The occupant of the balcony was after all quite another person, a person presented, on a second look, by a charming back and a slight shift of her position, as beautiful brilliant unconscious Mamie--Mamie alone at home, Mamie passing her time in her own innocent way, Mamie in short rather shabbily used, but Mamie absorbed interested and interesting.
With her arms on the balustrade and her attention dropped to the street she allowed Strether to watch her, to consider several things, without her turning round.
But the oddity was that when he HAD so watched and considered he simply stepped back into the room without following up his advantage.

He revolved there again for several minutes, quite as with something new to think of and as if the bearings of the possibility of Sarah had been superseded.

For frankly, yes, it HAD bearings thus to find the girl in solitary possession.

There was something in it that touched him to a point not to have been reckoned beforehand, something that softly but quite pressingly spoke to him, and that spoke the more each time he paused again at the edge of the balcony and saw her still unaware.

Her companions were plainly scattered; Sarah would be off somewhere with Waymarsh and Chad off somewhere with Jim.