In order to bring the external world to our consciousness, we are provided with various organs of sense; and as the eye is sensible to the light, the ear is sensible to sound, which comes to our consciousness either as noise (_Geräusch_) or as tone (_Klang_). The whistling of the wind, the plashing of water, the rattling of a wagon are noises, but musical instruments give us tones. When, however, many untuned instruments sound together, or when all the keys within an octave are struck on the same time, then it is a noise that we hear. Tones are therefore more simple and regular than noises. The ear perceives both by means of the agitation of the air that surrounds us. In the case of noise the agitation of the air is an irregularly changing motion. In musical sounds, on the other hand, there is a movement of the air in a continuously regular manner, which must be caused by a similar movement in the body which gives the sound. These so-called periodical movements of the sound in the body, rising, falling and repeated at equal intervals, are called vibrations. The length of the interval elapsing between one movement and the next succeeding repetition of the same movement is called the duration of vibration (_Schwingungsdauer_), or period of motion. TONE, AND ITS LAWS OF VIBRATION A _tone_ is produced by a periodical motion of the sounding body--a _noise_ by motions _not_ periodical. We can see and feel the sounding vibrations of stationary bodies. The eye can perceive the vibrations of a string, and a person playing on a clarionet, oboe, or any similar instrument, feels the vibration of the reed of the mouthpiece. How the movements of the air, agitated by the vibrations of the stationary body, are felt by the ear as tone (_Klang_), Helmholtz illustrates by the motion of waves of water in the following way: Imagine a stone thrown into perfectly smooth water. Around the point of the surface struck by the stone there is instantly formed a little ring, which, moving outwards equally in all directions, spreads to an ever-enlarging circle. Corresponding to this ring, sound goes out in the air from an agitated point, and enlarges in all directions as far as the limits of the atmosphere permit. What goes on in the air is essentially the same that takes place on the surface of the water; the chief difference only is that sound spreads out in the spacious sea of air like a sphere, while the waves on the surface of the water can extend only like a circle. At the surface the mass of the water is free to rise upward, where it is compressed and forms billows, or crests. In the interior of the aerial ocean the air must be condensed, because it cannot rise. For, “in fact, the condensation of the sound-wave corresponds to the crest, while the rarefaction of the sound-wave corresponds to the sinus of the water-wave.”[7]