Albert Barnes on Luke 1:78

Whereby the dayspring, &c. The word dayspring (7) means the morning light, the aurora, the rising of the sun. It is called the dayspring from on high because the light of the gospel shines forth from heaven. God is its author, and through his mercy it shines on men. There is here, doubtless, a reference to Isa 40:1,2; indeed, almost the very words of that place are quoted. Comp. also Rev 22:16.

(6) "tender mercy" or "bowels of the mercy" (7) "dayspring" or "sunrising" or "branch" Isa 11:1, Zech 3:8, 6:12