Among all our sense-perceptions, sound is unique in making itself perceptible in two quite different ways - via the ear as a direct sense experience and via the eye (potentially also via the senses of touch and movement) in the form of certain mechanical movements, such as those of a string or a tuning fork. Hence the world-spectator, as soon as he began to investigate acoustic phenomena scientifically, found himself in a unique position. In all other fields of perception, with the exception of the purely mechanical processes, the transition to non-stereoscopic colourless observation had the effect that the world-content of the naive consciousness simply ceased to exist, leaving the ensuing hiatus to be filled in by a pattern of imagined kinematic happenings - for example, colour by 'ether'-vibrations, heat by molecular movements. Not so in the sphere of acoustics. For here a part of the entire event, on account of its genuine kinetic character, remains a content of actual observation.