“Chick, you are a good pupil; I’m proud of you.” “Thanks. I will go further, and say that I believe you think Redway knows who killed the girl, or how she died, and that he is shielding somebody.” “You must have learned mind-reading with your other accomplishments,” smiled Nick. “Maybe you can tell me who it is that he is trying to shield?” “Not unless it be the strange woman.” “Humph! We must find that woman and learn more about her,” was the non-committal reply. “Chick, there is another character I want watched, and you’re the very one to do it.” “Who?” “The confidential clerk--Gay.” “Oh, ho!” “I don’t like him. Besides, there is a mystery about him.” “Well?” “He was absent from the building most of the day, but he managed to be in at the death.” “Did you ask him what had brought him back at that particular time?” “No.” “Why?” “It would have told him that he was under suspicion. Best to have him off his guard.” “You have other evidence to involve him?” “Yes.” “I thought so. What is it?” “I told you about my quick, hurried secret visit to the office of the senior member of the firm?” “Yes. What did you discover in there?” “Not much, and yet a good deal. There was a mirror so fixed near the ceiling that a person standing or sitting near the connecting door could see reflected everything which went on in the other room where the tragedy occurred.” “And, as the transom above the door was open, could hear as well?” “Yes.” “I see. That office was supposed to be deserted all the afternoon?” “Correct.” “Yet some one was concealed in there, and heard and saw all that was said and done in Redway’s office?” “I believe so. Whoever it was, he or she made a mistake.” “By forgetting to remove the mirror?” “Exactly. And unless I am much mistaken that mirror will trap the person we want.” “How?” “If it disappears from its place high up there on the wall before to-morrow--and I think it will.” “I see, I see. You have arranged to find out who goes into that office?” “Yes. The janitor will keep me posted.” “Can you trust him?” “I think so.” “But how about his seeing Redway go downstairs at the time he says he did?” “I believe he saw some one go down at that time.” “Who? Redway?” “That I am not ready to answer. It is one of the knotty places in the mystery.” “Nick, who wrote that note to you?” “I have no doubt it was the girl who was killed.” “What was her object?” “She had serious work for a detective, or she would not have sent for me. The girl wrote another note on her typewriter to-day besides the one sent to me.” “To whom?” “I don’t know; it is not addressed. I’ll show it to you.” Nick produced the dead girl’s pocketbook, from which he took a typewritten slip, on which was this message: