As chapter 10 of Agatha Christie's classic murder-mystery novel And Then There Were None opens, Vera Claythorne and Captain Philip Lombard are mulling over what Mr. Justice Lawrence Wargrave says at the end of chapter 9:

From now on, it is our task to suspect each and every one amongst us. (Chap. 9, VII)

There have been three murders so far, and Vera suspects Dr. Edward Armstrong:

Two of the deaths have been poison. That rather points to a doctor. (Chap. 10, I)

Anthony Marston had been killed by poisoned wine, and everyone knew that Dr. Armstrong gave Mrs. Rogers "a sleeping draught" to help her relax and get some rest.

Lombard admits that this could be true, but he doubts that Dr. Armstrong would have had time to commit the murder of General Macarthur while Dr. Armstrong was apart from him earlier in the day.

Vera suggests, however, that if Dr. Armstrong didn't commit the murder during the time he was away from Lombard, he had an opportunity to kill the General later in the day, when he went to call the General to lunch.

That would have given Dr. Armstrong ample time to go down to the sea where the General sat, lost in his own thoughts, hit the General "with a life preserver or some such thing on the back of the head” (Chap. 9, V), as Dr. Armstrong himself suggested, then get back to the others at the house.

Lombard found this idea intriguing:

“So you think he did it then? Pretty cool thing to do.”
“You know,” he said, “that’s a clever idea of yours. I wonder -” (Chap. 10, I)

By the end of chapter 10, Lombard isn't entirely convinced that Dr. Armstrong is the murderer, but he's definitely considering the possibility.