It was only a rudimentary idea, a surmise, a possibility; but still it was worth going into. It was not any cause of undue pride to me, for it came as a corollary to an established conclusion, rather than as a fine piece of reasoning from acute observation. The dates of the letters gave the period as the end of the sixteenth century, when one of the best ciphers of that time had been conceived, the "Biliteral Cipher" of Francis Bacon. To this my attention had been directed by the work of John Wilkins and I had followed it out with great care. As I was familiar with the principle and method of this cipher I was able to detect signs of its existence; and this being so, I had at once strong hopes of being able to find the key to it. The Biliteral cipher has as its great advantage, that it can be used in any ordinary writing, and that its forms and methods are simply endless. All that it requires in the first instance is that there be some method arranged on between the writer and the reader of distinguishing between different forms of the same letter. In my desk I had a typewritten copy of a monograph on the subject of the Biliteral cipher, in which I half suggested that possibly Bacon's idea might be worked out more fully so that a fewer number of symbols than his five would be sufficient. Leaving my present occupation for a moment I went and got it; for by reading it over I might get some clue to aid me. Some thought which had already come to me, or some conclusion at which I had already arrived might guide me in this new labyrinth of figures, words and symbols.[1] [1] See Appendix A. When I had carefully read the paper, occasionally referring to the documents before me, I sat down and wrote a letter to Miss Anita telling her that I had undertaken the task at once on her suggestion and that I surmised that the method of secret writing adopted if any, was probably a variant of the Biliteral cipher. I therefore sent her my own monograph on the subject so that if she chose she might study it and be prepared to go into the matter when we met. I studiously avoided saying anything which might frighten her or make any barrier between us; matters were shaping themselves too clearly for me to allow myself to fall into the folly of over-precipitation. It was only when I had placed the letter with its enclosure in the envelope and written Marjory's--Miss Anita's--name that I remembered that I had not got her address. I put it in my pocket to keep for her till we should meet on Tuesday.