“Yes; and I think we can distinguish between the two possibilities.  For, if he only passes the factory, the dust will be on the outside of the hat only; the inside will be protected by his head.  But if he is engaged in the works, the dust will be inside, too, as the hat will hang on a peg in the dust-laden atmosphere, and his head will also be powdered, and so convey the dust to the inside.”

He turned the hat over once more, and as I brought the powerful lens to bear upon the dark lining, I could clearly distinguish a number of white particles in the interstices of the fabric.

“The powder is on the inside, too,” I said.

He took the lens from me, and, having verified my statement, proceeded with the examination.  “You notice,” he said, “that the leather head-lining is stained with grease, and this staining is more pronounced at the sides and back.  His hair, therefore, is naturally greasy, or he greases it artificially; for if the staining were caused by perspiration, it would be most marked opposite the forehead.”

He peered anxiously into the interior of the hat, and eventually turned down the head-lining; and immediately there broke out upon his face a gleam of satisfaction.

“Ha!” he exclaimed.  “This is a stroke of luck.  I was afraid our neat and orderly friend had defeated us with his brush.  Pass me the small dissecting forceps, Jervis.”

I handed him the instrument, and he proceeded to pick out daintily from the space behind the head-lining some half a dozen short pieces of hair, which he laid, with infinite tenderness, on a sheet of white paper.

“There are several more on the other side,” I said, pointing them out to him.

“Yes, but we must leave some for the police,” he answered, with a smile.  “They must have the same chance as ourselves, you know.”

“But surely,” I said, as I bent down over the paper, “these are pieces of horsehair!”

“I think not,” he replied; “but the microscope will show.  At any rate, this is the kind of hair I should expect to find with a head of that shape.”

“Well, it is extraordinarily coarse,” said I, “and two of the hairs are nearly white.”

“Yes; black hairs beginning to turn grey.  And now, as our preliminary survey has given such encouraging results, we will proceed to more exact methods; and we must waste no time, for we shall have the police here presently to rob us of our treasure.”

He folded up carefully the paper containing the hairs, and taking the hat in both hands, as though it were some sacred vessel, ascended with me to the laboratory on the next floor.

“Now, Polton,” he said to his laboratory assistant, “we have here a specimen for examination, and time is precious.  First of all, we want your patent dust-extractor.”

The little man bustled to a cupboard and brought forth a singular appliance, of his own manufacture, somewhat like a miniature vacuum cleaner.  It had been made from a bicycle foot-pump, by reversing the piston-valve, and was fitted with a glass nozzle and a small detachable glass receiver for collecting the dust, at the end of a flexible metal tube.