§ 1. Nature of the Stimulus.—The physical stimulus which occasions sensations of sound consists of vibrations of the particles of the air. As in the case of light, we can distinguish wavelength or rapidity of vibration, amplitude, and complexity. Wavelength determines pitch; amplitude loudness, and complexity timbre.

§ 2. Organ of Hearing.—For anatomical details we must again refer to physiological textbooks. The drum of the ear is thrown into vibration by impact of soundwaves. This produces movements in certain small bones, and these movements in their turn give an impulse to a fluid, which by its impact throws into vibration a membrane called the basilar membrane. The vibrations of this membrane are the immediate stimulus exciting certain haircells, in which the fibres of the auditory nerve terminate.

§ 3. Noises and Musical Sounds.—Noises as immediate experiences are characterised by confusion and indefinite complexity, and for the most part by irregularity. A musical sound is marked by unity and uniformity of character. "The vibrations which constitute a musical sound are repeated at regular intervals, and thus possess a marked periodicity or rhythm."*

* Foster, TextBook of Physiology, book iii., chap, iv., p. 1361.

Musical sounds are also produced when the periodicity, instead of being regular, varies continuously. Regular vibrations which, would otherwise produce musical sounds, give rise to noises, when a large number of them, differing but little in wavelength, occur together, as when a number of adjoining keys of a piano are simultaneously touched. But in general, the stimulus which gives rise to noises is produced by a series of vibrations differing from one another in period. "There is, however, no abrupt line between" noises and musical sounds. "Between a pure and simple musical sound produced by a series of vibrations, each of which has exactly the same period, and a harsh noise in which no consecutive vibrations are alike, there are numerous intermediate stages. Much irregularity may present itself in a series of sounds called music, and in some of the roughest noises the regular repetition of one or more vibrations may be easily recognised."*