How in the world could Wethermill know of that clear picture in his mind?
"Yes.

You saw Celia Harland on the evening before the murder." Ricardo stared at his friend.

It seemed to him that Harry Wethermill had gone out of his mind.

Here he was corroborating the suspicions of the police by facts--damning and incontrovertible facts.
"On the night before the murder," continued Wethermill quietly, "Celia Harland lost money at the baccarat-table.

Ricardo saw her in the garden behind the rooms, and she was hysterical.