Definition of "fluctuating"

...they sat down beside the hearth in the adjoining room, over which the embers of the wood-fire cast a fluctuating light; now the long shadows falling duskily around—now dispersing them with bursts of brilliant flame, as the lighter wood kindled into a short-lived blaze.

1834, L[etitia] E[lizabeth] L[andon], chapter X, in Francesca Carrara. […], volume II, London: Richard Bentley, […], (successor to Henry Colburn), page 105