Final answer:
The mention of Lethe, Prosperone, and Psyche in Keats's "Ode on Melancholy" is an example of the literary device called allusion, which indirectly references mythological figures to add deeper meaning to the poem.
Step-by-step explanation:
In the excerpt from Keats's "Ode on Melancholy," the mention of Lethe, Prosperone (Proserpine/Persephone), and Psyche employs the literary device known as an allusion. An allusion is an indirect or implied reference to a person, event, thing, or a part of another text, typically with the intent to create an association or convey a message. Keats references Lethe, the river of forgetfulness in Greek mythology, along with Proserpine (Persephone) and Psyche, who are figures from classical mythology. These allusions are meant to enrich the poem by invoking the deeper symbolic meanings that these mythological figures carry.