On the 1st of May he repaired with his guards to a spot three miles out of Nicomedia, where he had thirteen years before proclaimed Galerius as Cæsar, and there addressing his officers and court, he said that the infirmities of age warned him to retire from power, and to deliver the administration of the state into stronger hands. He then proclaimed Galerius as Augustus, and Maximinus Daza as the new Cæsar. Constantine, who has given an account of the ceremony, which is quoted by Eusebius in his life of that prince, was present, and the troops fully expected that he would be the new Cæsar; when they heard another mentioned, they asked each other whether Constantine had changed his name. But Galerius did not leave them long in suspense; he pushed forward Maximinus and showed him to the assembly, and Diocletian clothed him with the purple vest, after which the old emperor returned privately in his carriage to Nicomedia, and immediately after set off for Salona in Dalmatia, near which he built himself an extensive palace by the sea-shore, in which he lived for the rest of his life, respected by the other emperors, without cares and without regret.
Part of the external walls which inclosed the area belonging to his palace and other buildings still remain, with three of the gates, as well as a temple, which is now a church at Spalatro, or Spalato, in Dalmatia, a comparatively modern town, grown out of the decay of the ancient Salona, and built in great part within the walls of Diocletian's residence, from the name of which, "Palatium," it is believed that "Spalato" is derived.
At the same time that Diocletian abdicated at Nicomedia, Maximianus, according to an agreement between them, performed a similar ceremony at Milan, proclaiming Constantius as Augustus, and Severus as Cæsar. Both Severus and Maximinus Daza were inferior persons, and creatures of Galerius, who insisted upon their nomination in preference to that of Maxentius and Constantine, whom Diocletian had at first proposed. Maximianus retired to his seat in Lucania, but not being endowed with the firmness of Diocletian he tried some time after to recover his former power, and wrote to his old colleague to induce him to do the same. "Were you but to come to Salona," answered Diocletian, "and see the vegetables which I grow in my garden with my own hands, you would no longer talk to me of empire." In his retirement he used to observe to his associates how difficult it is, even for the best-intentioned man, to govern well, as he cannot see everything with his own eyes, but must trust to others, who often deceive him.