Apart from our ear, every tone is nothing more than the condensation and rarefaction of a body repeated several times in a second (at least seven to eight times[15]). If this occurs in the same medium (the air) as that with which we are surrounded, then the membrane of our ear will be compressed toward the drum-cavity by every condensation, so that in the succeeding rarefaction it moves back in the opposite direction. These vibrations occasion a lifting-up and a falling-down of the “hammer” [_malleus_ bone] upon the “anvil” [_incus_ bone] with the same velocity, or, according to others, occasion an approach and a recession of the atoms of the auditory ossicles, and give rise, therefore, to exactly the same number of concussions in the fluid of the _cochlæa_, in which the auditory nerve and its terminals are spread out. The greater the condensation of the sound-conducting medium at any given moment, the greater will be the amplitude of vibration of the membrane and of the “hammer,” and the more powerful, therefore, the blow on the “anvil” and the concussion of the nerves through the intermediary action of the fluid. The function of the organs of hearing, therefore, is to impart faithfully to the auditory nerve, every condensation and rarefaction occurring in the surrounding medium. The function of the auditory nerve is to bring to our consciousness the vibrations of matter resulting at the given time, both according to their number and their magnitude. Here, first, certain combinations acquire a distinct name: here, first the vibrations become musical _tones_ or _discords_ (Misstöne). That which is perceived by the auditory nerve, is, therefore, merely the action of a _force_ affecting our consciousness, and as such may be represented graphically, according to its duration and magnitude, by a curve. [Illustration: Fig. 24.] Let the line _a, b_, indicate any given length of time, and the curve above the line a condensation (+), the curve below the line a rarefaction (-), then every ordinate erected at the end of an abscissa will give [according to the height of it], at a moment indicated by the position of the foot of the ordinate, the strength of the condensation that is causing the drum-skin to vibrate. Our ear can perceive absolutely nothing more than is capable of being represented by similar curves, and this method is completely sufficient to bring before our clear consciousness every tone and every combination of tones. If several tones are produced at the same time, then the medium that conducts sound is placed under the influence of several simultaneous forces; and the two following laws hold good:-- If all the forces operate in the same sense, the resultant motion is proportional in magnitude to the sum of the forces. If the forces operate in opposite senses, the resultant motion is proportional in magnitude to the difference of the opposing forces.