Final answer:
The compass metaphor in Donne's 'A Valediction Forbidding Mourning' illustrates the constancy and enduring nature of the lovers' bond despite physical separation, emphasizing that there is no need for mourning due to their inseparable connection.
Step-by-step explanation:
The poem 'A Valediction Forbidding Mourning' by John Donne uses the central metaphor of a compass to depict the steadfastness of the speaker's love. One way the metaphor works is by suggesting that though one foot of the compass moves (one lover departs), the other foot remains in the center (the lover left behind stays constant). Another aspect of the metaphor is that even as the moving foot seems to stray, it leans towards the other foot, demonstrating that the traveller's thoughts are still with the partner. Finally, as the moving foot comes in a full circle back to its starting point, it suggests the eventual reunion of the lovers. This image serves Donne's purpose by reinforcing the connection between the lovers despite their physical separation, and thus he 'forbids mourning' because their bond cannot be broken by mere distance.