(15-18) Diocletian became Augustus in 284, and co-opted Maximian as his colleague two years later.  About the same time Carausius, commander of the Channel fleet, crossed to Britain and had himself proclaimed independent emperor.  In 290 he was acknowledged as third colleague by the Augusti, but no place was found for him when in 293 the government of the Roman world was divided between Diocletian, Maximian, and two newly chosen Caesars—­Galerius and Flavius Valerius Constantius, later called Chlorus.  By this arrangement the recovery of Britain from Allectus—­who had murdered Carausius about 294—­fell to Constantius, and he accomplished this by a sudden attack in 296.  Constantius was twice married.  His first wife, Helena, bore him a son, Constantine the Great; his second was a step-daughter of Maximian, named Theodora, to whom coin 18 belongs.

Britain was now divided into four Diocletian provinces, to which a fifth—­Valentia—­was later added when the country north of Hadrian’s wall was re-occupied.  The only other event of Diocletian’s reign to be noticed is the persecution of Christians in which, according to tradition, St. Alban lost his life at Verulam about 303.

(19-20) On May 1, 305, Diocletian and Maximian abdicated.  Constantius and Galerius now became Augusti.  Trouble arose over the two vacant Caesarships.  It was the aim of Galerius to exclude Constantine, but the latter escaped to his father’s camp at York, a few weeks before Constantius died on July 25, 306, after a victory over the Picts and Scots.  Constantine was in power under various titles in Gaul and Britain for five years until, in 311, when Galerius died, he began his march on Rome, during which he is said to have had his vision of the cross with the words [Greek:  en touto nika].  In 314 the bishops of York, London, and some other uncertain British see attended the Council of Arles which sat to deal with the Donatist schism.  The British Church was also represented at the Council of Nicaea, called by Constantine in 325 to consider the Arian heresy, when the Nicene Creed in its original form was authorized; the British vote was orthodox.  It was Constantine who in 321 first made Sunday a holiday, but whether Christianity or Mithraism prompted him to this is doubtful.

(22-23) When Constantine the Great died in 337 the empire was divided between his sons.  Constantius II. received the east; Constans, Africa, Italy, and the Danuvian region; Constantine II., Gaul and Spain.  In 340 Constantine II. attacked Constans and was killed.  Constans then ruled the united west; it seems that Constans and Constantius II. visited Britain in 343.  Constans was assassinated in 350; this left Constantius II. alone.  His policy of toleration towards the Arians led to a great Church Council in 359.  The eastern bishops met at Seleucia, the western at Ariminum, where Britain was represented.  By a certain amount of coercion Constantius forced his views on the Western Council.  At this time the prosperity of Britain was great and corn was exported in large quantities.