This pre-Galenic astrological text by an unknown author from the later part of the 13th century concerns astrological prognostication. It was based upon Book III of Galen's De diebus criticis, but provided a "handy shortcut" to Galen's more convoluted text. Using it a skilled astrologer-physician could in theory calculate the critical days of an illness according to the length of the phases of the moon.