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likely your father's murderer had read 'The Sign of the Four；'
and； in the second place； even if he had； what assurance had I that
he had read this particular copy of it？ Notwithstanding this；
however； I felt impelled to give my synthetical theory a fair
experimental trial。 I was informed by the Library attendants that
the book had been much read； and given the list of some twenty
names of persons who had borrowed the book during the time I had
specified。 With these twenty…odd names before me； I sat down to
think what my next step should be。 I went carefully over this chain
of reasoning link by link。 'I wish to find a certain murderer； and
have adopted this method in the hope that it may help me。 If I
derive any assistance at all from it； it will be because my man has
read this particular copy of this work； therefore； I may as well
assume at the start that among these twenty…odd names is that of
the man I want。 Is there any possibility of this crime having been
committed by a woman？' was my next question； and my answer was； 'Yes；
a possibility； but it is so decidedly improbable that I may count it
out for the time being。' Accordingly； I set aside all the female
names； which cut my list down to eighteen。 Several of the applicants
had only signed the initials of their given names； and the attendant；
copying them from the slips； had done likewise； so I was obliged to
go to the registration clerk to determine this question of sex； and；
while there； I also ascertained the age of each applicant … that is；
of all but two。 The registrar could give me no information regarding
J。 Z。 Weltz； or B。 W。 Rizzi。 When I told him that one of the clerks
had copied the names for me from application slips； he informed me
that if I would go back to her I would undoubtedly find she had taken
the two last…mentioned names from the green slips used in applying
for books for hall use； as neither J。 Z。 Weltz nor B。 W。 Rizzi was a
card…holder。
〃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。
3。〃A Practical Essay on 3。〃A Practical Essay on
Cancer；〃 by C。 T。 Johnson。 Cancer；〃 by C。 T。 Johnson。
4。〃The Sharper Detected 4。〃The Sharper Detected
and Exposed；〃 by R。 Houdin。 and Exposed；〃 by R。 Houdin。
5。〃The Sign of the Four；〃 5。〃The Sign of the Four；〃
by A。 Conan Doyle。 by A。 Conan Doyle。
6。〃Cancer； a New Method of 6。〃Legal Chemistry： A
Treatment；〃 by W。 H。 Guide to the Detection of
Broadbent。 Poisons； Examinations of
Stains； etc。； as Applied to=20
Chemical Jurisprudence。〃
From the French of A。 Naquet=20
by J。 P。 Battershall； Nat。Sc。D。
7。〃Reports of Trials for 7。〃Traite Pratique des =20
Murder by Poisoning；〃 Maladies Cancerences；〃
by G。 L。 Browne and C。 par H。 Lebert。
G。Stewart。 =20
8。〃A Practical Treatise on 8。〃A Practical Treatise on
Poisons；〃 by 0。 H。 Costill。 Poisons；〃 by 0。 H。 Costill。
9。〃Poisons； Their Effects 9。〃A Treatise on Poisons in
and Detection；〃 by Alexander Relation to Medical=20
Wynter Blyth。 Jurisprudence； Physiology；
and the Practice of Physic；〃=20
by R。 Christison；M。D。； F。R。S。E。
10。〃Poisons； Their Effects 10。〃Poisons； Their Effects
and Detection；〃 by Alexander and Detection；〃 by Alexander =20
Wynter Blyth。 Wynter Blyth。
〃There； do you wonder that the perusal of that list excited me？
Come； now； before I go any further； tell me what you make of it；
Doc；〃 and he passed it to me。
〃There seems to me to be a singular unanimity of purpose existing
between these two men；〃 I said； 〃not only as regards the
subject…matter of their reading； but in no less than six cases they
have both perused the same volume。 This never happened by chance。
Clearly； they are acquaintances； and are working t