The Disappearing Eye by Fergus Hume — Read

Here was a discovery! Well might I talk about the disappearing eye, for it vanished every time it was found. It had disappeared out of Mrs. Caldershaw's head when she was murdered; it had disappeared from the drawing-room table, and now it had disappeared from the watch case of Mr. Walter Monk. And this final vanishing seemed to be the strangest of all. I could not understand how it had taken place since I was in the room and the closed case was on the table all the time. Striver could not have secured the eye, for I had held him in conversation.

Then I remembered that Mr. Monk had been hunting the smoking-room for a magnifying glass in order to decipher the inscription. Engaged with the repentant gardener, I had paid very little attention to his movements, so it was probable that when my back was turned he had taken the opportunity to slip the incriminating eye into his pocket. Also I recalled the fact that he had handed me the closed case himself, recommending me to get a magnifying glass from a jeweller. Had I been clever enough to mistrust him -- as I had every reason to -- I should there and then have opened the case to see that the eye was safe. But I had not done so, and now, in the train, when Monk was out of reach, I discovered the loss.