"A part to which you will not object, that of looker-on. Not from this room, but that"--pointing to the back room. "The panels of the door, you will observe, are of ground glass. Sitting within there, you can see all that passes in this room without being yourself seen. If you will keep quiet, no one will suspect that you are in hiding." "For the life of me," I said, "I cannot guess what good my sitting in there will do." "I do not suppose you can; but learn from me that I do nothing without a motive. I do not care to be questioned too closely. The promise I have made to you will be kept if you do not thwart it. You may see something that will surprise you. I say 'may,' because I have not the power to entirely rule men's movements. But I think it almost certain he will pay me a visit this morning." "He?" I cried. "Who?" "The man whose thoughts I read on Friday with respect to the girl who was murdered on that night." I started. If Devlin spoke the truth, and if the man came to his shop this morning, I should be in possession of a practical clue which would lead me to the goal I wished to reach. "He comes regularly," continued Devlin, "on Mondays, Wednesdays, and Fridays. This is his day." "Do you know his name?" I inquired, in great excitement. "I did not," replied Devlin, "the last time I saw him. How should I know it now?" "Nor where he lives?" "Nor where he lives." "I must obey you, I suppose," I said. "It will be advisable, and you must obey me implicitly. Deviate by a hair's breadth from what I require of you, and I withdraw my promise, which now exists in full integrity. Decide." "I have decided. I will remain in that room." "There is another point upon which I must insist positively. From that room you do not stir until I bid you; in that room you do not speak unless you receive a cue from me. Agreed?" "Agreed." "On your honour?" "On my honour." "Good. Now you can retire. You will find books in there to amuse you if you get wearied with your watch." He opened the door for me, and closed it upon me. He had spoken correctly. Through the ground glass I could see everything in the shop, and I took his word for it that I could not myself be seen.