“Dear Mr. Holmes [he says]: 

“I will go no further in this matter.  It is too dangerous—­he suspects me.  I can see that he suspects me.  He came to me quite unexpectedly after I had actually addressed this envelope with the intention of sending you the key to the cipher.  I was able to cover it up.  If he had seen it, it would have gone hard with me.  But I read suspicion in his eyes.  Please burn the cipher message, which can now be of no use to you.

Fred Porlock.”

Holmes sat for some little time twisting this letter between his fingers, and frowning, as he stared into the fire.

“After all,” he said at last, “there may be nothing in it.  It may be only his guilty conscience.  Knowing himself to be a traitor, he may have read the accusation in the other’s eyes.”

“The other being, I presume, Professor Moriarty.”

“No less!  When any of that party talk about ‘He’ you know whom they mean.  There is one predominant ‘He’ for all of them.”

“But what can he do?”

“Hum!  That’s a large question.  When you have one of the first brains of Europe up against you, and all the powers of darkness at his back, there are infinite possibilities.  Anyhow, Friend Porlock is evidently scared out of his senses—­kindly compare the writing in the note to that upon its envelope; which was done, he tells us, before this ill-omened visit.  The one is clear and firm.  The other hardly legible.”

“Why did he write at all?  Why did he not simply drop it?”

“Because he feared I would make some inquiry after him in that case, and possibly bring trouble on him.”

“No doubt,” said I.  “Of course.”  I had picked up the original cipher message and was bending my brows over it.  “It’s pretty maddening to think that an important secret may lie here on this slip of paper, and that it is beyond human power to penetrate it.”

Sherlock Holmes had pushed away his untasted breakfast and lit the unsavoury pipe which was the companion of his deepest meditations.  “I wonder!” said he, leaning back and staring at the ceiling.  “Perhaps there are points which have escaped your Machiavellian intellect.  Let us consider the problem in the light of pure reason.  This man’s reference is to a book.  That is our point of departure.”

“A somewhat vague one.”

“Let us see then if we can narrow it down.  As I focus my mind upon it, it seems rather less impenetrable.  What indications have we as to this book?”

“None.”

“Well, well, it is surely not quite so bad as that.  The cipher message begins with a large 534, does it not?  We may take it as a working hypothesis that 534 is the particular page to which the cipher refers.  So our book has already become a large book, which is surely something gained.  What other indications have we as to the nature of this large book?  The next sign is C2.  What do you make of that, Watson?”