“How was it done?” repeated Thorndyke in reply to Miller’s question, as we sat at a retired table in the “Lord Warden” Hotel. “Well, it was really very simple. I am afraid I shall disappoint you if you expect anything ingenious and recondite. Of course, it was obvious that Miss Dalby had not committed this atrocious murder and robbery; and it was profoundly improbable that this extremely incriminating letter had been dropped accidentally. That being so, it was almost certain that the letter had been ‘planted,’ as Pinder expressed it. But that was a mere opinion that helped us not at all. The actual solution turned upon a simple chemical fact with which I happened to be acquainted; which is this: that all the basic coal-tar dyes, and especially methylene blue, dye oxycellulose without requiring a mordant, but do not react in this way on cellulose. Now, good paper is practically pure cellulose; and if you dip a sheet of such paper into certain oxidizing liquids, such as a solution of potassium chlorate with a slight excess of hydrochloric acid, the paper is converted into oxycellulose. But if instead of immersing the paper, you write on it with a quill or glass pen dipped in the solution, only the part which has been touched by the pen is changed into oxycellulose. No change is visible to the eye: but if a sheet of paper written on with this colourless fluid is dipped in a solution of, say, methylene blue, the invisible writing immediately becomes visible. The oxycellulose takes up the blue dye. “Now, when I picked up that sheet of the letter in the railway carriage and noted that the ink used appeared to be methylene blue, this fact was recalled to my mind. Then, on looking at it closely, I seemed to detect a certain slight spottiness in the writing. There were points on some of the letters that were a little deeper in colour than the rest; and it occurred to me that it was possible that these circulars might be used to transmit secret messages of a less innocent kind than those that met the unaided eye, just as these political societies might form an excellent cover for the operations of criminal associations. But if the circulars had been so used, it is evident that the secret writing would not be on all the circulars. The prepared sheets would be used only for the circulars that were to be sent to particular persons, and in those cases the secret writing would probably be in the nature of a personal communication, either to a particular individual or to a small group. The possible presence of a secret message thus became of vital evidential importance; for if it could be shown that this letter was addressed to some person other than Miss Dalby, that would dispose of the only evidence connecting her with the crime. “It happened, most fortunately, that I was able to get possession of the final sheet of this letter----” “Of course it did,” growled Miller, with a sour smile.