Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
The same thing may be noted in music. For after assigning five notes to the lyre, musicians fill up the intervals between the strings by a variety of notes, and between these again they interpose yet others, so that the original divisions admit of a number of gradations.