John Calvin's Commentary on the Bible - Genesis 15:12
Verse 12 12.A deep sleep fell upon Abram. The vision is now mingled with a dream. Thus the Lord here joins those two kinds of communication together, which I have before related from Numbers 12:6, where it is said, ‘When I appear unto my servants the prophets, I speak to them in a vision or a dream.’ mention has already been made of a vision: Moses now relates, that a dream was superadded. A horrible darkness intervened, that Abram might know that the dream is not a common one, but that the... read more
Adam Clarke's Commentary on the Bible - Genesis 15:16
In the fourth generation - In former times most people counted by generations, to each of which was assigned a term of years amounting to 20, 25, 30, 33, 100, 108, or 110; for the generation was of various lengths among various people, at different times. It is probable that the fourth generation here means the same as the four hundred years in the preceding verse. Some think it refers to the time when Eleazar, the son of Aaron, the son of Amram, the son of Kohath, came out of Egypt, and... read more