Read the excerpt from "An Image of Africa: Racism in Conrad’s Heart of Darkness.”

It might be contended, of course, that the attitude to the African in Heart of Darkness is not Conrad's but that of his fictional narrator, Marlow, and that far from endorsing it Conrad might indeed be holding it up to irony and criticism. Certainly, Conrad appears to go to considerable pains to set up layers of insulation between himself and the moral universe of his history. He has, for example, a narrator behind a narrator. The primary narrator is Marlow, but his account is given to us through the filter of a second, shadowy person. But if Conrad's intention is to draw a cordon sanitaire between himself and the moral and psychological malaise of his narrator, his care seems to me totally wasted because he neglects to hint, clearly and adequately, at an alternative frame of reference by which we may judge the actions and opinions of his characters. It would not have been beyond Conrad's power to make that provision if he had thought it necessary. Conrad seems to me to approve of Marlow, with only minor reservations—a fact reinforced by the similarities between their two careers.

What is Achebe’s purpose in this passage?