“I’ve no idea,” said Roberts stiffly. “I myself merely went to the anteroom with Sir John, said what was necessary, and joined my patient in the anæsthetic-room.”

“Ah, well—we shall get a better idea of all your movements from the reconstruction.”

“I suppose so,” agreed Roberts, looking perturbed. “It will be a distressing experience for all of us. Except, no doubt, Mr. Thoms.”

He waited a moment and then said nervously:

“Perhaps this is a question that I should not ask, Inspector Alleyn, but I cannot help wondering if the police have a definite theory as regards this crime?”

Alleyn was used to this question.

“We’ve got several theories, Dr. Roberts, and all of them more or less fit. That’s the devil of it.”

“Have you explored the possibility of suicide?” asked Roberts wistfully.

“I have considered it.”

“Remember his heredity.”

“I have remembered it. After he had the attack in the House his physical condition would have rendered suicide impossible, and he could hardly have taken hyoscine while making his speech.”

“Again remember his heredity. He might have carried hyoscine tablets with him for some time and under the emotional stimulus of the occasion suffered a sudden ungovernable impulse. In the study of suicidal psychology one comes across many such cases. Did his hand go to his mouth while he was speaking? I see you look incredulous, Inspector Alleyn. Perhaps you even think it suspicious that I should urge the point. I—I—have a reason for hoping you find that O’Callaghan killed himself, but it does not spring from a sense of guilt.”

A strangely exalted look came into the little doctor’s eyes as he spoke. Alleyn regarded him intently.

“Dr. Roberts,” he said as last, “why not tell me what is in your mind?”

“No,” said Roberts emphatically, “no—not unless—unless the worst happens.”

“Well,” said Alleyn, “as you know, I can’t force you to give me your theory, but it’s a dangerous business, withholding information in a capital charge.”

“It may not be a capital charge,” cried Roberts in a hurry.

“Even suppose your suicide theory is possible, it seems to me that a man of Sir Derek’s stamp would not have done it in such a way as to cast suspicion upon other people.”

“No,” agreed Roberts. “No. That is undoubtedly a strong argument—and yet inherited suicidal mania sometimes manifests itself very abruptly and strangely. I have known instances…”