At that time Mr.Douglas entered the room.

It was a short interview, if there were any interview at all; for Mrs.Douglas declares that her husband had not left her more than a few minutes when she heard the shot." "The candle shows that," said Holmes.
"Exactly.

The candle, which was a new one, is not burned more than half an inch.

He must have placed it on the table before he was attacked; otherwise, of course, it would have fallen when he fell.

This shows that he was not attacked the instant that he entered the room.