Leviticus 16:13

He shall put the incense upon the fire The high-priest, having begun the solemn service with slaying the bullock, as a sin-offering of deprecation and atonement for himself and the other priests, lighted his incense-vessel, or censer, at the great altar of burnt-offering, and at his entrance into the holy of holies, threw the incense upon the burning coals, and so filled the place with a cloud of smoke, to prevent him from seeing the ark, and being struck with the glory issuing from between the cherubims, where the Shechinah or emblem of the divine presence resided: or, as others say, that he might not offend by too curiously gazing on the symbols of the divine glory. If we may believe the Jews, he entered sideways, as not daring to look directly on the glory of the place, and that, having filled the sanctuary with a cloud of smoke, he went out backward, having his face directed toward the mercy-seat, in reverence for the divine majesty, which was there represented.