is in the case of membranes that are not tightly stretched. A stretched membrane, like a drumhead, has its own vibration rate, but one that is not on the stretch is able to vibrate at almost any rate. This fact is taken advantage of in the telephone and the phonograph, both of which depend on being able to vibrate at various rates almost equally well. In the ear, also, an unstretched membrane is the vibrator. We are familiar with it as the eardrum. It is located at the bottom of the ear canal, but cannot be seen by looking therein, because of a slight curve at about the middle. When the ear specialist wants to examine the eardrum he thrusts a small metal tube into the canal. This straightens it out enough to bring the drum into view.
The eardrum does not act directly upon the sensitive hearing apparatus, but its vibrations are transmitted across a space known as the middle ear. The necessity for this space is found in the fact that atmospheric pressure is not constant, but changes frequently from day to day, besides falling off as one ascends higher above sea level. The free action of the eardrum depends on its not being stretched; if there were no means of readjustment it might be properly set for one air pressure, but greater pressures would bulge it inward, putting it on the stretch, and so cutting down its ability to respond to a wide range of tones. The middle ear, which is the space behind the drum, connects with the outside air by a tube leading from it to the back of the throat, which latter communicates freely with the air through the nose, as well as through the mouth whenever it is open. The tube is known as the Eustachian tube. Its walls are ordinarily collapsed, so it is not an open passage, but every time one swallows the tube is pulled open, thus allowing differences in air pressure on the two sides of the eardrum to equalize. Whenever one ascends a high hill quickly, as by train or automobile, or even in going to the top of a high building by elevator, the difference in air pressure behind and in front of the eardrum can be felt. The sensation is disagreeable, and there is definite impairment of hearing. Repeated swallowing gives relief.
The vibrations of the eardrum are transmitted across the space of the middle ear by a chain of three tiny bones; these are very irregular in shape, and are attached to one another in such a way that every movement of the drum is followed exactly as to time and direction, but with reduced size and increased power. The hearing apparatus, which is part of the internal ear, but not the same part as makes up the equilibrium organ, contains liquid which is set moving by the last of the chain of bones, and this liquid acts upon the actual sensitive cells which make up the sense organ proper. These latter are arranged in an exceedingly complicated fashion. There are various theories as to the precise manner in which the vibrations of the liquid in the inner ear arouse the sensitive cells. It is thought that different cells respond to tones of different pitch, but exactly how this is accomplished is not known.
Deafness may result from failure of the sensitive inner ear to respond, or from poor transmission of vibrations across the middle ear by the chain of bones, or by interference with the freedom of action of the eardrum, or by preventing the air waves from striking upon the drum. Injury to the inner ear is rare, because of its secure position within the bone. A common form of deafness is the result of hardening of the connections between the ear bones, so that the chain no longer follows well the vibrations of the drum. This usually begins to come on during the twenties or thirties, and causes almost complete deafness by the age of fifty. In most if not all cases it is hereditary. Interference with the action of the eardrum may be due to the partial destruction of the drum itself. Scarlet fever and measles are particularly likely to leave the drum in a delicate condition, and any strain upon it then may rupture it beyond repair. Continuous closing of the Eustachian tube, by preventing equalization of air pressure on the two sides of the drum, causes partial deafness. Inflammation accompanying a cold may cause this, or the growth of adenoids in the back of the throat. Adenoids will be described later; here they are mentioned only because they may press upon and close the Eustachian tubes. There is danger from the closure of the tubes by inflammation, because the inflammation may creep up to the middle ear and cause serious trouble, both there and in the mastoid region adjoining. Earache in children should be carefully watched, since it usually means that the middle ear has been invaded by the same inflammatory condition that is present in the throat in colds, and may do serious damage to the delicate structures there. Often in children, and sometimes in adults, the hearing is impaired by the accumulation of wax in the ear canal. This wax is a sticky secretion that serves to catch particles entering the ear canal and to prevent them from striking against the drum. Unless the ear canals are washed out frequently with hot water the wax accumulates and hardens into a plug which closes the ear canal, shutting off faint sounds. The wax dissolves in hot water, but not in cold, so its accumulation is to be prevented by taking pains that hot water actually gets to the bottom of the ear canal once in a while. Digging the wax out with hard instruments should be done only by an expert with greatest caution, lest the drum be injured.
The last of the senses to be described is sight; this is the one of which we make the most use ordinarily, and curiously enough is the only one that we can turn on and off. Loud noises or penetrating smells must be endured, but by shutting our eyes tightly we can escape sight whenever we want to. Altogether there are three kinds of information which the sense of sight brings to us; the first is the knowledge simply of light and darkness; the second is the knowledge of the shape and size of objects; the third is the knowledge of color. Nearly all animals seem to have some power of distinguishing between light and shade. Even the one-celled kinds, that have no eyes or anything corresponding to eyes, behave in a way that proves them to have this power. A lot of them can be put in a dish of water that is well lighted on one side and in shade on the other and in a few minutes all of them will be found to have traveled to one side or the other according as they happen to be a light-seeking kind or a dark-seeking species. As we go higher up the animal scale we find that parts of the body show this same power of distinguishing light from darkness. In the case of the common angleworm, or earthworm as it is more properly called, the front end has the power but the rear end has not. In very highly organized animals only the special organs known as eyes possess the power; all the rest of the body has lost it. The next feature of sight, the ability to perceive the shape and size of objects, requires a special apparatus, the eye, so animals that lack eyes cannot perceive objects, although they may be able to tell light from darkness.
In order to see an object it is necessary that a pattern or image of it be thrown on a sensitive surface; this surface registers the details of the pattern, and so the object is seen. What we have to do here is to find out how these patterns or images are formed in our eyes. In the first place we must realize that every visible object has rays of light going off from every part of its surface in every direction. So-called self-luminous objects, like lamps or the sun, produce the light within themselves; all others merely reflect light that falls on them from some source. Whenever light falls on any object, unless it is a perfect mirror, part is absorbed by the object and the rest is reflected; that which is reflected presently strikes against another object and is again in part absorbed and in part reflected; as this process is repeated over and over again the light becomes so broken up that rays of it are traveling in every direction from every point on every object. Any spot so protected that no rays can strike it evidently cannot reflect any out again, and such a spot will be absolutely dark.
For the formation of an image of any object all that is necessary is that some of the rays of light from every point on the object be caused to fall in exactly corresponding positions on the image. The simplest possible means of doing this takes advantage of the fact that rays of light travel in straight lines. If we inclose an incandescent bulb in a tight box with a round hole in one side of it, every spot on the incandescent filament will be giving off light in every direction, but all the light will be cut off by the box except that which happens to have the direction which takes it out through the hole. From every incandescent spot, then, there will be a beam of light in the form of a cone escaping from the box through the hole. The tip of the cone will be the incandescent spot; the slope of its sides will depend upon the size of the hole. If a screen is placed in front of the hole, all these cones of light will strike on it, and it will be illuminated in a pattern which is made up of all the cones from all the incandescent spots which make up the filament, but these will overlap so much that one cannot be told from another. Now if the hole in