For some mysterious reason Polly had forgotten to light her candle.

This was her rule, never broken before.
She stepped to the bellpull.

Her hand was on it, when she heard the girl's voice muttering in the next room--the boudoir.

At least, it sounded like Polly's voice, though its tone was strangely subdued and level.

"Talking to herself," Dorothea decided, and smiled, in spite of her annoyance, as everyone smiles who catches another in this trick.
She dropped the bellpull and opened the boudoir door.
Polly was not talking to herself.