There was a vacant chair (among many others) at one extremity of the row in front of them.

He took that place.

To look at her, without being discovered--there, so far, was the beginning and the end of his utmost desire.
The performances had already begun.

So long as her attention was directed to the singers and players on the platform, he could feast his eyes on her with impunity.

In an unoccupied interval, she looked at the audience--and discovered him.
Had he offended her?
If appearances were to be trusted, he had produced no impression of any sort.