the words in your book quite easily, without straining at all, when you hold the book either at arm’s length or a foot from your face; or if your head aches or your eyes begin to feel tired or uncomfortable, or the letters begin to blur, after you have read steadily—say, for half an hour,—it is a pretty sure sign that there is some trouble with your eyes. Then you had better have them examined at once by your family doctor or by the school doctor. In many schools now there are doctors to test the children’s eyes, and ears, too, so that each child may have a chance to see and hear everything that the other children can see and hear.
Not very many years ago people thought that glasses were only for old people, but now we know that many children’s eyes need glasses, too. I knew a little girl whose sight was so poor that when she was standing and looked down at the grass, she couldn’t see the green blades. She thought that the grass looked like a green blur to everyone, just as it did to her; and so she never said anything about it. She was twelve or thirteen years old before she found out that she couldn’t see clearly. Of course, trying hard to see things gave her a headache and made her tired and cross. So some one took her to a doctor, and he saw at once what was the matter and fitted her with glasses. Soon she was quite well and strong; and how glad she was to see the leaves and a hundred other things she had not seen before!
THE EYEBALL IN ITS SOCKET
The muscle from M to M, which helps to turn the eyeball, has been cut away to show the optic nerve.