That Affair Next Door by Anna Katharine Green

I had exhausted my wonder, so I accepted this statement with no more display of surprise than a grim smile.

"When you failed to identify Howard Van Burnam as the man who accompanied his wife into the adjacent house, I realized that I must look elsewhere for the murderer of Louise Van Burnam. You see I had more confidence in the excellence of your memory than you had yourself, so much indeed that I gave you more than one chance to exercise it, having, by certain little methods I sometimes employ, induced different moods in Mr. Van Burnam at the time of his several visits, so that his bearing might vary, and you have every opportunity to recognize him for the man you had seen on that fatal night."