Woman with Mirror - Unraveling the Clues
When this picture was painted, viewers would have understood that this man and woman are lovers—married men did not help their wives dress. Here, the man holds up a flat mirror so the lady can see herself simultaneously from the front and the back. Look carefully, and you may be able to make out the shadowy reflection of the lady’s back in the convex mirror.
Notice how the convex mirror also reflects a bright square of white: a window. And if you look at the bottom of the painting, you’ll see that the woman is daubing her finger with a perfume bottle, which is sitting on a ledge of a window. We are watching this intimate scene through an open window, as if we were voyeurs.Source