As I entered the Temple by the Tudor Street gate the aspect of the place smote my senses with an air of agreeable familiarity. Here had I spent many a delightful hour when working with Thorndyke at the remarkable Hornby case, which the newspapers had called "The Case of the Red Thumb Mark"; and here had I met the romance of my life, the story whereof is told elsewhere. The place was thus endeared to me by pleasant recollections of a happy past, and its associations suggested hopes of happiness yet to come and in the not too far distant future.  
My brisk tattoo on the little brass knocker brought to the door no less a person than Thorndyke himself; and the warmth of his greeting made me at once proud and ashamed. For I had not only been an absentee; I had been a very poor correspondent.
 
"The prodigal has returned, Polton," he exclaimed, looking into the room. "Here is Dr. Jervis."
 
I followed him into the room and found Polton—his confidential servant, laboratory assistant, artificer and general "familiar"—setting out the tea-tray on a small table. The little man shook hands cordially with me, and his face crinkled up into the sort of smile that one might expect to see on a benevolent walnut.
 
"We've often talked about you, sir," said he. "The doctor was wondering only yesterday when you were coming back to us."
 
As I was not "coming back to them" quite in the sense intended I felt a little guilty, but reserved my confidences for Thorndyke's ear and replied in polite generalities. Then Polton fetched the tea-pot from the laboratory, made up the fire and departed, and Thorndyke and I subsided, as of old, into our respective arm-chairs.
 
"And whence do you spring from in this unexpected fashion?" my colleague asked. "You look as if you had been making professional visits."
 
"I have. The base of operations is in Lower Kennington Lane."
 
"Ah! Then you are 'back once more on the old trail'?"
 
"Yes," I answered, with a laugh, "'the old trail, the long trail, the trail that is always new.'"
 
"And leads nowhere," Thorndyke added grimly.
 
I laughed again; not very heartily, for there was an uncomfortable element of truth in my friend's remark, to which my own experience bore only too complete testimony. The medical practitioner whose lack of means forces him to subsist by taking temporary charge of other men's practices is apt to find that the passing years bring him little but grey hairs and a wealth of disagreeable experience.
 
"You will have to drop it, Jervis; you will, indeed," Thorndyke resumed after a pause. "This casual employment is preposterous for a man of your class and professional attainments. Besides, are you not engaged to be married and to a most charming girl?"
 
"Yes, I know. I have been a fool. But I will really amend my ways. If necessary, I will pocket my pride and let Juliet advance the money to buy a practice."
 
"That," said Thorndyke, "is a very proper resolution. Pride and reserve between people who are going to be husband and wife, is an absurdity. But why buy a practice? Have you forgotten my proposal?"
 
"I should be an ungrateful brute if I had."
 
"Very well. I repeat it now. Come to me as my junior, read for the Bar and work with me, and, with your abilities, you will have a chance of something like a career. I want you, Jervis," he added, earnestly. "I must have a junior, with my increasing practice, and you are the junior I want. We are old and tried friends; we have worked together; we like and trust one another, and you are the best man for the job that I know. Come; I am not going to take a refusal. This is an ultimatum."
 
"And what is the alternative?" I asked with a smile at his eagerness.
 
"There isn't any. You are going to say yes."
 
"I believe I am," I answered, not without emotion; "and I am more rejoiced at your offer and more grateful than I can tell you. But we must leave the final arrangements for our next meeting—in a week or so, I hope—for I have to be back in an hour, and I want to consult you on a matter of some importance."
 
"Very well," said Thorndyke; "we will leave the formal agreement for consideration at our next meeting. What is it that you want my opinion on?"
 
"The fact is," I said, "I am in a rather awkward dilemma, and I want you to tell me what you think I ought to do."
 
Thorndyke paused in the act of refilling my cup and glanced at me with unmistakable anxiety.
 
"Nothing of an unpleasant nature, I hope," said he.
 
"No, no; nothing of that kind," I answered with a smile as I interpreted the euphemism; for "something unpleasant," in the case of a young and reasonably presentable medical man is ordinarily the equivalent of trouble with the female of his species. "It is nothing that concerns me personally at all," I continued; "it is a question of professional responsibility. But I had better give you an account of the affair in a complete narrative, as I know that you like to have your data in a regular and consecutive order."
 
Thereupon I proceeded to relate the history of my visit to the mysterious Mr. Graves, not omitting any single circumstance or detail that I could recollect.
 
Thorndyke listened from the very beginning of my story with the closest attention. His face was the most impassive that I have ever seen; ordinarily as inscrutable as a bronze mask; but to me, who knew him intimately, there was a certain something—a change of colour, perhaps, or an additional sparkle of the eye—that told me when his curious passion for investigation was fully aroused. And now, as I told him of that weird journey and the strange, secret house to which it had brought me, I could see that it offered a problem after his very heart. During the whole of my narration he sat as motionless as a statue, evidently committing the whole story to memory, detail by detail; and even when I had finished he remained for an appreciable time without moving or speaking.
 
At length he looked up at me. "This is a very extraordinary affair, Jervis," he said.
 
"Very," I agreed; "and the question that is agitating me is, what is to be done?"
 
"Yes," he said, meditatively, "that is the question; and an uncommonly difficult question it is. It really involves the settlement of the antecedent question: What is it that is happening at that house?"
 
"What do you think is happening at that house?" I asked.
 
"We must go slow, Jervis," he replied. "We must carefully separate the legal tissues from the medical, and avoid confusing what we know with what we suspect. Now, with reference to the medical aspects of the case. The first question that confronts us is that of sleeping sickness, or negro-lethargy as it is sometimes called; and here we are in a difficulty. We have not enough knowledge. Neither of us, I take it, has ever seen a case, and the extant descriptions are inadequate. From what I know of the disease, its symptoms agree with those in your case in respect of the alleged moroseness and in the gradually increasing periods of lethargy alternating with periods of apparent recovery. On the other hand, the disease is said to be confined to negroes; but that probably means only that negroes alone have hitherto been exposed to the conditions that produce it. A more important fact is that, as far as I know, extreme contraction of the pupils is not a symptom of sleeping sickness. To sum up, the probabilities are against sleeping sickness, but with our insufficient knowledge, we cannot definitely exclude it."
 
"You think that it may really be sleeping sickness?"
 
"No; personally I do not entertain that theory for a moment. But I am considering the evidence apart from our opinions on the subject. We have to accept it as a conceivable hypothesis that it may be sleeping sickness because we cannot positively prove that it is not. That is all. But when we come to the hypothesis of morphine poisoning, the case is different. The symptoms agree with those of morphine poisoning in every respect. There is no exception or disagreement whatever. The common sense of the matter is therefore that we adopt morphine poisoning as our working diagnosis; which is what you seem to have done."
 
"Yes. For purposes of treatment."
 
"Exactly. For medical purposes you adopted the more probable view and dismissed the less probable. That was the reasonable thing to do. But for legal purposes you must entertain both possibilities; for the hypothesis of poisoning involves serious legal issues, whereas the hypothesis of disease involves no legal issues at all."
 
"That doesn't sound very helpful," I remarked.
 
"It indicates the necessity for caution," he retorted.
 
"Yes, I see that. But what is your own opinion of the case?"
 
"Well," he said, "let us consider the facts in order. Here is a man who, we assume, is under the influence of a poisonous dose of morphine. The question is, did he take that dose himself or was it administered to him by some other person? If he took it himself, with what object did he take it? The history that was given to you seems completely to exclude the idea of suicide. But the patient's condition seems equally to exclude the idea of morphinomania. Your opium-eater does not reduce himself to a state of coma. He usually keeps well within the limits of the tolerance that has been established. The conclusion that emerges is, I think, that the drug was administered by some other person; and the most likely person seems to be Mr. Weiss."
 
"Isn't morphine a very unusual poison?"
 
"Very; and most inconvenient except in a single, fatal dose, by reason of the rapidity with which tolerance of the drug is established. But we must not forget that slow morphine poisoning might be eminently suitable in certain cases. The manner in which it enfeebles the will, confuses the judgment and debilitates the body might make it very useful to a poisoner whose aim was to get some instrument or document executed, such as a will, deed or assignment. And death could be produced afterwards by other means. You see the important bearing of this?"
 
"You mean in respect of a death certificate?"
 
"Yes. Suppose Mr. Weiss to have given a large dose of morphine. He then sends for you and throws out a suggestion of sleeping sickness. If you accept the suggestion he is pretty safe. He can repeat the process until he kills his victim and then get a certificate from you which will cover the murder. It was quite an ingenious scheme—which, by the way, is characteristic of intricate crimes; your subtle criminal often plans his crime like a genius, but he generally executes it like a fool—as this man seems to have done, if we are not doing him an injustice."
 
"How has he acted like a fool?"
 
"In several respects. In the first place, he should have chosen his doctor. A good, brisk, confident man who 'knows his own mind' is the sort of person who would have suited him; a man who would have jumped at a diagnosis and stuck to it; or else an ignorant weakling of alcoholic tendencies. It was shockingly bad luck to run against a cautious scientific practitioner like my learned friend. Then, of course, all this secrecy was sheer tomfoolery, exactly calculated to put a careful man on his guard; as it has actually done. If Mr. Weiss is really a criminal, he has mismanaged his affairs badly."
 
"And you apparently think that he is a criminal?"
 
"I suspect him deeply. But I should like to ask you one or two questions about him. You say he spoke with a German accent. What command of English had he? Was his vocabulary good? Did he use any German idioms?"
 
"No. I should say that his English was perfect, and I noticed that his phrases were quite well chosen even for an Englishman."
 
"Did he seem to you 'made up' in any way; disguised, I mean?"
 
"I couldn't say. The light was so very feeble."
 
"You couldn't see the colour of his eyes, for instance?"
 
"No. I think they were grey, but I couldn't be sure."
 
"And as to the coachman. He wore a wig, you said. Could you see the colour of his eyes? Or any peculiarity by which you could recognize him?"
 
"He had a malformed thumb-nail on his right hand. That is all I can say about him."
 
"He didn't strike you as resembling Weiss in any way; in voice or features?"
 
"Not at all; and he spoke, as I told you, with a distinct Scotch accent."
 
"The reason I ask is that if Weiss is attempting to poison this man, the coachman is almost certain to be a confederate and might be a relative. You had better examine him closely if you get another chance."
 
"I will. And that brings me back to the question, What am I to do? Ought I to report the case to the police?"
 
"I am inclined to think not. You have hardly enough facts. Of course, if Mr. Weiss has administered poison 'unlawfully and maliciously' he has committed a felony, and is liable under the Consolidation Acts of 1861 to ten years' penal servitude. But I do not see how you could swear an information. You don't know that he administered the poison—if poison has really been administered—and you cannot give any reliable name or any address whatever. Then there is the question of sleeping sickness. You reject it for medical purposes, but you could not swear, in a court of law, that this is not a case of sleeping sickness."
 
"No," I admitted, "I could not."
 
"Then I think the police would decline to move in the matter, and you might find that you had raised a scandal in Dr. Stillbury's practice to no purpose."
 
"So you think I had better do nothing in the matter?"
 
"For the present. It is, of course, a medical man's duty to assist justice in any way that is possible. But a doctor is not a detective; he should not go out of his way to assume police functions. He should keep his eyes and ears open, and, though, in general, he should keep his own counsel, it is his duty to note very carefully anything that seems to him likely to bear on any important legal issues. It is not his business officiously to initiate criminal inquiries, but it is emphatically his business to be ready, if called upon, to assist justice with information that his special knowledge and opportunities have rendered accessible to him. You see the bearing of this?"
 
"You mean that I should note down what I have seen and heard and say nothing about it until I am asked."
 
"Yes; if nothing further happens. But if you should be sent for again, I think it is your duty to make further observations with a view, if necessary, to informing the police. It may be, for instance, of vital importance to identify the house, and it is your duty to secure the means of doing so."
 
"But, my dear Thorndyke," I expostulated, "I have told you how I was conveyed to the house. Now, will you kindly explain to me how a man, boxed up in a pitch-dark carriage, is going to identify any place to which he may be carried?"
 
"The problem doesn't appear to me to present any serious difficulties," he replied.
 
"Doesn't it?" said I. "To me it looks like a pretty solid impossibility. But what do you suggest? Should I break out of the house and run away up the street? Or should I bore a hole through the shutter of the carriage and peep out?"
 
Thorndyke smiled indulgently. "The methods proposed by my learned friend display a certain crudity inappropriate to the character of a man of science; to say nothing of the disadvantage of letting the enemy into our counsels. No, no, Jervis; we can do something better than that. Just excuse me for a minute while I run up to the laboratory."
 
He hurried away to Polton's sanctum on the upper floor, leaving me to speculate on the method by which he proposed that a man should be enabled, as Sam Weller would express it, "to see through a flight of stairs and a deal door"; or, what was equally opaque, the wooden shutters of a closed carriage.
 
"Now," he said, when he returned a couple of minutes later with a small, paper-covered notebook in his hand, "I have set Polton to work on a little appliance that will, I think, solve our difficulty, and I will show you how I propose that you should make your observations. First of all, we have to rule the pages of this book into columns."
 
He sat down at the table and began methodically to rule the pages each into three columns, two quite narrow and one broad. The process occupied some time, during which I sat and watched with impatient curiosity the unhurried, precise movements of Thorndyke's pencil, all agog to hear the promised explanation. He was just finishing the last page when there came a gentle tap at the door, and Polton entered with a satisfied smile on his dry, shrewd-looking face and a small board in his hand.
 
"Will this do, sir?" he asked.
 
As he spoke he handed the little board to Thorndyke, who looked at it and passed it to me.
 
"The very thing, Polton," my friend replied. "Where did you find it? It's of no use for you to pretend that you've made it in about two minutes and a half."
 
Polton smiled one of his queer crinkly smiles, and remarking that "it didn't take much making," departed much gratified by the compliment.
 
"What a wonderful old fellow that is, Jervis," Thorndyke observed as his factotum retired. "He took in the idea instantly and seems to have produced the finished article by magic, as the conjurers bring forth rabbits and bowls of goldfish at a moment's notice. I suppose you see what your modus operandi is to be?"
 
I had gathered a clue from the little appliance—a plate of white fret-wood about seven inches by five, to one corner of which a pocket-compass had been fixed with shellac—but was not quite clear as to the details of the method.
 
"You can read a compass pretty quickly, I think?" Thorndyke said.
 
"Of course I can. Used we not to sail a yacht together when we were students?"
 
"To be sure we did; and we will again before we die. And now as to your method of locating this house. Here is a pocket reading-lamp which you can hook on the carriage lining. This notebook can be fixed to the board with an india-rubber band—thus. You observe that the thoughtful Polton has stuck a piece of thread on the glass of the compass to serve as a lubber's line. This is how you will proceed. As soon as you are locked in the carriage, light your lamp—better have a book with you in case the light is noticed—take out your watch and put the board on your knee, keeping its long side exactly in a line with the axis of the carriage. Then enter in one narrow column of your notebook the time, in the other the direction shown by the compass, and in the broad column any particulars, including the number of steps the horse makes in a minute. Like this."
 
He took a loose sheet of paper and made one or two sample entries on it in pencil, thus—
 
    "9.40. S.E. Start from home.
    9.41 S.W. Granite setts.
    9.43. S.W. Wood pavement. Hoofs 104.
    9.47. W. by S Granite crossing. Macadam— 
 
and so on. Note every change of direction, with the time; and whenever you hear or feel anything from outside, note it, with the time and direction; and don't forget to note any variations in the horse's pace. You follow the process?"
 
"Perfectly. But do you think the method is accurate enough to fix the position of a house? Remember, this is only a pocket-compass with no dial, and it will jump frightfully. And the mode of estimating distance is very rough."
 
"That is all perfectly true," Thorndyke answered. "But you are overlooking certain important facts The track-chart that you will produce can be checked by other data. The house, for instance, has a covered way by which you could identify it if you knew approximately where to look for it. Then you must remember that your carriage is not travelling over a featureless plain. It is passing through streets which have a determined position and direction and which are accurately represented on the ordnance map. I think, Jervis, that, in spite of the apparent roughness of the method, if you make your observations carefully, we shall have no trouble in narrowing down the inquiry to a quite small area. If we get the chance, that is to say."
 
"Yes, if we do. I am doubtful whether Mr. Weiss will require my services again, but I sincerely hope he will. It would be rare sport to locate his secret burrow, all unsuspected. But now I must really be off."
 
"Good-bye, then," said Thorndyke, slipping a well-sharpened pencil through the rubber band that fixed the notebook to the board. "Let me know how the adventure progresses—if it progresses at all—and remember, I hold your promise to come and see me again quite soon in any case."
 
He handed me the board and the lamp, and, when I had slipped them into my pocket, we shook hands and I hurried away, a little uneasy at having left my charge so long.