Iamblichus' Life of Pythagoras, or Pythagoric Life / Accompanied by Fragments of the Ethical Writings of certain Pythagoreans in the Doric dialect; and a collection of Pythagoric Sentences from Stobaeus and others, which are omitted by Gale in his Opuscula Mythologica, and have not been noticed by any editor - Iamblichus

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“Let these chords be perfectly equal, and equally stretched, so as to be in unison, i. e. so that there may be only one sound, though there are two strings. But it is requisite that they should be placed upon some oblong and polished rule. The ancients called this rule an harmonic rule, or also a monochord, by which instrument all consonances and dissonances, and likewise musical intervals, were tried. Let now one of these chords be bisected in E. Afterwards under the point E place what is vulgarly called the tactus, but which was denominated by the ancients, from its figure, a hemisphere. The tactus, therefore, being placed under E, press there the chord, so that one half of it only, as for instance ED, may be wholly struck and resound. Having therefore struck each of the chords at the same time, viz. the whole of AB, and the half ED, so that they may resound at one and the same time, you will hear the sweetest of all consonances, composed from the sound of the whole chord AB, and the sound of the half ED. This consonance the ancients called diapason, i. e. through all [the chords], because in the musical instruments of the ancients, the two extreme chords, i. e. the most grave, and the most acute of all the chords, contained this consonance; so that, from the gravest chord having made a transition through all the chords to the supreme and most acute of all, they would hear this sweetest consonance. It was, likewise, said to be in a duple ratio of the proportion of one sound to the other. For the sound of the chord AB is doubly greater or more grave than the sound of the half ED. For as sounding bodies are to each other, so are their sounds. But the chord AB is the double of ED. This, however, is now commonly called the octave, because from the first sound, and that the gravest, which is called ut, as far as to that sound which corresponds to it in the consonance diapason, there are these eight sounds, ut, re, mi, fa, sol, re, mi, fa. And of these the first ut, and the last fa, which is the eighth, produce the consonance diapason, or the double, or the octave.

“Again, let the same chord CD be divided into three equal parts in the points F, G.

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“FD, therefore, will be two-thirds as well of the whole CD as of the whole AB. Let the tactus now be placed in F, and let AB and FD be struck at the same time, and a consonance very sweet and perfect will indeed be heard, yet not so sweet as the diapason. This the ancients called diapente (i. e. through five chords), because the first and the fifth chord produce this consonance. But according to proportion it is called sesquialter, because the chord AB is sesquialter to FD, and consequently the sounds of these chords also are in the same ratio. But sesquialter ratio is when the greater quantity AB contains the less FD once, and the half of it besides. It is, indeed, commonly called the fifth, because it is composed from the first sound ut, and the fifth, sol.

“Again, let the same chord be cut into four equal parts in the points H, E, I,

“so that the chord HD, may be three-fourths of the whole CD. The tactus, therefore, being placed in H, let AB and HD be struck at one and the same time, and a consonance will be heard, indeed, yet more imperfect than the preceding two. This was called by the ancients diatessaron, i. e. through four chords or sounds, for a similar reason to that by which the former were denominated. With reference, however, to the ratio of the chords and sounds, it is called sesquitertian, because the greater AB contains the less once, and a third part of it besides. But it is now commonly called a fourth, because it is found between the first sound ut, and the fourth fa. If now the point F be added in the preceding figure, and at one and the same time two chords HD and FD are compared in arithmetical ratios, we shall find that the greater HD will have to the less FD a sesquioctave[105] ratio, and the sound of the greater HD to the less FD will have the same ratio, i. e. in modern terms, that between fa and sol there is a sesquioctave ratio. But if these two sounds are heard together, they will be discordant to the ear. Again, the distance between these sounds fa, sol, or between the chords HD and FD, or between the two harmonic intervals HD and FD, the ratio of which was sesquioctave, was called by the ancients a tone. Afterwards they divided the whole of CD into nine equal parts, the first of which is divided in K, so that the whole CD may have to the remainder KD, which contains eight of those parts, a sesquioctave ratio. This, in like manner, will be the interval of a tone, the first sound of which, i. e, of the whole CD, is now called ut, but the second sound of the rest of the chord KD is called re. Afterwards they in a similar manner divided the remainder KD into nine parts, the first part of which is marked in the point L. And for the same reason between the chord KD and the chord KD, and their sounds, there will be a sesquioctave ratio. The sound of the chord LD is now called mi; but the interval which remains between the chord LD and the chord HD has not a sesquioctave ratio, but less than it almost by half, and therefore an interval of this kind was called a semitone, and also diesis or a division. But that interval which remains between the points F and E they divided after the same manner, as the space between C and H was divided, and they again found the same sounds. Let those divisions be marked by the points M and N; and here, also, between N and E, or between mi and fa, there is in like manner another semitone. These eight sounds, therefore, are ut, re, mi, fa, sol, re, mi, fa, which compose the whole diapason. For as we have before observed, between ut and the last fa is the consonance diapason, or between the chord CD or AB, and the chord ED. But from the intervals which are between the sounds there are two semitones, viz. one between mi and fa, denoted by the letters L, N, and the other between the last mi and fa, denoted by the letters N, E. The remaining five intervals are entire tones. It must, also, be observed, that from ut to the first sol is the consonance diapente, which contains three tonic intervals, and one semitone; nevertheless in all there are five sounds, ut, re, mi, fa, sol.