Apparently without guiding principle, the perfect participles passive of certain verbs in Latin have a short radical vowel, while others have a long vowel: thus factus, spectus, scissus, to faciō, speciō, scindō; but āctus, rēctus, vīsus, to agō, regō, videō. An excellent summary of the discussion which has grown up around this point was given by Sommer in 1914; but for the proper understanding of the problem it will be necessary briefly to repeat the main points.