Before the description of the necessary apparatus is followed out, it might be necessary, however, to go back to the point how our ear perceives the vibrations of a given tone, and the total vibrations of all the tones simultaneously acting upon it; because by this means will be determined the various requisite conditions which must be fulfilled by the transmitting and receiving apparatus for the solution of the problem that has been set.
Let us consider first the processes which take place in order that a single tone should be perceived by the human ear; so shall we find that each tone is the result of a condensation and rarefaction several times repeated in a certain period of time. If this process is going on in the same medium (the air) in which our ear is situated, the membrane will at every condensation be forced toward the hollow of the drum, and at every rarefaction will move itself in the opposite direction.
These vibrations necessitate a similar motion of the auditory ossicles, and thereby a transference to the auditory nerves is effected.
The greater the condensation of a sound-conducting medium at any given moment, the greater also will be the amplitude of vibration of the membrane and of the auditory ossicles and of their action; and in the converse case the action will be proportionally less. It is, therefore, the function of the organs of hearing to communicate with fidelity to the auditory nerves every condensation and rarefaction occurring in the surrounding medium; whilst it remains to be the function of the auditory nerves to bring to our consciousness the number as well as the magnitude of the vibrations ensuing in a given time.
Here in our consciousness a definite name is given to a certain composition, and here the vibrations brought to the consciousness become “tones.”
That which is perceived by our auditory nerves is consequently the effect upon our consciousness of a force which, according to its duration and magnitude, may for the sake of better comprehension, be exhibited graphically.
Let, for example, the length of the line _a b_ be any definite duration of time, and let the curves above this line denote the condensations (+), and the curves below this line the rarefactions (-); then every ordinate erected at the extremity of an abscissa gives us the strength of the condensation in consequence of which the drum-skin vibrates, at the moment indicated by the position of the foot of the ordinate.