"You mean the man we sat opposite at Luzatti's, of course?"
"Yes, I got his name, of course, from the reverse of that gold medal he was good enough to show me. But I fear he has bilked me over the address. He suspected me, that was plain, and left his umbrella by way of experiment, to see if I were watching him sharply enough to notice the circumstance, and to avail myself of it to follow him. I was hasty and fell into the trap. He cabbed it away from Luzatti's, and I cabbed it after him. He has led me a pretty dance up and down London to-night, and two cabbies have made quite a stroke of business out of us. In the end he entered a house of which, of course, I have taken the address, but I expect he doesn't live there. He is too smart a man to lead me to his den; but the police can certainly find something of him at the house he went in at—and, I expect, left by the back way. By the way, you never guessed that simple little puzzle as to how I found that this was a murder, did you? You see it now, of course?"
"Something to do with that apple you stole, I suppose?"
"Something to do with it? I should think so, you worthy innocent. Just ring your bell—we'll borrow Mrs. Clayton's sewing-machine oil again. On the night we broke into Foggatt's room you saw the nutshells and the bitten remains of an apple on the sideboard, and you remembered it; and yet you couldn't see that in that piece of apple possibly lay an important piece of evidence. Of course, I never expected you to have arrived at any conclusion, as I had, because I had ten minutes in which to examine that apple, and to do what I did with it. But at least you should have seen the possibility of evidence in it.
"First, now, the apple was white. A bitten apple, as you must have observed, turns of a reddish-brown colour if left to stand long. Different kinds of apples brown with different rapidities, and the browning always begins at the core. This is one of the twenty thousand tiny things that few people take the trouble to notice, but which it is useful for a man in my position to know. A russet will brown quite quickly. The apple on the sideboard was, as near as I could tell, a Newtown pippin or other apple of that kind, which will brown at the core in from twenty minutes to half an hour, and in other parts in a quarter of an hour more. When we saw it, it was white, with barely a tinge of brown about the exposed core. Inference—somebody had been eating it fifteen or twenty minutes before—perhaps a little longer; an inference supported by the fact that it was only partly eaten.
"I examined that apple, and found it bore marks of very irregular teeth. While you were gone I oiled it over, and, rushing down to my rooms, where I always have a little plaster of Paris handy for such work, took a mould of the part where the teeth had left the clearest marks. I then returned the apple to its place, for the police to use if they thought fit. Looking at my mould, it was plain that the person who had bitten that apple had lost two teeth, one at top and one below, not exactly opposite, but nearly so. The other teeth, although they would appear to have been fairly sound, were irregular in size and line. Now the dead man had, as I saw, a very excellent set of false teeth, regular and sharp, with none missing. Therefore it was plain that somebody else had been eating that apple. Do I make myself clear?"
"Quite. Go on."
"There were other inferences to be made—slighter, but all pointing the same way. For instance, a man of Foggatt's age does not as a rule munch an unpeeled apple like a schoolboy—inference, a young man, and healthy. Why I came to the conclusion that he was tall, active, a gymnast, and perhaps a sailor, I have already told you, when we examined the outside of Foggatt's window. It was also pretty clear that robbery was not the motive, since nothing was disturbed, and that a friendly conversation had preceded the murder—witness the drinking and the eating of the apple. Whether or not the police noticed these things I can't say. If they had had their best men on they certainly would, I think; but the case, to a rough observer, looked so clearly one of accident or suicide, that possibly they didn't.
"As I said, after the inquest I was unable to devote any immediate time to the case, but I resolved to keep my eyes open. The man to look for was tall, young, strong and active, with a very irregular set of teeth, a tooth missing from the lower jaw just to the left of the centre, and another from the upper jaw a little further still toward the left. He might possibly be a person I had seen about the premises (I have a good memory for faces), or, of course, he possibly might not.
"Just before you returned from your holiday I noticed a young man at Luzatti's whom I remembered to have seen somewhere about the offices in this building. He was tall, young, and so on, but I had a client with me, and was unable to examine him more narrowly—indeed, as I was not exactly engaged on the case, and as there are several tall young men about, I took little trouble. But to-day, finding the same young man with a vacant seat opposite him, I took the opportunity of making a closer acquaintance."