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! [Illustration: 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.] Here we have a picture of the _eyeball_, as we call it. The little bands fastened to it are the bands of muscle; and as soon as I say _muscle_ you know what they are for--to move the eyeball about, up and down and from side to side. There are muscles outside the eye as well as inside. Coming out from the back of the eyeball is a pearly white cord quite different from the muscle bands. This is what we call a _nerve_. This nerve in your eye carries to your _brain_, or thinking machine, picture-messages of whatever you look at.