A leaking tap in a harshly lit bathroom becomes the poem's picture of eternity. Each drip repeats the last, so the moment seems to stretch without end. The speaker watches this and feels small, even threatened: "I will fall void / down the drain," as if they might vanish the way the water does.
The poem suggests that eternity isn't grand or heavenly here. It is "composed of so little," just shadows ("penumbras") on white porcelain. The last lines carry the sharpest feeling of being trapped. Every attempt to escape or finish something is "hemmed so round / as to manifest another," so each effort only leads to another cycle.
The mood is cold, sterile, and quietly anxious. The bright fluorescent light and bare white surfaces offer no comfort, and the short, broken lines mimic the slow drip. Judging by the imagery, the speaker feels stuck in a loop of repetition, where time passes but nothing changes.