If it be asked why no more primes than 2, 3, and 5 are admitted into musical ratios, one reason is that consonances whose vibrations are in ratios whose terms involve 7, 11, 13, etc., would be less simple and harmonious than those whose terms involve the lesser primes only. Another reason is this - as perfect fifths and other intervals resulting from the number 3 make the schism of a comma with perfect thirds and other intervals resulting from the number 5, so intervals resulting from the numbers 7, 11, 13, etc., would make other schisms with both those kinds of intervals.
There is no one musical interval which is the perfect measuring rule for the others. But the octave has been divided into 53 parts called commas, and these commas are as near a commensurable rule as we need seek for measuring the musical intervals; always remembering that, strictly speaking, these intervals are incommensurable. The large second has 9 commas; the medium second has 8; and the small second 5; and all other intervals, being of course composed of some of these seconds, can be measured accordingly. Thus the comma, though not itself an interval of our musical system, is the handy and sufficiently perfect inch, let us call it, for practical purposes in music.
In getting the length of a string, in inches or otherwise, to produce the scale of music, any number may be fixed on for the unit; or for the vibrations of the root note any number may be fixed on for the unit; but in the fractions which show the proportions of the notes of the scale, there is no coming and going here; this belongs to the invariables; there is just one way of it. Whatever is not sense here is nonsense. It is here we are to look for the truth. The numbers which express the quantities and the numbers which express the motions are always related as being of the same kind. The fractions bring their characters with them, and we know by this where they come from. 1/4 of a string gives a note 2 octaves above the whole string, no matter what may be its length; 2 has exactly the same character as 1; 2/4 gives the note which is 1 octave above the whole string; but in the case of 3/4 here is a new ingredient, 3; 3/4 of a string gives a note which is a fifth below the [Scientific Basis and Build of Music, page 75]