Again Godfrey paused.  I was following his reasoning with such absorbed attention that I could feel my brain crinkle with the effort.

“Now, listen,” said Godfrey, and I could have smiled at the uselessness of the admonition—­as if I were not already listening with all my faculties!  “There is only one way in which the murderer could have known that it was Swain’s right hand, and that was by overhearing the conversation in the arbour.  But if he overheard that much, he overheard it all, and he knew therefore what it was Swain proposed to do.  He knew that Vaughan’s sanity was to be questioned; he knew that he would probably be placed in a sanitarium; he knew that Miss Vaughan would probably marry Swain.  Presuming that it was Silva, he knew that, unless something was done to stop it, a very few days would place both Vaughan and his daughter beyond his reach.”

“That is true,” I admitted; “but Vaughan was beyond his reach a good deal more certainly dead than he would have been in a sanitarium.  Besides, it isn’t at all certain that he would have been sent to a sanitarium.”

“That’s an objection, surely,” Godfrey agreed; “but I must find out if Vaughan is really beyond his reach dead.”

I stared at him.

“You don’t mean....”

“I don’t know what I mean, Lester.  I can feel a sort of dim meaning at the back of my mind, but I can’t get it out into the light.”

“Besides,” I went on, “if the yogi did it, how did he get back into the house before we got there?”

“He peeped in at the door, saw the coast was clear, and went back through the library.  Remember, Miss Vaughan was unconscious.  That doesn’t bother me.  And another thing, Lester.  How did Miss Vaughan’s father come to burst in on her and Swain like that?  How did he know they were in the arbour?  It was dark and he couldn’t have seen either of them.”

“He might have been walking about the grounds and overheard them.”

“I don’t believe it.  I believe somebody told him they were there.  And only one person could have told him—­that is Silva.  No—­there’s only one point I can’t get past—­that’s the finger-prints.”

And then I remembered.

“Godfrey,” I cried, “there’s one thing—­I forgot to tell you.  You heard Swain remark that Vaughan was a collector of finger-prints?”

“Yes.”

“And that he had a set of Swain’s?”

“Yes.”

“Well, when I told Miss Vaughan about the prints on her father’s robe, she ran to a book-case and got out a book.  It had Vaughan’s collection in it, all bound together.  But the page on which Swain’s were had been torn out.”

Godfrey sat for a moment, staring at me spell-bound.  Then he began pacing up and down the study, like a tiger in its cage; up and down, up and down.

“I’m bound to add,” I went on finally, “that Hinman suggested a very plausible reason for their disappearance.”