Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
It may also be important to remark that there are degrees of length in long syllables and of shortness in short. Consequently, although syllables may be thought never to involve more than two time-beats or less than one, and although for that reason in metre all shorts and all longs are regarded as equal to other shorts and longs, they none the less possess some undefinable and secret quality, which makes some seem longer and others shorter than the normal. Verse, on the other hand, has its own peculiar features, and consequently some syllables may be either long or short.
Indeed, since strict law allows a vowel to be long or short, as the case may be, when it stands alone, no less than when one or