If we wish to arrange associations according to their simplicity and the closeness of their combinations, we can start with the following simple experiment. If we make the string of a piano sound by plucking it in the middle, then, as the science of physics teaches us, not only does the whole string vibrate, but each half vibrates as well in a smaller degree, and in general each third part, each fourth, &c., in ever decreasing amplitudes. These segments, which decrease in length according to the numerical series 1, 2, 3, &c., correspond to tones of increasing pitch--the half string corresponds to the octave, the third part to the fifth of the octave, the fourth to the double octave, and so on. If these high tones are then produced alone, one after another, by making the corresponding part of the string vibrate each time, and if we then return to the tone of the whole string, we can then, if we listen attentively, hear clearly, along with the stronger sounding fundamental tone, these overtones, or at least those nearest to the fundamental. We therefore say that the clang of a string, or of any other musical source, does not only consist of the one tone according to which we determine its pitch, but also of a series of overtones, which give it its timbre or clang-colour. This expression itself points to the fact that in hearing a clang there takes place psychologically an association, which is of a specially intimate kind. The above-described experiment of comparing a clang with some of its overtones teaches us that these latter really exist in sensation, and that we can perceive them with very intense attention. Nevertheless under ordinary circumstances we do not perceive them as independent tones, but they appear to us massed together only as a specific modification of the fundamental tone, and we call this its clang-colour or timbre. An association of this kind, in which the sensation-components are so fused into the resulting product that they can no longer be clearly perceived as isolated component parts, is called a fusion. Such a fusion can be either a very close one or a very loose one. A single clang is for example a close fusion, a chord is a loose one. The separate fundamental tones of a chord are bound fairly closely into one whole, but we can hear at least some of them quite plainly.