The imagery in "A Valediction: Forbidding Mourning" serves to create what sort of tone for the poem?
Throughout Donne's poem "A Valediction: Forbidding Mourning" the speaker attempts to comfort his beloved who is upset about their impending separation. The complex imagery helps convey this tone of consolation, as the speaker assures his beloved that their separation is only physical; it is not spiritual. He uses several images of constancy and unity to convey this idea and this tone. He compares their love to the heavenly bodies moving around the sun--"trepidation of the spheres"-- explaining that such a movement is "innocent" because it is constant and circular. He will return to her just as the heavenly bodies continue their orbit.
He later compares their separation to gold that can be beaten to "airy thinness." Their separation is not a "breach but an expansion." In other words, gold is so malleable that it can be stretched quite thinly without breaking. This image is quite comforting, for it shows how a separation need not be a permanent break, that the two can be connected over a wide expanse of time and space.
Lastly, Donne uses the image of the compass to represent the idea of spiritual unity (the compass is joined at the top) even though the two legs are separate, just as the two lovers are separate on earth.
All these images combine to show that since the two lovers' love is spiritual, it can withstand a temporary physical separation. The images used to depict this idea are quite reassuring in that they are heavenly, precious, and united.