“I decided to let these two names rest a while, and to give my attention to the others.  After careful deliberation I felt reasonably sure your father’s assassin could not fail to be a man of mature judgment and extraordinary cunning, probably a man past middle life—­at all events, I could safely say he was over twenty-one years of age.  Proceeding upon this assumption my list was reduced to ten names.  But how should I further continue this process of exclusion?  This was the question which now confronted me.  I could think of but one way, apart from personally making the gentlemen’s acquaintance, which I did not then wish to do, and that was to ascertain what other books they had borrowed immediately before and after they had read ‘The Sign of the Four.’  This was the course I determined to pursue.

“If you ask me why I so persistently followed an investigation, a successful outcome of which anyone must recognise would be little short of miraculous, I can only say that I felt impelled to do so.  Perhaps the impulse was due to my habit of testing patiently and thoroughly each new theory which impresses me as having any degree of probability, and perhaps it was due to something else—­Cleopatra, perhaps, eh, Doctor?—­I don’t know.  I determined, however, to thoroughly satisfy myself regarding these ten men.  I made a careful list, with the assistance of an attendant, of ten books taken by each man, five taken just prior to ‘The Sign of the Four,’ and the other five just following it.  I made no deductions until the list was completed, although I began to see certain things of interest as we worked upon it.  At length the whole hundred titles were spread before me, and I sat down to see what I could make of them.  I purposely reserved consideration of the books borrowed by Weltz and Rizzi until the last, because I had been able to learn nothing of them, and considered, therefore, that they were the most difficult persons in the list about whom to satisfy myself.  I found the other eight exhibited no system in their reading.  One had read —­I think I can remember the books in the order in which they were borrowed—­’Thelma,’ ‘Under Two Flags,’ ‘David Copperfield,’ ’The Story of an African Farm,’ ‘A Study in Scarlet,’ ’The Sign of the Four,’ ‘The Prisoner of Zenda,’ ‘The Dolly Dialogues,’ ’The Yellow Aster,’ ‘The Superfluous Woman,’ and ‘Ideala.’  This is a fair sample of the other seven.  Not so, however, with Messrs. Weltz and Rizzi.  The reading of these men at once impressed me as having a purpose behind it.

“I will read you a list of the books taken by Weltz and Rizzi, just to see what you will make out of it: 

         Weltz Rizzi

I."Lecons de Toxicologic,” 1."Traite de Toxicologic,”
   par M. Orifia. par C. P. Galtier.

2."The Poisons of Asps and 2."The Poisons of Asps and
   Other Stories,” by Florence Other Stories,” by Florence
   Marryat.  Marryat.