It is an ordinary mirror that needs quicksilver (mercury) to reflect, whereas for a pure heart, a single tremor of restlessness is enough.

An ordinary glass mirror needs a backing of quicksilver (mercury) to cast a reflection, and mercury is known for its trembling restlessness. The poet beautifully suggests that a pure, conscious heart doesn't need artificial silvering—its own tender yearning and restlessness of spirit are enough to make it clear and reflective. In this view, the ache in a sensitive soul is not a flaw, but the very sheen that mirrors truth.