Final answer:
Dupin conducts an experiment with a drawing to prove a point about the limitations of the Prefect and his cohort's investigation methods.
Step-by-step explanation:
Dupin conducts an experiment with a drawing to prove a point about the limitations of the Prefect and his cohort's investigation methods. He explains that they fail to consider the diverse cunning of the individual felon, beyond their own ideas of ingenuity. The experiment involves Dupin imitating the facial expression of the schoolboy in order to match his thoughts and sentiments. The conclusion is that the robber's knowledge of the loser's knowledge is key to understanding the situation.