Princess Mary has previously been described in the story as "plain", and has always seen herself so. She has lived a life of profound suffering at the hands of an eccentric and cruel father, whom she deeply loved and was thoroughly devoted to, and who has recently died. She is intensely religious, kind-hearted, and good. And she had long given up any hope of marrying, seeing herself as she had, and confirmed by the cruel remarks of her father.
This scene is the second meeting between her and Nicholas Rostov (brother of the previously-mentioned Natasha). On the occasion of the first meeting, he saved her from a very dangerous situation resulting from the French invasion of Russia, and each was favorably impressed with the other, as well as having developed the feelings naturally occurring on both sides from such an action.
"When Rostov entered the room, the princess dropped her eyes for an instant, as if to give the visitor time to greet her aunt, and then just as Nicholas turned to her she raised her head and met his look with shining eyes. With a movement full of dignity and grace she half rose with a smile of pleasure, held out her slender, delicate hand to him, and began to speak in a voice in which for the first time new deep womanly notes vibrated. Mademoiselle Bourienne, who was in the drawing room, looked at Princess Mary in bewildered surprise. Herself a consummate coquette, she could not have maneuvered better on meeting a man she wished to attract.
'Either black is particularly becoming to her or she really has greatly improved without my having noticed it. And above all, what tact and grace!' thought Mademoiselle Bourienne.
Had Princess Mary been capable of reflection at that moment, she would have been more surprised than Mademoiselle Bourienne at the change that had taken place in herself. From the moment she recognized that dear, loved face, a new life force took possession of her and compelled her to speak and act apart from her own will. From the time Rostov entered, her face became suddenly transformed. It was as if a light had been kindled in a carved and painted lantern and the intricate, skillful, artistic work on its sides, that previously seemed dark, coarse, and meaningless, was suddenly shown up in unexpected and striking beauty. For the first time all that pure, spiritual, inward travail through which she had lived appeared on the surface. All her inward labor, her dissatisfaction with herself, her sufferings, her strivings after goodness, her meekness, love, and self-sacrifice--all this now shone in those radiant eyes, in her delicate smile, and in every trait of her gentle face.
Rostov saw all this as clearly as if he had known her whole life. He felt that the being before him was quite different from, and better than, anyone he had met before, and above all better than himself."
This is the aspect of your beauty which most makes me love you, and the one I saw from the very first time I met you: your spiritual beauty. I saw it as clearly as Nicholas saw Mary's; as clearly as if it had been written down and set before me on a page in words describing all the details of your life. This is what I meant when I said that I saw your woundedness, and loved you for it. That is, I saw the spiritual beauty and strength of the person you had become because of whatever hurt or suffering you had experienced.
And I saw, too, that same light in your eyes, felt that same grace in your actions toward me whenever we met, from the first time you glanced at me, wearing that black skirt and burgundy velvet blouse, with your hair done up in braids on your head, to the day I gave you that little bit of ribbon that matched your eyes.
Tolstoy uses two characters to portray two different types of true beauty: one a beauty of soul, the other a beauty of spirit. But you encompass both in one inexpressibly beautiful person.