Humphrey de Bohun very slowly pushed back the crystal, and almost immediately the needle trembled again. "There!" said the Professor in happy confidence, leaning back. "There! What did I tell you?" "Well, what of it, Bill?" said the harassed master. "What of it?" answered his cousin. "The Emerald. Ah! the Emerald!" and he rubbed his hands together. "I don't understand a word you're saying," said poor Humphrey. The Professor leaned forward and tapped his cousin twice upon the shoulder with that knotted forefinger. "That instrument," he said, as solemnly as such a voice can say anything, "tells a crystal close at hand. According to the cube of the distance. I have to use it perpetually. Very well known. German, you know--wonderful people, the Germans. It was Meitz's idea," he went on, adding verisimilitude by the effective use of detail. "But _he_ couldn't have done it without Speitzer. Often like that in research work. Any doubt about a crystal's character. Even amorphous--put that thing close enough, and it points at once. Now do you see? Eh! Now do you see?" "Not exactly," said Humphrey de Bohun. "Why, it's plain enough! I hadn't thought of it. It suddenly occurred to me. It suddenly came to me while you were off to London. Here I had what could solve all our troubles. I put it first here, then there. Everywhere I could. Went on for an hour--all over the room! All over the rug where it dropped. Then one of your guests came in. I didn't want to be seen at it. I was putting it back into my pocket when my hand came close by the side of his coat. Bless my heart! It pointed!" He leant forward again and tapped his cousin more solemnly still, this time on the chest. "Mark my words! That young man's got it!" "Which young man?" said Humphrey, remembering what counter accusation the Professor would naturally make, and thinking at once of Galton. "That young writing fellow," said Cousin Bill. "That newspaper chap McTaggart. McTaggart, McTaggart, McTaggart, McTaggart, McTaggart." Humphrey de Bohun hesitated. "My dear Bill," he said, "you never know. He might have had something else in his pocket--also crystal, or--I don't know ... something." The Professor wagged his head with all the dignity of a goat. "Won't work, Humphrey!" he said. "Won't work! One can always tell the size by the distance. It wasn't some ring or small thing of that kind. Besides which, he wouldn't have such a small thing of his own in his pocket. No, the Emerald's there all right. And I'll tell you something that makes me surer still. I took occasion to brush up against him--there was a hard slab in that pocket, Humphrey. In that pocket. A small, hard slab! Slab! ... Hard slab! ..." An awful task arose in the conscience of Humphrey de Bohun. He must play the spy again. He must mistrust yet another guest.