A simile is a literary device that usually uses the words "like" or "as" to compare one idea or image to another, in order to both substitute a more familiar image in place of the less common one, and to emphasize a mood.
In the opening scene of chapter 17, Doctor Manette and his daughter, Lucie, are sitting in the moonlight, discussing her engagement to Charles. The narrator says, "In the sad moonlight, she clasped him by the neck, and laid her face upon his breast. In the moonlight which is always sad, as the light of the sun itself is- as the light called human life is- at its coming and its going." In this double-simile, the narrator likens the sadness of the moonlight to the sun and life itself, using the image to emphasize the somberness of the mood.
At the end of this chapter, her father is sleeping and she slips in to watch him. And so "the sunrise came, and the shadows of the leaves of the plane-tree moved upon his face, as softly as her lips had moved in praying for him." Here, the play of the shadows upon her father's face--a peaceful, calming image--is compared to the gentleness of Lucie's prayers for her father.