De institutione musica V.5: De sonorum differentiis Ptolomaei sententia.
These matters being thus dispatched, Ptolemy divides the differences of sounds in this way. Of voices, some are unisonous, others not. Unisonous are those whose sound is one, whether in the low or in the high; not unisonous, when one is lower and another higher. Of these, some are such that their difference is joined by a common boundary. For it is not divided, but is led from the low to the high in such a way that it seems continuous. Others, however, are not unisonous in such a way that their difference is distinguished by an intervening silence. That voices should be joined by a common boundary happens in this way. For just as when a rainbow is seen in the clouds the colors are so close to one another that there is no definite limit where one is separated from another, but, for example, it passes from red to pale so that by continuous change it turns into the following color with no definite middle term intervening to distinguish the two, so it also usually happens in voices: if someone strikes a string and, while he strikes it, twists it, it happens that at the beginning the stroke is lower, but while it is twisted the voice is made thin, and the sounds of the low voice and of the high become continuous.