Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
Similarly it has been held that we should observe distinctions such as the following: if the preposition ex is compounded with specto, there will be an s in the second syllable, while there will be no s if it is compounded with pecto.