"He does a simple little act," I said to myself as I watched the figure seat itself again. "His thoughts may be as simple. But the consequences of either! Who can say? Life itself is all on one side of a blue wall!" * * * * * Physicians, however, make good detectives. I mention this not to point out my own case particularly, but merely to call your attention to the fact that a good surgeon or practitioner has a training in those qualities of mind which produce a great solver of mysteries. A good physician must develop the powers of observation. In any physical disorder, knowing the cause, he must forecast the effect, or with the evidences of some effect before him, he must deduce the cause. Above all he must keep his mind from jumping at false conclusions, even though these conclusions are in line with all his former experiences. Physicians learn these principles by their mistakes in following clues. A good diagnostician has in him the material for an immortal police inspector. I speak modestly, and yet I must say that the next morning proved that I was not mistaken in these theories. Before nine o'clock I had arrived at the Marburys'. The banker himself opened the door. "Doctor!" he cried, his face drawn out of its mask of eternal shrewdness and suspicion by a beaming smile, "what can I say? How can we ever show our gratitude?" "Not so fast!" I reproved him. "There is danger in too much optimism. The disease is treacherous." "But Miss Peters, the nurse--she sees it, too! There can be no doubt. Our little Virginia is saved! You have done it!" I shook my head. "Not I." "Not you? Who, then?" "Marbury," said I, "I am just beginning to learn that there are other contagions than those of the body. Can we be sure, my good sir, that fear is not a disease? Do we know that love is not an infection? Can the criminal's gloves, saturated with his personality, be safe for the hands of an honest man? Don't we weaken by rubbing elbows with the weak? Are there not contagious germs of thought?" He raised his eyebrows. Finance he knew well. Otherwise he was a stupid man. "I do not believe I follow you," he said nervously. "I was speaking of Virginia. She is so much better!" I bowed to him politely, and, instead of entering the open door, descended the steps. "You're not coming in?" he exclaimed. "Not yet," said I. "To tell you the truth, I am looking in that grass plot next door for something dropped there. I see that no one has disturbed the grass. It has not even been cut. Hello! What's this?" I had reached down, picked up a metal cylinder and showed it to him. "It looks like a rifle cartridge--one of those murderous steel-nosed bullet affairs," said he. "Something even more dangerous!" said I, thrusting it into my pocket. "Much more dangerous! Possibly you will believe that I am ungracious--rather odd as it were--not to mention its name."