举出文中对居里夫人外貌描写的例子，分析其作用？

“一个苍白羞怯的女士走进来，其神色之忧郁……她身穿黑色棉布衣衫，她那雍容、坚韧、温良的脸上带着专心做学问的人常有的那种心不在焉的神情。”

    William Henry Harrison, the ninth President of the USA, was born in a small town. When he was a boy, he was very quiet and shy. Because he was so quiet, everyone thought that he wasn't bright. People in the town often put a nickel(五分镍币)and a dime(十分硬币)before him and asked him which to choose. He always chose the nickel and they laughed.

    One day a woman felt sorry for him. She asked, "Why do you never take a dime and always choose a nickel? Do you know a nickel is worth(值……钱) much less than a dime?"

    "Certainly I know it.” William answered slowly. "But if I choose the dime, they will not ask me to choose them any more.”