In A Doll's House, what risqué item does Nora reveal to Dr. Rank that eventually prompts him to disclose his own secret?

Henrik Ibsen wrote A Doll's House in 1879, a time when people in polite society followed strict codes of behavior, including dressing modestly. "Modest dress" for women meant clothing that covered the body properly, such as gloved hands and skirts that covered the ankles.
In A Doll's House, Nora removes a pair of silk stockings from a box and playfully shows Dr. Rank these "unmentionables." She flirts with him by giving him permission to look at her legs (a very shocking offer, indeed!). Spurred on by her daring move (and his fatal illness), Dr. Rank reveals his deep secret: He loves her.