Also, here is a short video reading of the passage from Eusebius detailing the miracle of the Cross in the sky and its immediate aftermath, done by Arx Publishing:
Constantine's miraculous vision of a cross in the heavens with the words, "Conquer by this" is an epochal event in history. This video offers an account of the vision as related to Eusebius Pamphilus, an historian and bishop, who was acquainted with the Emperor and heard the account first-hand from Constantine himself. He recorded the story in his Life of the Blessed Emperor Constantine which was penned shortly after Constantine's death in AD 337.
The complete Life of the Blessed Emperor Constantine may be found here in book form: http://www.evolpub.com/CRE/CREseries.html#CRE8