If he mean this ironically, it may be truer than he thinks. He points it, however, by no deviation from his straightforward manner of speech, though in saying it he turns towards that part of the dim room where my Lady sits. Sir Leicester stands to return his parting salutation, Mr. Tulkinghorn again rings, Mercury takes another flight, and Mr. Rouncewell and Rosa leave the house.
Then lights are brought in, discovering Mr. Tulkinghorn still standing in his window with his hands behind him and my Lady still sitting with his figure before her, closing up her view of the night as well as of the day. She is very pale. Mr. Tulkinghorn, observing it as she rises to retire, thinks, "Well she may be! The power of this woman is astonishing. She has been acting a part the whole time." But he can act a part too--his one unchanging character--and as he holds the door open for this woman, fifty pairs of eyes, each fifty times sharper than Sir Leicester's pair, should find no flaw in him.

Sir Leicester stands to return his parting salutation, and Mr. Rouncewell and Rosa leave the house.
Then lights are brought in, discovering Mr. Tulkinghorn still standing in his window with his hands behind him and my Lady still sitting with his figure before her. She is very pale. Mr. Tulkinghorn thinks, "The power of this woman is astonishing. She has been acting a part the whole time." But he can act a part too - his one unchanging character - and as he holds the door open for her to leave the room, fifty pairs of eyes should find no flaw in him.
