=168. How to use the eyes.=--If using your eyes makes them painful or gives you a headache, you are straining your eyes. Facing a bright light strains the eyes. Shade your eyes while you study. A cap may be used as a shade if you cannot get anything else. Never try to look at the sun or a very bright light. You should have the light at one side or behind you. The light should be steady. Reading in a dim light will harm the eyes. =169. Near sight.=--If you cannot read without holding your book less than a foot from your eyes, you are nearsighted, and should wear glasses all the time. If you do this, your eyes may be strong, and you may be able to see well. =170. Far sight.=--If you cannot read without holding your book at arm's length, you are farsighted and need glasses. Most old persons are farsighted. =171. Alcohol and the eyes.=--Alcohol makes the eyes red. It weakens the eyes and may produce blindness. A drunken person often sees double. =172. Tobacco= causes dimness of sight and sometimes produces blindness. =173. Hearing.=--We hear with the ears. Sound is made by waves in the air. The part of the ear on the outside of the head catches the air waves and throws them inside the ear. These air waves strike against a little drum. Three little bones then carry the waves on to nerves farther inside the head. Animals can turn their ears and catch sound from any direction. [Illustration: =Diagram of the ear.= _a_ outer ear. _b_ drum head. _c_ _d_ and _e_ bones to carry sound to inner ear. _f_ _g_ and _h_ inner ear. _i_ tube to the mouth. _j_ middle ear.] =174. Ear wax.=--Wax is formed just inside the ear. It keeps flies and insects from crawling into the ear. Boys in swimming sometimes get cold water into their ears. This may make them have an earache. =175. How the throat affects the ear.=--An air tube runs from the inside of the ear to the mouth. Sometimes when you blow your nose, you blow air into the ear. This makes you partly deaf and you hear a roaring in your ears. Sometimes when you have a cold in your throat, this little tube is stopped. Then your ear may ache and may even discharge matter. This may make you somewhat deaf. Earache and deafness are most often due to a cold in the throat and a stoppage of this tube. Many little boys and girls are deaf and do not know it. They cannot hear the teacher well, and sometimes the teacher thinks they are bad or careless because they do not answer. =176. Care of the ears.=--Very loud noises may harm the ear and make you deaf. When you expect a very loud noise, put your fingers in your ears to shut out the sound. Boxing the ears may break their tiny drums and make you deaf. Do not get cold water in your ear. This may cause an earache and make you deaf. If you get water in your ear while you are in swimming, turn your head to one side and shake it. This will get the water out. Do not put cotton or anything else into your ears.