The nerve in your eye gets messages of light much as the nerve deep in your ear gets its messages of sound--from tiny waves in the air. The light waves are smaller and faster even than the sound waves, and the eye nerve is the only nerve that can get pictures of them. You know that, for wireless messages, the receiving machines are not all alike and cannot all take the same messages, if the messages are sent with different sorts of electric waves; and neither can our receiving machines. Some get messages of sight, and some of sound, and some of touch, or taste, or smell. Now shut your eyes as quickly as you can. How long did it take you? A minute? No, not a quarter of a second. It is about the quickest thing you can think of--"the twinkling of an eye." You shut your eyes "quick as a wink" whenever anything seems likely to fly or splash into them, and this is what the eyelids are for. If anything gets into the eye before the lids can shut, the eye "waters," and _tears_ pour out of it. These are made by a gland-sponge up under the upper lid, so as to wash any dust or sand or other harmful speck out of the eye before it can hurt the sensitive eyeball. Now look at some one's eyeball. It is like the picture, isn't it?--bright white around the edge and then a ring of color, brown or blue or gray; and inside the color-ring, or _iris_, a little round black hole that we call the _pupil_. Watch the little hole change as you turn the face toward the window. It becomes ever so much smaller. Now turn the face away from the window, back again into the shadow. How did the pupil change this time? [Illustration: EYES PROTECT THEMSELVES AGAINST THE LIGHT] The iris, or color-ring, acts like a curtain, like the ring-shutter of a camera, and closes up the hole, or pupil, when the light is too bright and would dazzle or burn the inside of the eye; but when the light is dim, the iris opens again, so as to let in light enough with which to see. Look at the little window in your kitten's eyes. It is not the same shape as yours; but when you carry her to the light, you see how the iris closes in and leaves just a little black slit or line. You remember the blind children? Isn't it wonderful how they can play games and study, too, even though they are blind! They have to make their senses of touch and hearing tell them many things that you learn through your sense of sight. Many of these children _need not have been blind_, if the nurse who first took care of them when they were born had known enough to wash their eyes properly, not with soap and water, of course, but with just one or two drops of a kind of medicine--an _antiseptic_, as we call it--that makes the eye perfectly clean.