Self-reproaches, however, were of little use now, and with a tremendous effort he wrenched his mind back to the immediate problem that confronted it. Why, in God's name, should any one have wished to kill the Professor, and how had it come about that the latter's body was lying where it did? In order to reach the study from the laboratory one had to pass through the whole length of the hall. Colin's hearing was particularly acute, and he felt positive that the creak of footsteps or the opening or shutting of a door would instantly have attracted his attention. Nothing of the sort had happened. Until that one crash of breaking glass the whole house had been absolutely silent. His eyes fell upon the damaged lock, and another question suddenly presented itself. Who had been responsible for turning the key? Surely it could not have been the Professor. If he had entered the room expecting to find it empty, what conceivable reason could he have had for fastening himself in? If, on the other hand, he had entertained even the remotest suspicion that somebody was hiding on the premises, he would certainly have come upstairs before attempting to approach the study. It seemed more likely that the murderer had locked the door after committing the crime, so that he might have a better chance of making his escape. There was a coolness about the proceeding which suggested that he was fully aware of Colin's presence in the house, and a conviction that the whole thing had been planned and carried out with the most cold-blooded deliberation forced itself gradually upon the young surgeon's mind. Had Mrs. Ramsay been right? Was it the same man who had ransacked the Professor's desk? If it were so--and all the circumstances seemed to point to that conclusion--burglary and not murder had probably been the real object of his visit. There was evidently something in the place, some document or paper, of which he was desperately anxious to obtain possession. Having failed to find it at his first attempt, he had apparently returned to the house a second time in order to make another and more exhaustive search. By some fatal chance the Professor must have taken it into his head to enter the study just after the intruder had succeeded in gaining admittance. On finding the window open he had naturally stepped forward to close it, only to receive a murderous blow out of the darkness, which had sent him crashing into the glass. The one fact which refused to fit in with this theory was the entire absence of any sound right up to the actual moment of the crime. There must, of course, be some explanation, and Colin was puzzling his brains in a vain attempt to discover it when the loud peal of a bell suddenly jangled out from the kitchen.