It is perhaps impossible to determine with precision when the rule came to be generally accepted that the full moon, which was to regulate the date of Easter, was the first full moon _after_ the vernal equinox. We find that this is the rule which governs the Paschal Tables of Hippolytus (of which more will be said hereafter), and we find it expressly enjoined in that ancient collection of Church law which goes under the name of the Apostolic Canons. The Tables of Hippolytus can, with reasonable certainty, be assigned to A.D. 222. In the Apostolic Constitutions, the date of which it is impossible to determine with any close approach to certainty[147], the rule runs, ‘Observe the days of the Pascha with all care after the vernal equinox, that ye keep not the memorial of the one passion twice in a year. Keep it once only in a year for Him who died but once[148].’ The mystical reason assigned here also appears in the letter of the Emperor Constantine, announcing the decision to which the Nicene Council came upon the Paschal question[149]. Later on the reader will find what is probably meant by keeping the Pascha twice in the same year[150]. It would not perhaps be fitting to pass over in silence the attempt made in the early part of the third century by the Roman ecclesiastic, Hippolytus, to construct a cycle which would make it possible to predict the day on which Easter would fall in any future year. As to who this Hippolytus was, Eusebius and subsequent students among the Fathers appear to have known scarcely anything. Eusebius speaks of the many writings of Hippolytus, and gives the titles of some of them, and describes one more particularly. This was a treatise _Concerning the Pascha_, in which was to be found a certain sixteen-year rule (canon) about the Pascha, the boundary of the writer’s computation being the first year of the Emperor Alexander[151], _i.e._ Alexander Severus, whose first year was A.D. 222.