Why did Virginia's eyes became dim with tears after the funeral?

Virginia's eyes grow dim with tears after the funeral for the Canterville ghost because she remembers his description of the Garden of Death. The ghost had earlier painted a picture for her of this garden that he longed to enter, a nighttime setting where the nightingale sings, the hemlock blooms and the moon shines. The Canterville ghost had said to her:

"Yes, death. Death must be so beautiful. To lie in the soft brown earth, with the grasses waving above one's head, and listen to silence. To have no yesterday, and no to-morrow. To forget time, to forget life, to be at peace. You can help me. You can open for me the portals of death's house, for love is always with you, and love is stronger than death is."

Virginia, the aptly named virginal symbol of purity and goodness in the story, has helped the ghost to die, an act of love that relieves him of the burden of his sins. For the Victorian middle-class, children often represented innocence, and Virginia fits perfectly this notion of  the child as the representation of purity, and aligns as well with the Victorian concept of the good female as the redemptive angel of the home. She is the one person in the story who has truly treated the ghost with compassion and humanity.

Now that he is buried her eyes dim with tears for two reasons: she both sorrows in losing him and yet knows, happily, he is in a place of peace. Her tears are bittersweet.