“The Black Sailor went out this way, waited in the shadows at the corner of the back street until he saw Mr. Tovey pass, then followed him and caught him up among the crowd outside the Express Train. The knife required no violence in its use, merely a firm push in one of the spots which Dr. Morlandson had shown me. The actual stabbing was a very simple matter, and the Black Sailor disappeared in the crowd, to return here by the way he went out and transform himself into Mr. Ludgrove. “But I realized that this method, although it had served its purpose once, was inartistic and dangerous. There was always the risk that the Black Sailor might be recognized and traced back here. As a matter of fact, Wal Snyder had noticed him, but his evidence was discredited. The police, however, were compelled to offer a reward for him, and I determined that the Black Sailor must be reserved for special purposes. “In the case of Mr. Colburn I adopted quite a different method. I had become a customer of his, and had encouraged his son to confide in me. From him I gradually learnt his father’s habits and peculiarities, particularly that of smoking a pipe in the shop after lunch, and, when he had finished it, of refilling it and putting it away on a shelf. I examined the pipe in young Colburn’s absence, and noticed that it was cracked. That evening, I visited a tobacconist at some distance from here and bought a pipe of similar make and shape. “Within a few days, as I had expected, I saw Mr. Colburn enter Copperdock’s shop. It was a fair assumption that he had gone in to buy a new pipe to replace the old one, since I had learnt that his tobacco was always delivered to him. I took a very small splinter of glass, covered it with a preparation which Dr. Morlandson had given me, and drove one end of it into a notch I had made in the mouthpiece of the pipe I had bought. I then went to Colburn’s shop, on an errand which necessitated young Colburn’s going to the bakehouse and leaving me alone. I saw, as I had expected, a new pipe lying filled on the shelf. I substituted mine for it, put Mr. Colburn’s pipe in my pocket, waited until young Colburn came back with my purchase, and left the shop. I was in the country when Mr. Colburn died, but I heard the details later, as no doubt you have heard them, Dr. Priestley.” “Yes, the deaths of Colburn and Martin strengthened my theory that Dr. Morlandson was at the bottom of the affair,” replied the Professor calmly. “I guessed that there was extensive medical knowledge behind both of them.” His one idea was to prolong this recital as much as possible.