“The writing on the wall was done with a man’s forefinger dipped in blood. My glass allowed me to observe that the plaster was slightly scratched in doing it, which would not have been the case if the man’s nail had been trimmed. I gathered up some scattered ash from the floor. It was dark in colour and flakey—such an ash as is only made by a Trichinopoly. I have made a special study of cigar ashes—in fact, I have written a monograph upon the subject. I flatter myself that I can distinguish at a glance the ash of any known brand, either of cigar or of tobacco. It is just in such details that the skilled detective differs from the Gregson and Lestrade type.”

“And the florid face?” I asked.

“Ah, that was a more daring shot, though I have no doubt that I was right. You must not ask me that at the present state of the affair.”

I passed my hand over my brow. “My head is in a whirl,” I remarked; “the more one thinks of it the more mysterious it grows. How came these two men—if there were two men—into an empty house? What has become of the cabman who drove them? How could one man compel another to take poison? Where did the blood come from? What was the object of the murderer, since robbery had no part in it? How came the woman’s ring there? Above all, why should the second man write up the German word RACHE before decamping? I confess that I cannot see any possible way of reconciling all these facts.”

My companion smiled approvingly.

“You sum up the difficulties of the situation succinctly and well,” he said. “There is much that is still obscure, though I have quite made up my mind on the main facts. As to poor Lestrade’s discovery it was simply a blind intended to put the police upon a wrong track, by suggesting Socialism and secret societies. It was not done by a German. The A, if you noticed, was printed somewhat after the German fashion. Now, a real German invariably prints in the Latin character, so that we may safely say that this was not written by one, but by a clumsy imitator who overdid his part. It was simply a ruse to divert inquiry into a wrong channel. I’m not going to tell you much more of the case, Doctor. You know a conjuror gets no credit when once he has explained his trick, and if I show you too much of my method of working, you will come to the conclusion that I am a very ordinary individual after all.”

“I shall never do that,” I answered; “you have brought detection as near an exact science as it ever will be brought in this world.”

My companion flushed up with pleasure at my words, and the earnest way in which I uttered them. I had already observed that he was as sensitive to flattery on the score of his art as any girl could be of her beauty.