The policeman's hands shook as he opened the letter. He read it quickly. Then he put it in his pocket and ran out of the room without saying a word.
"Dupin!" I said, as I turned to my friend. "How did you solve the mystery?"
"It was simple, my friend," he said. "Germont and his policemen could not find the letter, because they did not try to understand the mind of the man who stole it. Instead, they looked for the letter where they would have hidden it.
"Mister D'Arcy is not a policeman. He is, however, very intelligent. He knew the police would search his apartment. He also knew how police think. So, he did not hide the letter where he knew they would look for it.
"Do you remember how Germont laughed when I said the mystery was difficult for him to solve because it was so simple?"
Dupin filled his pipe with tobacco and lit it. "Well, the more I thought about it, the more I realized the police could not find the letter because D'Arcy had not hidden it at all.
"So I went to visit D'Arcy in his apartment. I took a pair of dark green eyeglasses with me. I explained to him that I was having trouble with my eyes and needed to wear the dark glasses at all times. He believed me. The glasses permitted me to look around the apartment while I seemed only to be talking to him.
"I paid special attention to a large desk where there were a lot of papers and books. However, I saw nothing suspicious there. After a few minutes, however, I noticed a small shelf over the fireplace. A few postcards and a letter were lying on the shelf. The letter looked very old and dirty.
"As soon as I saw this letter, I decided it must be the one I was looking for. It must be, even though it was completely different from the one Germont had described.