He produced from his pocket an irregular lump of plaster, about three inches long. On one side of this appeared in relief the likeness of two irregular rows of six or eight teeth, minus one in each row, where a deep gap was seen, in the position spoken of by my friend. He proceeded: "This was enough at least to set me after this young man. But he gave me the greatest chance of all when he turned and left his apple (eaten unpeeled, remember!--another important triviality) on his plate. I'm afraid I wasn't at all polite, and I ran the risk of arousing his suspicions, but I couldn't resist the temptation to steal it. I did, as you saw, and here it is." He brought the apple from his coat-pocket. One bitten side, placed against the upper half of the mold, fitted precisely, a projection of apple filling exactly the deep gap. The other side similarly fitted the lower half. "There's no getting behind that, you see," Hewitt remarked. "Merely observing the man's teeth was a guide, to some extent, but this is as plain as his signature or his thumb impression. You'll never find two men _bite_ exactly alike, no matter whether they leave distinct teeth-marks or not. Here, by the by, is Mrs. Clayton's oil. We'll take another mold from this apple, and compare _them_." He oiled the apple, heaped a little plaster in a newspaper, took my water-jug, and rapidly pulled off a hard mold. The parts corresponding to the merely broken places in the apple were, of course, dissimilar; but as to the teeth-marks, the impressions were identical. "That will do, I think," Hewitt said. "Tomorrow morning, Brett, I shall put up these things in a small parcel, and take them round to Bow Street." "But are they sufficient evidence?" "Quite sufficient for the police purpose. There is the man, and all the rest--his movements on the day and so forth--are simple matters of inquiry; at any rate, that is police business." * * * * * I had scarcely sat down to my breakfast on the following morning when Hewitt came into the room and put a long letter before me. "From our friend of last night," he said; "read it." This letter began abruptly, and undated, and was as follows: "TO MARTIN HEWITT, ESQ.