It is clearly a reference to the words in a page of some book.
Until I am told which page and which book I am powerless."
"But why 'Douglas' and 'Birlstone'?"
"Clearly because those are words which were not contained in the page in question."
"Then why has he not indicated the book?"
"Yow native shrewdness, my dear Watson, that innate cunning which is the delight of your friends, would surely prevent you from inclosing cipher and message in the same envelope.
Should it miscarry, you are undone.
As it is, both have to go wrong before any harm comes from it.
Our second post is now overdue, and I shall be surprised if it does not bring us either a further letter of explanation, or, as is more probable, the very volume to which these figures refer."
Holmes's calculation was fulfilled within a very few minutes by the appearance of Billy, the page, with the very letter which we were expecting.