“I remember,” I said, still not able to detach the evil-eye idea quite from my mind.  “How about the Senora’s eyes?  What makes them so—­well, effective?”

“Oh,” Craig answered quickly, “her pupils were normal enough.  Didn’t you notice that?  It was the difference in Whitney’s and the others’ that first suggested making some tests.”

“What is the effect?” I asked, wondering whether it might have contributed to the cause of Mendoza’s death.

“The concentrated poison which has been used in these cigarettes does not kill—­at least not outright.  It is worse than that.  Slowly it accumulates in the system.  It acts on the brain.”

I was listening, spellbound, as he made his disclosure.  No wonder, I thought, even a scientific criminal stood in awe of Craig.

“Of all the dangers to be met with in superstitious countries, these mydratic alkaloids are among the worst.  They offer a chance for crimes of the most fiendish nature—­worse than with the gun or the stiletto.  They are worse because there is so little fear of detection.  That crime is the production of insanity!”

Horrible though the idea, and repulsive, I could not doubt it in the face of Craig’s investigations and what I had already seen with my own eyes.  In fact, it was necessary for me only to recall the mild sensations I myself had experienced, in order to be convinced of the possible effect intended by the insidious poison contained in the many cigarettes which Whitney, for instance, had smoked.

“But don’t you suppose they know it?” I wondered.  “Can’t they tell it?”

“I suppose they have gradually become accustomed to it,” Craig ventured.  “If you have ever smoked one particular brand of cigarette you must have noticed how the manufacturer can gradually substitute a cheaper grade of tobacco without any large number of his patrons knowing anything about it.  I imagine it might have been done in some way like that.”

“But you would think they’d feel the effect and attribute it to smoking.”

“Perhaps they do feel the effect.  But when it comes to tracing causes, some people are loath to admit that tobacco and liquor can be the root of the evil.  No, some one is slipping these cigarettes in on them, perhaps substituting the doped brand for those that are ordered.  If you will notice, both Whitney and Lockwood have cigarettes that are made especially for them.  So had Mendoza.  It is a circumstance which some one has turned to account, though how and by whom the substitution has been made I cannot say yet.  I wish I had time to follow out this one line, to the exclusion of everything else.  But I’ve got to keep my fingers on every rope at once, else the thing will pull away from me.  It is enough for the present that we know what the poison is.  I shall take up the tracing of the person who is administering it the moment I get a hint.”

It was almost daylight before Craig and I left the laboratory after his discovery of the manner of the cigarette poisoning by stramonium.  But that was the only way in which he was able to make progress—­taking time for each separate point by main force.