Why the Incense Atones for the Sin of Slander Done in Secret

And Rabbi Hanina said: we have learned that the incense atones, as it is said, "and he put on the incense" and so forth (Numbers 17:12). The school of Rabbi Yishmael taught: the incense atones for slander. The Holy One, blessed be He, said: let a thing done in secret come and atone for a deed done in secret. There is a difficulty of bloodshed against bloodshed, and a difficulty of slander against slander. Bloodshed against bloodshed is not difficult: this is where the killer is known, that is where the killer is not known. Where the killer is known, he is liable to death. Where it was done deliberately but no one warned him [the broken-necked heifer atones]. Slander against slander is not difficult: this is in private, that is in public.