All this was very plain, but the main point was still dark.
"You say you know that another man was in the room," I said; "how do you know that?"
"As I said, by an obvious inference. Come now, you shall guess how I arrived at that inference. You often speak of your interest in my work, and the attention with which you follow it. This shall be a simple exercise for you. You saw everything in the room as plainly as I myself. Bring the scene back to your memory, and think over the various small objects littering about, and how they would affect the case. Quick observation is the first essential for my work. Did you see a newspaper, for instance?"
"Yes. There was an evening paper on the floor, but I didn't examine it."
"Anything else?"
"On the table there was a whisky decanter, taken from the tantalus stand on the sideboard, and one glass. That, by-the-bye," I added, "looked as though only one person were present."
"So it did, perhaps, although the inference wouldn't be very strong. Go on."
"There was a fruit-stand on the sideboard, with a plate beside it, containing a few nutshells, a piece of apple, a pair of nutcrackers, and, I think, some orange peel. There was, of course, all the ordinary furniture, but no chair pulled up to the table except that used by Foggatt himself. That's all I noticed, I think. Stay—there was an ash-tray on the table, and a partly-burned cigar near it—only one cigar, though."
"Excellent—excellent, indeed, as far as memory and simple observation go. You saw everything plainly, and you remember everything. Surely now you know how I found out that another man had just left?"
"No, I don't; unless there were different kinds of ash in the ash-tray."