J. B. Bury 
History of the Roman Empire 27 BC to 180 AD [EPUB ebook] 

JULIUS CESAR, the triumvir and the founder of the Roman Empire, was the grandnephew of C. Julius Caesar, the dictator, his adoptive father. Originally named, like his true father, C. Octavius, he entered the Julian family after the dictator’s death, and, according to the usual practice of adopted sons, called himself C. Julius Caesar Octavianus. But the name Octavianus soon fell into disuse, and by his contemporaries he was commonly spoken of as Caesar, just as Scipio Emilianus was commonly called Scipio.