BOUBLES AFTER THE ABDICATION OF DIOCLETIAN. — DEATH OF CONSTAN- TIUS. ELEVATION OF CONSTANTINE AND MAXENTIUS. -SIX EMPER- ORS AT THE SAME TIME. DEATH OF MAXIMIAN AND GALERIUS. - VIC- TORIES OF CONSTANTINE OVER MAXENTIUS AND LICINIUS. — REUNION OF THE EMPIRE UNDEL THE AUTHORITY OF CONSTANTINE.