How does Arthur react to the reminder that he employed Eva Smith?

The Inspector reminds Birling that Eva Smith was employed in his works at one point, and, when Birling does still not remember, the Inspector shows him a photograph of her that he says he found in her room.

Birling stares hard at the photograph and recognizes the young girl. Immediately after this recognition, he seems a little nervous and admits the girl had once workied for him, and that he had discharched her.